summaryrefslogtreecommitdiffstats
path: root/Documentation/trace/histogram.rst
blob: b71e09f745c3dae4511c36beb8ccf290f5f01c3e (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
================
Event Histograms
================

Documentation written by Tom Zanussi

1. Introduction
===============

  Histogram triggers are special event triggers that can be used to
  aggregate trace event data into histograms.  For information on
  trace events and event triggers, see Documentation/trace/events.rst.


2. Histogram Trigger Command
============================

  A histogram trigger command is an event trigger command that
  aggregates event hits into a hash table keyed on one or more trace
  event format fields (or stacktrace) and a set of running totals
  derived from one or more trace event format fields and/or event
  counts (hitcount).

  The format of a hist trigger is as follows::

        hist:keys=<field1[,field2,...]>[:values=<field1[,field2,...]>]
          [:sort=<field1[,field2,...]>][:size=#entries][:pause][:continue]
          [:clear][:name=histname1][:<handler>.<action>] [if <filter>]

  When a matching event is hit, an entry is added to a hash table
  using the key(s) and value(s) named.  Keys and values correspond to
  fields in the event's format description.  Values must correspond to
  numeric fields - on an event hit, the value(s) will be added to a
  sum kept for that field.  The special string 'hitcount' can be used
  in place of an explicit value field - this is simply a count of
  event hits.  If 'values' isn't specified, an implicit 'hitcount'
  value will be automatically created and used as the only value.
  Keys can be any field, or the special string 'stacktrace', which
  will use the event's kernel stacktrace as the key.  The keywords
  'keys' or 'key' can be used to specify keys, and the keywords
  'values', 'vals', or 'val' can be used to specify values.  Compound
  keys consisting of up to two fields can be specified by the 'keys'
  keyword.  Hashing a compound key produces a unique entry in the
  table for each unique combination of component keys, and can be
  useful for providing more fine-grained summaries of event data.
  Additionally, sort keys consisting of up to two fields can be
  specified by the 'sort' keyword.  If more than one field is
  specified, the result will be a 'sort within a sort': the first key
  is taken to be the primary sort key and the second the secondary
  key.  If a hist trigger is given a name using the 'name' parameter,
  its histogram data will be shared with other triggers of the same
  name, and trigger hits will update this common data.  Only triggers
  with 'compatible' fields can be combined in this way; triggers are
  'compatible' if the fields named in the trigger share the same
  number and type of fields and those fields also have the same names.
  Note that any two events always share the compatible 'hitcount' and
  'stacktrace' fields and can therefore be combined using those
  fields, however pointless that may be.

  'hist' triggers add a 'hist' file to each event's subdirectory.
  Reading the 'hist' file for the event will dump the hash table in
  its entirety to stdout.  If there are multiple hist triggers
  attached to an event, there will be a table for each trigger in the
  output.  The table displayed for a named trigger will be the same as
  any other instance having the same name. Each printed hash table
  entry is a simple list of the keys and values comprising the entry;
  keys are printed first and are delineated by curly braces, and are
  followed by the set of value fields for the entry.  By default,
  numeric fields are displayed as base-10 integers.  This can be
  modified by appending any of the following modifiers to the field
  name:

	=========== ==========================================
        .hex        display a number as a hex value
	.sym        display an address as a symbol
	.sym-offset display an address as a symbol and offset
	.syscall    display a syscall id as a system call name
	.execname   display a common_pid as a program name
	.log2       display log2 value rather than raw number
	.usecs      display a common_timestamp in microseconds
	=========== ==========================================

  Note that in general the semantics of a given field aren't
  interpreted when applying a modifier to it, but there are some
  restrictions to be aware of in this regard:

    - only the 'hex' modifier can be used for values (because values
      are essentially sums, and the other modifiers don't make sense
      in that context).
    - the 'execname' modifier can only be used on a 'common_pid'.  The
      reason for this is that the execname is simply the 'comm' value
      saved for the 'current' process when an event was triggered,
      which is the same as the common_pid value saved by the event
      tracing code.  Trying to apply that comm value to other pid
      values wouldn't be correct, and typically events that care save
      pid-specific comm fields in the event itself.

  A typical usage scenario would be the following to enable a hist
  trigger, read its current contents, and then turn it off::

    # echo 'hist:keys=skbaddr.hex:vals=len' > \
      /sys/kernel/debug/tracing/events/net/netif_rx/trigger

    # cat /sys/kernel/debug/tracing/events/net/netif_rx/hist

    # echo '!hist:keys=skbaddr.hex:vals=len' > \
      /sys/kernel/debug/tracing/events/net/netif_rx/trigger

  The trigger file itself can be read to show the details of the
  currently attached hist trigger.  This information is also displayed
  at the top of the 'hist' file when read.

  By default, the size of the hash table is 2048 entries.  The 'size'
  parameter can be used to specify more or fewer than that.  The units
  are in terms of hashtable entries - if a run uses more entries than
  specified, the results will show the number of 'drops', the number
  of hits that were ignored.  The size should be a power of 2 between
  128 and 131072 (any non- power-of-2 number specified will be rounded
  up).

  The 'sort' parameter can be used to specify a value field to sort
  on.  The default if unspecified is 'hitcount' and the default sort
  order is 'ascending'.  To sort in the opposite direction, append
  .descending' to the sort key.

  The 'pause' parameter can be used to pause an existing hist trigger
  or to start a hist trigger but not log any events until told to do
  so.  'continue' or 'cont' can be used to start or restart a paused
  hist trigger.

  The 'clear' parameter will clear the contents of a running hist
  trigger and leave its current paused/active state.

  Note that the 'pause', 'cont', and 'clear' parameters should be
  applied using 'append' shell operator ('>>') if applied to an
  existing trigger, rather than via the '>' operator, which will cause
  the trigger to be removed through truncation.

- enable_hist/disable_hist

  The enable_hist and disable_hist triggers can be used to have one
  event conditionally start and stop another event's already-attached
  hist trigger.  Any number of enable_hist and disable_hist triggers
  can be attached to a given event, allowing that event to kick off
  and stop aggregations on a host of other events.

  The format is very similar to the enable/disable_event triggers::

      enable_hist:<system>:<event>[:count]
      disable_hist:<system>:<event>[:count]

  Instead of enabling or disabling the tracing of the target event
  into the trace buffer as the enable/disable_event triggers do, the
  enable/disable_hist triggers enable or disable the aggregation of
  the target event into a hash table.

  A typical usage scenario for the enable_hist/disable_hist triggers
  would be to first set up a paused hist trigger on some event,
  followed by an enable_hist/disable_hist pair that turns the hist
  aggregation on and off when conditions of interest are hit::

   # echo 'hist:keys=skbaddr.hex:vals=len:pause' > \
      /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger

    # echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \
      /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger

    # echo 'disable_hist:net:netif_receive_skb if comm==wget' > \
      /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger

  The above sets up an initially paused hist trigger which is unpaused
  and starts aggregating events when a given program is executed, and
  which stops aggregating when the process exits and the hist trigger
  is paused again.

  The examples below provide a more concrete illustration of the
  concepts and typical usage patterns discussed above.

'special' event fields
------------------------

  There are a number of 'special event fields' available for use as
  keys or values in a hist trigger.  These look like and behave as if
  they were actual event fields, but aren't really part of the event's
  field definition or format file.  They are however available for any
  event, and can be used anywhere an actual event field could be.
  They are:

    ====================== ==== =======================================
    common_timestamp       u64  timestamp (from ring buffer) associated
                                with the event, in nanoseconds.  May be
			        modified by .usecs to have timestamps
			        interpreted as microseconds.
    cpu                    int  the cpu on which the event occurred.
    ====================== ==== =======================================

Extended error information
--------------------------

  For some error conditions encountered when invoking a hist trigger
  command, extended error information is available via the
  tracing/error_log file.  See Error Conditions in
  :file:`Documentation/trace/ftrace.rst` for details.

6.2 'hist' trigger examples
---------------------------

  The first set of examples creates aggregations using the kmalloc
  event.  The fields that can be used for the hist trigger are listed
  in the kmalloc event's format file::

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/format
    name: kmalloc
    ID: 374
    format:
	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
	field:unsigned char common_preempt_count;		offset:3;	size:1;	signed:0;
	field:int common_pid;					offset:4;	size:4;	signed:1;

	field:unsigned long call_site;				offset:8;	size:8;	signed:0;
	field:const void * ptr;					offset:16;	size:8;	signed:0;
	field:size_t bytes_req;					offset:24;	size:8;	signed:0;
	field:size_t bytes_alloc;				offset:32;	size:8;	signed:0;
	field:gfp_t gfp_flags;					offset:40;	size:4;	signed:0;

  We'll start by creating a hist trigger that generates a simple table
  that lists the total number of bytes requested for each function in
  the kernel that made one or more calls to kmalloc::

    # echo 'hist:key=call_site:val=bytes_req' > \
            /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

  This tells the tracing system to create a 'hist' trigger using the
  call_site field of the kmalloc event as the key for the table, which
  just means that each unique call_site address will have an entry
  created for it in the table.  The 'val=bytes_req' parameter tells
  the hist trigger that for each unique entry (call_site) in the
  table, it should keep a running total of the number of bytes
  requested by that call_site.

  We'll let it run for awhile and then dump the contents of the 'hist'
  file in the kmalloc event's subdirectory (for readability, a number
  of entries have been omitted)::

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]

    { call_site: 18446744072106379007 } hitcount:          1  bytes_req:        176
    { call_site: 18446744071579557049 } hitcount:          1  bytes_req:       1024
    { call_site: 18446744071580608289 } hitcount:          1  bytes_req:      16384
    { call_site: 18446744071581827654 } hitcount:          1  bytes_req:         24
    { call_site: 18446744071580700980 } hitcount:          1  bytes_req:          8
    { call_site: 18446744071579359876 } hitcount:          1  bytes_req:        152
    { call_site: 18446744071580795365 } hitcount:          3  bytes_req:        144
    { call_site: 18446744071581303129 } hitcount:          3  bytes_req:        144
    { call_site: 18446744071580713234 } hitcount:          4  bytes_req:       2560
    { call_site: 18446744071580933750 } hitcount:          4  bytes_req:        736
    .
    .
    .
    { call_site: 18446744072106047046 } hitcount:         69  bytes_req:       5576
    { call_site: 18446744071582116407 } hitcount:         73  bytes_req:       2336
    { call_site: 18446744072106054684 } hitcount:        136  bytes_req:     140504
    { call_site: 18446744072106224230 } hitcount:        136  bytes_req:      19584
    { call_site: 18446744072106078074 } hitcount:        153  bytes_req:       2448
    { call_site: 18446744072106062406 } hitcount:        153  bytes_req:      36720
    { call_site: 18446744071582507929 } hitcount:        153  bytes_req:      37088
    { call_site: 18446744072102520590 } hitcount:        273  bytes_req:      10920
    { call_site: 18446744071582143559 } hitcount:        358  bytes_req:        716
    { call_site: 18446744072106465852 } hitcount:        417  bytes_req:      56712
    { call_site: 18446744072102523378 } hitcount:        485  bytes_req:      27160
    { call_site: 18446744072099568646 } hitcount:       1676  bytes_req:      33520

    Totals:
        Hits: 4610
        Entries: 45
        Dropped: 0

  The output displays a line for each entry, beginning with the key
  specified in the trigger, followed by the value(s) also specified in
  the trigger.  At the beginning of the output is a line that displays
  the trigger info, which can also be displayed by reading the
  'trigger' file::

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
    hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]

  At the end of the output are a few lines that display the overall
  totals for the run.  The 'Hits' field shows the total number of
  times the event trigger was hit, the 'Entries' field shows the total
  number of used entries in the hash table, and the 'Dropped' field
  shows the number of hits that were dropped because the number of
  used entries for the run exceeded the maximum number of entries
  allowed for the table (normally 0, but if not a hint that you may
  want to increase the size of the table using the 'size' parameter).

  Notice in the above output that there's an extra field, 'hitcount',
  which wasn't specified in the trigger.  Also notice that in the
  trigger info output, there's a parameter, 'sort=hitcount', which
  wasn't specified in the trigger either.  The reason for that is that
  every trigger implicitly keeps a count of the total number of hits
  attributed to a given entry, called the 'hitcount'.  That hitcount
  information is explicitly displayed in the output, and in the
  absence of a user-specified sort parameter, is used as the default
  sort field.

  The value 'hitcount' can be used in place of an explicit value in
  the 'values' parameter if you don't really need to have any
  particular field summed and are mainly interested in hit
  frequencies.

  To turn the hist trigger off, simply call up the trigger in the
  command history and re-execute it with a '!' prepended::

    # echo '!hist:key=call_site:val=bytes_req' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

  Finally, notice that the call_site as displayed in the output above
  isn't really very useful.  It's an address, but normally addresses
  are displayed in hex.  To have a numeric field displayed as a hex
  value, simply append '.hex' to the field name in the trigger::

    # echo 'hist:key=call_site.hex:val=bytes_req' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.hex:vals=bytes_req:sort=hitcount:size=2048 [active]

    { call_site: ffffffffa026b291 } hitcount:          1  bytes_req:        433
    { call_site: ffffffffa07186ff } hitcount:          1  bytes_req:        176
    { call_site: ffffffff811ae721 } hitcount:          1  bytes_req:      16384
    { call_site: ffffffff811c5134 } hitcount:          1  bytes_req:          8
    { call_site: ffffffffa04a9ebb } hitcount:          1  bytes_req:        511
    { call_site: ffffffff8122e0a6 } hitcount:          1  bytes_req:         12
    { call_site: ffffffff8107da84 } hitcount:          1  bytes_req:        152
    { call_site: ffffffff812d8246 } hitcount:          1  bytes_req:         24
    { call_site: ffffffff811dc1e5 } hitcount:          3  bytes_req:        144
    { call_site: ffffffffa02515e8 } hitcount:          3  bytes_req:        648
    { call_site: ffffffff81258159 } hitcount:          3  bytes_req:        144
    { call_site: ffffffff811c80f4 } hitcount:          4  bytes_req:        544
    .
    .
    .
    { call_site: ffffffffa06c7646 } hitcount:        106  bytes_req:       8024
    { call_site: ffffffffa06cb246 } hitcount:        132  bytes_req:      31680
    { call_site: ffffffffa06cef7a } hitcount:        132  bytes_req:       2112
    { call_site: ffffffff8137e399 } hitcount:        132  bytes_req:      23232
    { call_site: ffffffffa06c941c } hitcount:        185  bytes_req:     171360
    { call_site: ffffffffa06f2a66 } hitcount:        185  bytes_req:      26640
    { call_site: ffffffffa036a70e } hitcount:        265  bytes_req:      10600
    { call_site: ffffffff81325447 } hitcount:        292  bytes_req:        584
    { call_site: ffffffffa072da3c } hitcount:        446  bytes_req:      60656
    { call_site: ffffffffa036b1f2 } hitcount:        526  bytes_req:      29456
    { call_site: ffffffffa0099c06 } hitcount:       1780  bytes_req:      35600

    Totals:
        Hits: 4775
        Entries: 46
        Dropped: 0

  Even that's only marginally more useful - while hex values do look
  more like addresses, what users are typically more interested in
  when looking at text addresses are the corresponding symbols
  instead.  To have an address displayed as symbolic value instead,
  simply append '.sym' or '.sym-offset' to the field name in the
  trigger::

    # echo 'hist:key=call_site.sym:val=bytes_req' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=hitcount:size=2048 [active]

    { call_site: [ffffffff810adcb9] syslog_print_all                              } hitcount:          1  bytes_req:       1024
    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8
    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7
    { call_site: [ffffffff8154acbe] usb_alloc_urb                                 } hitcount:          1  bytes_req:        192
    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7
    { call_site: [ffffffff811e3a25] __seq_open_private                            } hitcount:          1  bytes_req:         40
    { call_site: [ffffffff8109524a] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128
    { call_site: [ffffffff811febd5] fsnotify_alloc_group                          } hitcount:          2  bytes_req:        528
    { call_site: [ffffffff81440f58] __tty_buffer_request_room                     } hitcount:          2  bytes_req:       2624
    { call_site: [ffffffff81200ba6] inotify_new_group                             } hitcount:          2  bytes_req:         96
    { call_site: [ffffffffa05e19af] ieee80211_start_tx_ba_session [mac80211]      } hitcount:          2  bytes_req:        464
    { call_site: [ffffffff81672406] tcp_get_metrics                               } hitcount:          2  bytes_req:        304
    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128
    { call_site: [ffffffff81089b05] sched_create_group                            } hitcount:          2  bytes_req:       1424
    .
    .
    .
    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:       1185  bytes_req:     123240
    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm]                } hitcount:       1185  bytes_req:     104280
    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:       1402  bytes_req:     190672
    { call_site: [ffffffff812891ca] ext4_find_extent                              } hitcount:       1518  bytes_req:     146208
    { call_site: [ffffffffa029070e] drm_vma_node_allow [drm]                      } hitcount:       1746  bytes_req:      69840
    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       2021  bytes_req:     792312
    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       2592  bytes_req:     145152
    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       2629  bytes_req:     378576
    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       2629  bytes_req:    3783248
    { call_site: [ffffffff81325607] apparmor_file_alloc_security                  } hitcount:       5192  bytes_req:      10384
    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       5529  bytes_req:     110584
    { call_site: [ffffffff8131ebf7] aa_alloc_task_context                         } hitcount:      21943  bytes_req:     702176
    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:      55759  bytes_req:    5074265

    Totals:
        Hits: 109928
        Entries: 71
        Dropped: 0

  Because the default sort key above is 'hitcount', the above shows a
  the list of call_sites by increasing hitcount, so that at the bottom
  we see the functions that made the most kmalloc calls during the
  run.  If instead we we wanted to see the top kmalloc callers in
  terms of the number of bytes requested rather than the number of
  calls, and we wanted the top caller to appear at the top, we can use
  the 'sort' parameter, along with the 'descending' modifier::

    # echo 'hist:key=call_site.sym:val=bytes_req:sort=bytes_req.descending' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]

    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       2186  bytes_req:    3397464
    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       1790  bytes_req:     712176
    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:       8132  bytes_req:     513135
    { call_site: [ffffffff811e2a1b] seq_buf_alloc                                 } hitcount:        106  bytes_req:     440128
    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       2186  bytes_req:     314784
    { call_site: [ffffffff812891ca] ext4_find_extent                              } hitcount:       2174  bytes_req:     208992
    { call_site: [ffffffff811ae8e1] __kmalloc                                     } hitcount:          8  bytes_req:     131072
    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:        859  bytes_req:     116824
    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       1834  bytes_req:     102704
    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:        972  bytes_req:     101088
    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm]                } hitcount:        972  bytes_req:      85536
    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       3333  bytes_req:      66664
    { call_site: [ffffffff8137e559] sg_kmalloc                                    } hitcount:        209  bytes_req:      61632
    .
    .
    .
    { call_site: [ffffffff81095225] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128
    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128
    { call_site: [ffffffff812d8406] copy_semundo                                  } hitcount:          2  bytes_req:         48
    { call_site: [ffffffff81200ba6] inotify_new_group                             } hitcount:          1  bytes_req:         48
    { call_site: [ffffffffa027121a] drm_getmagic [drm]                            } hitcount:          1  bytes_req:         48
    { call_site: [ffffffff811e3a25] __seq_open_private                            } hitcount:          1  bytes_req:         40
    { call_site: [ffffffff811c52f4] bprm_change_interp                            } hitcount:          2  bytes_req:         16
    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8
    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7
    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7

    Totals:
        Hits: 32133
        Entries: 81
        Dropped: 0

  To display the offset and size information in addition to the symbol
  name, just use 'sym-offset' instead::

    # echo 'hist:key=call_site.sym-offset:val=bytes_req:sort=bytes_req.descending' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.sym-offset:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]

    { call_site: [ffffffffa046041c] i915_gem_execbuffer2+0x6c/0x2c0 [i915]                  } hitcount:       4569  bytes_req:    3163720
    { call_site: [ffffffffa0489a66] intel_ring_begin+0xc6/0x1f0 [i915]                      } hitcount:       4569  bytes_req:     657936
    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23+0x694/0x1020 [i915]      } hitcount:       1519  bytes_req:     472936
    { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23+0x516/0x1020 [i915]      } hitcount:       3050  bytes_req:     211832
    { call_site: [ffffffff811e2a1b] seq_buf_alloc+0x1b/0x50                                 } hitcount:         34  bytes_req:     148384
    { call_site: [ffffffffa04a580c] intel_crtc_page_flip+0xbc/0x870 [i915]                  } hitcount:       1385  bytes_req:     144040
    { call_site: [ffffffff811ae8e1] __kmalloc+0x191/0x1b0                                   } hitcount:          8  bytes_req:     131072
    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl+0x282/0x360 [drm]              } hitcount:       1385  bytes_req:     121880
    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc+0x32/0x100 [drm]                  } hitcount:       1848  bytes_req:     103488
    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state+0x2c/0xa0 [i915]            } hitcount:        461  bytes_req:      62696
    { call_site: [ffffffffa029070e] drm_vma_node_allow+0x2e/0xd0 [drm]                      } hitcount:       1541  bytes_req:      61640
    { call_site: [ffffffff815f8d7b] sk_prot_alloc+0xcb/0x1b0                                } hitcount:         57  bytes_req:      57456
    .
    .
    .
    { call_site: [ffffffff8109524a] alloc_fair_sched_group+0x5a/0x1a0                       } hitcount:          2  bytes_req:        128
    { call_site: [ffffffffa027b921] drm_vm_open_locked+0x31/0xa0 [drm]                      } hitcount:          3  bytes_req:         96
    { call_site: [ffffffff8122e266] proc_self_follow_link+0x76/0xb0                         } hitcount:          8  bytes_req:         96
    { call_site: [ffffffff81213e80] load_elf_binary+0x240/0x1650                            } hitcount:          3  bytes_req:         84
    { call_site: [ffffffff8154bc62] usb_control_msg+0x42/0x110                              } hitcount:          1  bytes_req:          8
    { call_site: [ffffffffa00bf6fe] hidraw_send_report+0x7e/0x1a0 [hid]                     } hitcount:          1  bytes_req:          7
    { call_site: [ffffffffa00bf1ca] hidraw_report_event+0x8a/0x120 [hid]                    } hitcount:          1  bytes_req:          7

    Totals:
        Hits: 26098
        Entries: 64
        Dropped: 0

  We can also add multiple fields to the 'values' parameter.  For
  example, we might want to see the total number of bytes allocated
  alongside bytes requested, and display the result sorted by bytes
  allocated in a descending order::

    # echo 'hist:keys=call_site.sym:values=bytes_req,bytes_alloc:sort=bytes_alloc.descending' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.sym:vals=bytes_req,bytes_alloc:sort=bytes_alloc.descending:size=2048 [active]

    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       7403  bytes_req:    4084360  bytes_alloc:    5958016
    { call_site: [ffffffff811e2a1b] seq_buf_alloc                                 } hitcount:        541  bytes_req:    2213968  bytes_alloc:    2228224
    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       7404  bytes_req:    1066176  bytes_alloc:    1421568
    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       1565  bytes_req:     557368  bytes_alloc:    1037760
    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:       9557  bytes_req:     595778  bytes_alloc:     695744
    { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       5839  bytes_req:     430680  bytes_alloc:     470400
    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:       2388  bytes_req:     324768  bytes_alloc:     458496
    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       3911  bytes_req:     219016  bytes_alloc:     250304
    { call_site: [ffffffff815f8d7b] sk_prot_alloc                                 } hitcount:        235  bytes_req:     236880  bytes_alloc:     240640
    { call_site: [ffffffff8137e559] sg_kmalloc                                    } hitcount:        557  bytes_req:     169024  bytes_alloc:     221760
    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       9378  bytes_req:     187548  bytes_alloc:     206312
    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:       1519  bytes_req:     157976  bytes_alloc:     194432
    .
    .
    .
    { call_site: [ffffffff8109bd3b] sched_autogroup_create_attach                 } hitcount:          2  bytes_req:        144  bytes_alloc:        192
    { call_site: [ffffffff81097ee8] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128  bytes_alloc:        128
    { call_site: [ffffffff8109524a] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128  bytes_alloc:        128
    { call_site: [ffffffff81095225] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128  bytes_alloc:        128
    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128  bytes_alloc:        128
    { call_site: [ffffffff81213e80] load_elf_binary                               } hitcount:          3  bytes_req:         84  bytes_alloc:         96
    { call_site: [ffffffff81079a2e] kthread_create_on_node                        } hitcount:          1  bytes_req:         56  bytes_alloc:         64
    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7  bytes_alloc:          8
    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8  bytes_alloc:          8
    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7  bytes_alloc:          8

    Totals:
        Hits: 66598
        Entries: 65
        Dropped: 0

  Finally, to finish off our kmalloc example, instead of simply having
  the hist trigger display symbolic call_sites, we can have the hist
  trigger additionally display the complete set of kernel stack traces
  that led to each call_site.  To do that, we simply use the special
  value 'stacktrace' for the key parameter::

    # echo 'hist:keys=stacktrace:values=bytes_req,bytes_alloc:sort=bytes_alloc' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

  The above trigger will use the kernel stack trace in effect when an
  event is triggered as the key for the hash table.  This allows the
  enumeration of every kernel callpath that led up to a particular
  event, along with a running total of any of the event fields for
  that event.  Here we tally bytes requested and bytes allocated for
  every callpath in the system that led up to a kmalloc (in this case
  every callpath to a kmalloc for a kernel compile)::

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=stacktrace:vals=bytes_req,bytes_alloc:sort=bytes_alloc:size=2048 [active]

    { stacktrace:
         __kmalloc_track_caller+0x10b/0x1a0
         kmemdup+0x20/0x50
         hidraw_report_event+0x8a/0x120 [hid]
         hid_report_raw_event+0x3ea/0x440 [hid]
         hid_input_report+0x112/0x190 [hid]
         hid_irq_in+0xc2/0x260 [usbhid]
         __usb_hcd_giveback_urb+0x72/0x120
         usb_giveback_urb_bh+0x9e/0xe0
         tasklet_hi_action+0xf8/0x100
         __do_softirq+0x114/0x2c0
         irq_exit+0xa5/0xb0
         do_IRQ+0x5a/0xf0
         ret_from_intr+0x0/0x30
         cpuidle_enter+0x17/0x20
         cpu_startup_entry+0x315/0x3e0
         rest_init+0x7c/0x80
    } hitcount:          3  bytes_req:         21  bytes_alloc:         24
    { stacktrace:
         __kmalloc_track_caller+0x10b/0x1a0
         kmemdup+0x20/0x50
         hidraw_report_event+0x8a/0x120 [hid]
         hid_report_raw_event+0x3ea/0x440 [hid]
         hid_input_report+0x112/0x190 [hid]
         hid_irq_in+0xc2/0x260 [usbhid]
         __usb_hcd_giveback_urb+0x72/0x120
         usb_giveback_urb_bh+0x9e/0xe0
         tasklet_hi_action+0xf8/0x100
         __do_softirq+0x114/0x2c0
         irq_exit+0xa5/0xb0
         do_IRQ+0x5a/0xf0
         ret_from_intr+0x0/0x30
    } hitcount:          3  bytes_req:         21  bytes_alloc:         24
    { stacktrace:
         kmem_cache_alloc_trace+0xeb/0x150
         aa_alloc_task_context+0x27/0x40
         apparmor_cred_prepare+0x1f/0x50
         security_prepare_creds+0x16/0x20
         prepare_creds+0xdf/0x1a0
         SyS_capset+0xb5/0x200
         system_call_fastpath+0x12/0x6a
    } hitcount:          1  bytes_req:         32  bytes_alloc:         32
    .
    .
    .
    { stacktrace:
         __kmalloc+0x11b/0x1b0
         i915_gem_execbuffer2+0x6c/0x2c0 [i915]
         drm_ioctl+0x349/0x670 [drm]
         do_vfs_ioctl+0x2f0/0x4f0
         SyS_ioctl+0x81/0xa0
         system_call_fastpath+0x12/0x6a
    } hitcount:      17726  bytes_req:   13944120  bytes_alloc:   19593808
    { stacktrace:
         __kmalloc+0x11b/0x1b0
         load_elf_phdrs+0x76/0xa0
         load_elf_binary+0x102/0x1650
         search_binary_handler+0x97/0x1d0
         do_execveat_common.isra.34+0x551/0x6e0
         SyS_execve+0x3a/0x50
         return_from_execve+0x0/0x23
    } hitcount:      33348  bytes_req:   17152128  bytes_alloc:   20226048
    { stacktrace:
         kmem_cache_alloc_trace+0xeb/0x150
         apparmor_file_alloc_security+0x27/0x40
         security_file_alloc+0x16/0x20
         get_empty_filp+0x93/0x1c0
         path_openat+0x31/0x5f0
         do_filp_open+0x3a/0x90
         do_sys_open+0x128/0x220
         SyS_open+0x1e/0x20
         system_call_fastpath+0x12/0x6a
    } hitcount:    4766422  bytes_req:    9532844  bytes_alloc:   38131376
    { stacktrace:
         __kmalloc+0x11b/0x1b0
         seq_buf_alloc+0x1b/0x50
         seq_read+0x2cc/0x370
         proc_reg_read+0x3d/0x80
         __vfs_read+0x28/0xe0
         vfs_read+0x86/0x140
         SyS_read+0x46/0xb0
         system_call_fastpath+0x12/0x6a
    } hitcount:      19133  bytes_req:   78368768  bytes_alloc:   78368768

    Totals:
        Hits: 6085872
        Entries: 253
        Dropped: 0

  If you key a hist trigger on common_pid, in order for example to
  gather and display sorted totals for each process, you can use the
  special .execname modifier to display the executable names for the
  processes in the table rather than raw pids.  The example below
  keeps a per-process sum of total bytes read::

    # echo 'hist:key=common_pid.execname:val=count:sort=count.descending' > \
           /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/trigger

    # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/hist
    # trigger info: hist:keys=common_pid.execname:vals=count:sort=count.descending:size=2048 [active]

    { common_pid: gnome-terminal  [      3196] } hitcount:        280  count:    1093512
    { common_pid: Xorg            [      1309] } hitcount:        525  count:     256640
    { common_pid: compiz          [      2889] } hitcount:         59  count:     254400
    { common_pid: bash            [      8710] } hitcount:          3  count:      66369
    { common_pid: dbus-daemon-lau [      8703] } hitcount:         49  count:      47739
    { common_pid: irqbalance      [      1252] } hitcount:         27  count:      27648
    { common_pid: 01ifupdown      [      8705] } hitcount:          3  count:      17216
    { common_pid: dbus-daemon     [       772] } hitcount:         10  count:      12396
    { common_pid: Socket Thread   [      8342] } hitcount:         11  count:      11264
    { common_pid: nm-dhcp-client. [      8701] } hitcount:          6  count:       7424
    { common_pid: gmain           [      1315] } hitcount:         18  count:       6336
    .
    .
    .
    { common_pid: postgres        [      1892] } hitcount:          2  count:         32
    { common_pid: postgres        [      1891] } hitcount:          2  count:         32
    { common_pid: gmain           [      8704] } hitcount:          2  count:         32
    { common_pid: upstart-dbus-br [      2740] } hitcount:         21  count:         21
    { common_pid: nm-dispatcher.a [      8696] } hitcount:          1  count:         16
    { common_pid: indicator-datet [      2904] } hitcount:          1  count:         16
    { common_pid: gdbus           [      2998] } hitcount:          1  count:         16
    { common_pid: rtkit-daemon    [      2052] } hitcount:          1  count:          8
    { common_pid: init            [         1] } hitcount:          2  count:          2

    Totals:
        Hits: 2116
        Entries: 51
        Dropped: 0

  Similarly, if you key a hist trigger on syscall id, for example to
  gather and display a list of systemwide syscall hits, you can use
  the special .syscall modifier to display the syscall names rather
  than raw ids.  The example below keeps a running total of syscall
  counts for the system during the run::

    # echo 'hist:key=id.syscall:val=hitcount' > \
           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger

    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
    # trigger info: hist:keys=id.syscall:vals=hitcount:sort=hitcount:size=2048 [active]

    { id: sys_fsync                     [ 74] } hitcount:          1
    { id: sys_newuname                  [ 63] } hitcount:          1
    { id: sys_prctl                     [157] } hitcount:          1
    { id: sys_statfs                    [137] } hitcount:          1
    { id: sys_symlink                   [ 88] } hitcount:          1
    { id: sys_sendmmsg                  [307] } hitcount:          1
    { id: sys_semctl                    [ 66] } hitcount:          1
    { id: sys_readlink                  [ 89] } hitcount:          3
    { id: sys_bind                      [ 49] } hitcount:          3
    { id: sys_getsockname               [ 51] } hitcount:          3
    { id: sys_unlink                    [ 87] } hitcount:          3
    { id: sys_rename                    [ 82] } hitcount:          4
    { id: unknown_syscall               [ 58] } hitcount:          4
    { id: sys_connect                   [ 42] } hitcount:          4
    { id: sys_getpid                    [ 39] } hitcount:          4
    .
    .
    .
    { id: sys_rt_sigprocmask            [ 14] } hitcount:        952
    { id: sys_futex                     [202] } hitcount:       1534
    { id: sys_write                     [  1] } hitcount:       2689
    { id: sys_setitimer                 [ 38] } hitcount:       2797
    { id: sys_read                      [  0] } hitcount:       3202
    { id: sys_select                    [ 23] } hitcount:       3773
    { id: sys_writev                    [ 20] } hitcount:       4531
    { id: sys_poll                      [  7] } hitcount:       8314
    { id: sys_recvmsg                   [ 47] } hitcount:      13738
    { id: sys_ioctl                     [ 16] } hitcount:      21843

    Totals:
        Hits: 67612
        Entries: 72
        Dropped: 0

  The syscall counts above provide a rough overall picture of system
  call activity on the system; we can see for example that the most
  popular system call on this system was the 'sys_ioctl' system call.

  We can use 'compound' keys to refine that number and provide some
  further insight as to which processes exactly contribute to the
  overall ioctl count.

  The command below keeps a hitcount for every unique combination of
  system call id and pid - the end result is essentially a table
  that keeps a per-pid sum of system call hits.  The results are
  sorted using the system call id as the primary key, and the
  hitcount sum as the secondary key::

    # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount' > \
           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger

    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
    # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 [active]

    { id: sys_read                      [  0], common_pid: rtkit-daemon    [      1877] } hitcount:          1
    { id: sys_read                      [  0], common_pid: gdbus           [      2976] } hitcount:          1
    { id: sys_read                      [  0], common_pid: console-kit-dae [      3400] } hitcount:          1
    { id: sys_read                      [  0], common_pid: postgres        [      1865] } hitcount:          1
    { id: sys_read                      [  0], common_pid: deja-dup-monito [      3543] } hitcount:          2
    { id: sys_read                      [  0], common_pid: NetworkManager  [       890] } hitcount:          2
    { id: sys_read                      [  0], common_pid: evolution-calen [      3048] } hitcount:          2
    { id: sys_read                      [  0], common_pid: postgres        [      1864] } hitcount:          2
    { id: sys_read                      [  0], common_pid: nm-applet       [      3022] } hitcount:          2
    { id: sys_read                      [  0], common_pid: whoopsie        [      1212] } hitcount:          2
    .
    .
    .
    { id: sys_ioctl                     [ 16], common_pid: bash            [      8479] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: bash            [      3472] } hitcount:         12
    { id: sys_ioctl                     [ 16], common_pid: gnome-terminal  [      3199] } hitcount:         16
    { id: sys_ioctl                     [ 16], common_pid: Xorg            [      1267] } hitcount:       1808
    { id: sys_ioctl                     [ 16], common_pid: compiz          [      2994] } hitcount:       5580
    .
    .
    .
    { id: sys_waitid                    [247], common_pid: upstart-dbus-br [      2690] } hitcount:          3
    { id: sys_waitid                    [247], common_pid: upstart-dbus-br [      2688] } hitcount:         16
    { id: sys_inotify_add_watch         [254], common_pid: gmain           [       975] } hitcount:          2
    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3204] } hitcount:          4
    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      2888] } hitcount:          4
    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3003] } hitcount:          4
    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      2873] } hitcount:          4
    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3196] } hitcount:          6
    { id: sys_openat                    [257], common_pid: java            [      2623] } hitcount:          2
    { id: sys_eventfd2                  [290], common_pid: ibus-ui-gtk3    [      2760] } hitcount:          4
    { id: sys_eventfd2                  [290], common_pid: compiz          [      2994] } hitcount:          6

    Totals:
        Hits: 31536
        Entries: 323
        Dropped: 0

  The above list does give us a breakdown of the ioctl syscall by
  pid, but it also gives us quite a bit more than that, which we
  don't really care about at the moment.  Since we know the syscall
  id for sys_ioctl (16, displayed next to the sys_ioctl name), we
  can use that to filter out all the other syscalls::

    # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount if id == 16' > \
           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger

    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
    # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 if id == 16 [active]

    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2769] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: evolution-addre [      8571] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: gmain           [      3003] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2781] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2829] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: bash            [      8726] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: bash            [      8508] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2970] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2768] } hitcount:          1
    .
    .
    .
    { id: sys_ioctl                     [ 16], common_pid: pool            [      8559] } hitcount:         45
    { id: sys_ioctl                     [ 16], common_pid: pool            [      8555] } hitcount:         48
    { id: sys_ioctl                     [ 16], common_pid: pool            [      8551] } hitcount:         48
    { id: sys_ioctl                     [ 16], common_pid: avahi-daemon    [       896] } hitcount:         66
    { id: sys_ioctl                     [ 16], common_pid: Xorg            [      1267] } hitcount:      26674
    { id: sys_ioctl                     [ 16], common_pid: compiz          [      2994] } hitcount:      73443

    Totals:
        Hits: 101162
        Entries: 103
        Dropped: 0

  The above output shows that 'compiz' and 'Xorg' are far and away
  the heaviest ioctl callers (which might lead to questions about
  whether they really need to be making all those calls and to
  possible avenues for further investigation.)

  The compound key examples used a key and a sum value (hitcount) to
  sort the output, but we can just as easily use two keys instead.
  Here's an example where we use a compound key composed of the the
  common_pid and size event fields.  Sorting with pid as the primary
  key and 'size' as the secondary key allows us to display an
  ordered summary of the recvfrom sizes, with counts, received by
  each process::

    # echo 'hist:key=common_pid.execname,size:val=hitcount:sort=common_pid,size' > \
           /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/trigger

    # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/hist
    # trigger info: hist:keys=common_pid.execname,size:vals=hitcount:sort=common_pid.execname,size:size=2048 [active]

    { common_pid: smbd            [       784], size:          4 } hitcount:          1
    { common_pid: dnsmasq         [      1412], size:       4096 } hitcount:        672
    { common_pid: postgres        [      1796], size:       1000 } hitcount:          6
    { common_pid: postgres        [      1867], size:       1000 } hitcount:         10
    { common_pid: bamfdaemon      [      2787], size:         28 } hitcount:          2
    { common_pid: bamfdaemon      [      2787], size:      14360 } hitcount:          1
    { common_pid: compiz          [      2994], size:          8 } hitcount:          1
    { common_pid: compiz          [      2994], size:         20 } hitcount:         11
    { common_pid: gnome-terminal  [      3199], size:          4 } hitcount:          2
    { common_pid: firefox         [      8817], size:          4 } hitcount:          1
    { common_pid: firefox         [      8817], size:          8 } hitcount:          5
    { common_pid: firefox         [      8817], size:        588 } hitcount:          2
    { common_pid: firefox         [      8817], size:        628 } hitcount:          1
    { common_pid: firefox         [      8817], size:       6944 } hitcount:          1
    { common_pid: firefox         [      8817], size:     408880 } hitcount:          2
    { common_pid: firefox         [      8822], size:          8 } hitcount:          2
    { common_pid: firefox         [      8822], size:        160 } hitcount:          2
    { common_pid: firefox         [      8822], size:        320 } hitcount:          2
    { common_pid: firefox         [      8822], size:        352 } hitcount:          1
    .
    .
    .
    { common_pid: pool            [      8923], size:       1960 } hitcount:         10
    { common_pid: pool            [      8923], size:       2048 } hitcount:         10
    { common_pid: pool            [      8924], size:       1960 } hitcount:         10
    { common_pid: pool            [      8924], size:       2048 } hitcount:         10
    { common_pid: pool            [      8928], size:       1964 } hitcount:          4
    { common_pid: pool            [      8928], size:       1965 } hitcount:          2
    { common_pid: pool            [      8928], size:       2048 } hitcount:          6
    { common_pid: pool            [      8929], size:       1982 } hitcount:          1
    { common_pid: pool            [      8929], size:       2048 } hitcount:          1

    Totals:
        Hits: 2016
        Entries: 224
        Dropped: 0

  The above example also illustrates the fact that although a compound
  key is treated as a single entity for hashing purposes, the sub-keys
  it's composed of can be accessed independently.

  The next example uses a string field as the hash key and
  demonstrates how you can manually pause and continue a hist trigger.
  In this example, we'll aggregate fork counts and don't expect a
  large number of entries in the hash table, so we'll drop it to a
  much smaller number, say 256::

    # echo 'hist:key=child_comm:val=hitcount:size=256' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]

    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: ibus-daemon                         } hitcount:          1
    { child_comm: whoopsie                            } hitcount:          1
    { child_comm: smbd                                } hitcount:          1
    { child_comm: gdbus                               } hitcount:          1
    { child_comm: kthreadd                            } hitcount:          1
    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: evolution-alarm                     } hitcount:          2
    { child_comm: Socket Thread                       } hitcount:          2
    { child_comm: postgres                            } hitcount:          2
    { child_comm: bash                                } hitcount:          3
    { child_comm: compiz                              } hitcount:          3
    { child_comm: evolution-sourc                     } hitcount:          4
    { child_comm: dhclient                            } hitcount:          4
    { child_comm: pool                                } hitcount:          5
    { child_comm: nm-dispatcher.a                     } hitcount:          8
    { child_comm: firefox                             } hitcount:          8
    { child_comm: dbus-daemon                         } hitcount:          8
    { child_comm: glib-pacrunner                      } hitcount:         10
    { child_comm: evolution                           } hitcount:         23

    Totals:
        Hits: 89
        Entries: 20
        Dropped: 0

  If we want to pause the hist trigger, we can simply append :pause to
  the command that started the trigger.  Notice that the trigger info
  displays as [paused]::

    # echo 'hist:key=child_comm:val=hitcount:size=256:pause' >> \
           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [paused]

    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: kthreadd                            } hitcount:          1
    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: gdbus                               } hitcount:          1
    { child_comm: ibus-daemon                         } hitcount:          1
    { child_comm: Socket Thread                       } hitcount:          2
    { child_comm: evolution-alarm                     } hitcount:          2
    { child_comm: smbd                                } hitcount:          2
    { child_comm: bash                                } hitcount:          3
    { child_comm: whoopsie                            } hitcount:          3
    { child_comm: compiz                              } hitcount:          3
    { child_comm: evolution-sourc                     } hitcount:          4
    { child_comm: pool                                } hitcount:          5
    { child_comm: postgres                            } hitcount:          6
    { child_comm: firefox                             } hitcount:          8
    { child_comm: dhclient                            } hitcount:         10
    { child_comm: emacs                               } hitcount:         12
    { child_comm: dbus-daemon                         } hitcount:         20
    { child_comm: nm-dispatcher.a                     } hitcount:         20
    { child_comm: evolution                           } hitcount:         35
    { child_comm: glib-pacrunner                      } hitcount:         59

    Totals:
        Hits: 199
        Entries: 21
        Dropped: 0

  To manually continue having the trigger aggregate events, append
  :cont instead.  Notice that the trigger info displays as [active]
  again, and the data has changed::

    # echo 'hist:key=child_comm:val=hitcount:size=256:cont' >> \
           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]

    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: kthreadd                            } hitcount:          1
    { child_comm: gdbus                               } hitcount:          1
    { child_comm: ibus-daemon                         } hitcount:          1
    { child_comm: Socket Thread                       } hitcount:          2
    { child_comm: evolution-alarm                     } hitcount:          2
    { child_comm: smbd                                } hitcount:          2
    { child_comm: whoopsie                            } hitcount:          3
    { child_comm: compiz                              } hitcount:          3
    { child_comm: evolution-sourc                     } hitcount:          4
    { child_comm: bash                                } hitcount:          5
    { child_comm: pool                                } hitcount:          5
    { child_comm: postgres                            } hitcount:          6
    { child_comm: firefox                             } hitcount:          8
    { child_comm: dhclient                            } hitcount:         11
    { child_comm: emacs                               } hitcount:         12
    { child_comm: dbus-daemon                         } hitcount:         22
    { child_comm: nm-dispatcher.a                     } hitcount:         22
    { child_comm: evolution                           } hitcount:         35
    { child_comm: glib-pacrunner                      } hitcount:         59

    Totals:
        Hits: 206
        Entries: 21
        Dropped: 0

  The previous example showed how to start and stop a hist trigger by
  appending 'pause' and 'continue' to the hist trigger command.  A
  hist trigger can also be started in a paused state by initially
  starting the trigger with ':pause' appended.  This allows you to
  start the trigger only when you're ready to start collecting data
  and not before.  For example, you could start the trigger in a
  paused state, then unpause it and do something you want to measure,
  then pause the trigger again when done.

  Of course, doing this manually can be difficult and error-prone, but
  it is possible to automatically start and stop a hist trigger based
  on some condition, via the enable_hist and disable_hist triggers.

  For example, suppose we wanted to take a look at the relative
  weights in terms of skb length for each callpath that leads to a
  netif_receive_skb event when downloading a decent-sized file using
  wget.

  First we set up an initially paused stacktrace trigger on the
  netif_receive_skb event::

    # echo 'hist:key=stacktrace:vals=len:pause' > \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger

  Next, we set up an 'enable_hist' trigger on the sched_process_exec
  event, with an 'if filename==/usr/bin/wget' filter.  The effect of
  this new trigger is that it will 'unpause' the hist trigger we just
  set up on netif_receive_skb if and only if it sees a
  sched_process_exec event with a filename of '/usr/bin/wget'.  When
  that happens, all netif_receive_skb events are aggregated into a
  hash table keyed on stacktrace::

    # echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger

  The aggregation continues until the netif_receive_skb is paused
  again, which is what the following disable_hist event does by
  creating a similar setup on the sched_process_exit event, using the
  filter 'comm==wget'::

    # echo 'disable_hist:net:netif_receive_skb if comm==wget' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger

  Whenever a process exits and the comm field of the disable_hist
  trigger filter matches 'comm==wget', the netif_receive_skb hist
  trigger is disabled.

  The overall effect is that netif_receive_skb events are aggregated
  into the hash table for only the duration of the wget.  Executing a
  wget command and then listing the 'hist' file will display the
  output generated by the wget command::

    $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz

    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
    # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]

    { stacktrace:
         __netif_receive_skb_core+0x46d/0x990
         __netif_receive_skb+0x18/0x60
         netif_receive_skb_internal+0x23/0x90
         napi_gro_receive+0xc8/0x100
         ieee80211_deliver_skb+0xd6/0x270 [mac80211]
         ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
         ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
         ieee80211_rx+0x31d/0x900 [mac80211]
         iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
         iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
         iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
         irq_thread_fn+0x20/0x50
         irq_thread+0x11f/0x150
         kthread+0xd2/0xf0
         ret_from_fork+0x42/0x70
    } hitcount:         85  len:      28884
    { stacktrace:
         __netif_receive_skb_core+0x46d/0x990
         __netif_receive_skb+0x18/0x60
         netif_receive_skb_internal+0x23/0x90
         napi_gro_complete+0xa4/0xe0
         dev_gro_receive+0x23a/0x360
         napi_gro_receive+0x30/0x100
         ieee80211_deliver_skb+0xd6/0x270 [mac80211]
         ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
         ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
         ieee80211_rx+0x31d/0x900 [mac80211]
         iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
         iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
         iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
         irq_thread_fn+0x20/0x50
         irq_thread+0x11f/0x150
         kthread+0xd2/0xf0
    } hitcount:         98  len:     664329
    { stacktrace:
         __netif_receive_skb_core+0x46d/0x990
         __netif_receive_skb+0x18/0x60
         process_backlog+0xa8/0x150
         net_rx_action+0x15d/0x340
         __do_softirq+0x114/0x2c0
         do_softirq_own_stack+0x1c/0x30
         do_softirq+0x65/0x70
         __local_bh_enable_ip+0xb5/0xc0
         ip_finish_output+0x1f4/0x840
         ip_output+0x6b/0xc0
         ip_local_out_sk+0x31/0x40
         ip_send_skb+0x1a/0x50
         udp_send_skb+0x173/0x2a0
         udp_sendmsg+0x2bf/0x9f0
         inet_sendmsg+0x64/0xa0
         sock_sendmsg+0x3d/0x50
    } hitcount:        115  len:      13030
    { stacktrace:
         __netif_receive_skb_core+0x46d/0x990
         __netif_receive_skb+0x18/0x60
         netif_receive_skb_internal+0x23/0x90
         napi_gro_complete+0xa4/0xe0
         napi_gro_flush+0x6d/0x90
         iwl_pcie_irq_handler+0x92a/0x12f0 [iwlwifi]
         irq_thread_fn+0x20/0x50
         irq_thread+0x11f/0x150
         kthread+0xd2/0xf0
         ret_from_fork+0x42/0x70
    } hitcount:        934  len:    5512212

    Totals:
        Hits: 1232
        Entries: 4
        Dropped: 0

  The above shows all the netif_receive_skb callpaths and their total
  lengths for the duration of the wget command.

  The 'clear' hist trigger param can be used to clear the hash table.
  Suppose we wanted to try another run of the previous example but
  this time also wanted to see the complete list of events that went
  into the histogram.  In order to avoid having to set everything up
  again, we can just clear the histogram first::

    # echo 'hist:key=stacktrace:vals=len:clear' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger

  Just to verify that it is in fact cleared, here's what we now see in
  the hist file::

    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
    # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]

    Totals:
        Hits: 0
        Entries: 0
        Dropped: 0

  Since we want to see the detailed list of every netif_receive_skb
  event occurring during the new run, which are in fact the same
  events being aggregated into the hash table, we add some additional
  'enable_event' events to the triggering sched_process_exec and
  sched_process_exit events as such::

    # echo 'enable_event:net:netif_receive_skb if filename==/usr/bin/wget' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger

    # echo 'disable_event:net:netif_receive_skb if comm==wget' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger

  If you read the trigger files for the sched_process_exec and
  sched_process_exit triggers, you should see two triggers for each:
  one enabling/disabling the hist aggregation and the other
  enabling/disabling the logging of events::

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
    enable_event:net:netif_receive_skb:unlimited if filename==/usr/bin/wget
    enable_hist:net:netif_receive_skb:unlimited if filename==/usr/bin/wget

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
    enable_event:net:netif_receive_skb:unlimited if comm==wget
    disable_hist:net:netif_receive_skb:unlimited if comm==wget

  In other words, whenever either of the sched_process_exec or
  sched_process_exit events is hit and matches 'wget', it enables or
  disables both the histogram and the event log, and what you end up
  with is a hash table and set of events just covering the specified
  duration.  Run the wget command again::

    $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz

  Displaying the 'hist' file should show something similar to what you
  saw in the last run, but this time you should also see the
  individual events in the trace file::

    # cat /sys/kernel/debug/tracing/trace

    # tracer: nop
    #
    # entries-in-buffer/entries-written: 183/1426   #P:4
    #
    #                              _-----=> irqs-off
    #                             / _----=> need-resched
    #                            | / _---=> hardirq/softirq
    #                            || / _--=> preempt-depth
    #                            ||| /     delay
    #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
    #              | |       |   ||||       |         |
                wget-15108 [000] ..s1 31769.606929: netif_receive_skb: dev=lo skbaddr=ffff88009c353100 len=60
                wget-15108 [000] ..s1 31769.606999: netif_receive_skb: dev=lo skbaddr=ffff88009c353200 len=60
             dnsmasq-1382  [000] ..s1 31769.677652: netif_receive_skb: dev=lo skbaddr=ffff88009c352b00 len=130
             dnsmasq-1382  [000] ..s1 31769.685917: netif_receive_skb: dev=lo skbaddr=ffff88009c352200 len=138
    ##### CPU 2 buffer started ####
      irq/29-iwlwifi-559   [002] ..s. 31772.031529: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433d00 len=2948
      irq/29-iwlwifi-559   [002] ..s. 31772.031572: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432200 len=1500
      irq/29-iwlwifi-559   [002] ..s. 31772.032196: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433100 len=2948
      irq/29-iwlwifi-559   [002] ..s. 31772.032761: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433000 len=2948
      irq/29-iwlwifi-559   [002] ..s. 31772.033220: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432e00 len=1500
    .
    .
    .

  The following example demonstrates how multiple hist triggers can be
  attached to a given event.  This capability can be useful for
  creating a set of different summaries derived from the same set of
  events, or for comparing the effects of different filters, among
  other things::

    # echo 'hist:keys=skbaddr.hex:vals=len if len < 0' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:keys=skbaddr.hex:vals=len if len > 4096' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:keys=skbaddr.hex:vals=len if len == 256' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:keys=skbaddr.hex:vals=len' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:keys=len:vals=common_preempt_count' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger

  The above set of commands create four triggers differing only in
  their filters, along with a completely different though fairly
  nonsensical trigger.  Note that in order to append multiple hist
  triggers to the same file, you should use the '>>' operator to
  append them ('>' will also add the new hist trigger, but will remove
  any existing hist triggers beforehand).

  Displaying the contents of the 'hist' file for the event shows the
  contents of all five histograms::

    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist

    # event histogram
    #
    # trigger info: hist:keys=len:vals=hitcount,common_preempt_count:sort=hitcount:size=2048 [active]
    #

    { len:        176 } hitcount:          1  common_preempt_count:          0
    { len:        223 } hitcount:          1  common_preempt_count:          0
    { len:       4854 } hitcount:          1  common_preempt_count:          0
    { len:        395 } hitcount:          1  common_preempt_count:          0
    { len:        177 } hitcount:          1  common_preempt_count:          0
    { len:        446 } hitcount:          1  common_preempt_count:          0
    { len:       1601 } hitcount:          1  common_preempt_count:          0
    .
    .
    .
    { len:       1280 } hitcount:         66  common_preempt_count:          0
    { len:        116 } hitcount:         81  common_preempt_count:         40
    { len:        708 } hitcount:        112  common_preempt_count:          0
    { len:         46 } hitcount:        221  common_preempt_count:          0
    { len:       1264 } hitcount:        458  common_preempt_count:          0

    Totals:
        Hits: 1428
        Entries: 147
        Dropped: 0


    # event histogram
    #
    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
    #

    { skbaddr: ffff8800baee5e00 } hitcount:          1  len:        130
    { skbaddr: ffff88005f3d5600 } hitcount:          1  len:       1280
    { skbaddr: ffff88005f3d4900 } hitcount:          1  len:       1280
    { skbaddr: ffff88009fed6300 } hitcount:          1  len:        115
    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:        115
    { skbaddr: ffff88008cdb1900 } hitcount:          1  len:         46
    { skbaddr: ffff880064b5ef00 } hitcount:          1  len:        118
    { skbaddr: ffff880044e3c700 } hitcount:          1  len:         60
    { skbaddr: ffff880100065900 } hitcount:          1  len:         46
    { skbaddr: ffff8800d46bd500 } hitcount:          1  len:        116
    { skbaddr: ffff88005f3d5f00 } hitcount:          1  len:       1280
    { skbaddr: ffff880100064700 } hitcount:          1  len:        365
    { skbaddr: ffff8800badb6f00 } hitcount:          1  len:         60
    .
    .
    .
    { skbaddr: ffff88009fe0be00 } hitcount:         27  len:      24677
    { skbaddr: ffff88009fe0a400 } hitcount:         27  len:      23052
    { skbaddr: ffff88009fe0b700 } hitcount:         31  len:      25589
    { skbaddr: ffff88009fe0b600 } hitcount:         32  len:      27326
    { skbaddr: ffff88006a462800 } hitcount:         68  len:      71678
    { skbaddr: ffff88006a463700 } hitcount:         70  len:      72678
    { skbaddr: ffff88006a462b00 } hitcount:         71  len:      77589
    { skbaddr: ffff88006a463600 } hitcount:         73  len:      71307
    { skbaddr: ffff88006a462200 } hitcount:         81  len:      81032

    Totals:
        Hits: 1451
        Entries: 318
        Dropped: 0


    # event histogram
    #
    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len == 256 [active]
    #


    Totals:
        Hits: 0
        Entries: 0
        Dropped: 0


    # event histogram
    #
    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len > 4096 [active]
    #

    { skbaddr: ffff88009fd2c300 } hitcount:          1  len:       7212
    { skbaddr: ffff8800d2bcce00 } hitcount:          1  len:       7212
    { skbaddr: ffff8800d2bcd700 } hitcount:          1  len:       7212
    { skbaddr: ffff8800d2bcda00 } hitcount:          1  len:      21492
    { skbaddr: ffff8800ae2e2d00 } hitcount:          1  len:       7212
    { skbaddr: ffff8800d2bcdb00 } hitcount:          1  len:       7212
    { skbaddr: ffff88006a4df500 } hitcount:          1  len:       4854
    { skbaddr: ffff88008ce47b00 } hitcount:          1  len:      18636
    { skbaddr: ffff8800ae2e2200 } hitcount:          1  len:      12924
    { skbaddr: ffff88005f3e1000 } hitcount:          1  len:       4356
    { skbaddr: ffff8800d2bcdc00 } hitcount:          2  len:      24420
    { skbaddr: ffff8800d2bcc200 } hitcount:          2  len:      12996

    Totals:
        Hits: 14
        Entries: 12
        Dropped: 0


    # event histogram
    #
    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len < 0 [active]
    #


    Totals:
        Hits: 0
        Entries: 0
        Dropped: 0

  Named triggers can be used to have triggers share a common set of
  histogram data.  This capability is mostly useful for combining the
  output of events generated by tracepoints contained inside inline
  functions, but names can be used in a hist trigger on any event.
  For example, these two triggers when hit will update the same 'len'
  field in the shared 'foo' histogram data::

    # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
           /sys/kernel/debug/tracing/events/net/netif_rx/trigger

  You can see that they're updating common histogram data by reading
  each event's hist files at the same time::

    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist;
      cat /sys/kernel/debug/tracing/events/net/netif_rx/hist

    # event histogram
    #
    # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
    #

    { skbaddr: ffff88000ad53500 } hitcount:          1  len:         46
    { skbaddr: ffff8800af5a1500 } hitcount:          1  len:         76
    { skbaddr: ffff8800d62a1900 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bccb00 } hitcount:          1  len:        468
    { skbaddr: ffff8800d3c69900 } hitcount:          1  len:         46
    { skbaddr: ffff88009ff09100 } hitcount:          1  len:         52
    { skbaddr: ffff88010f13ab00 } hitcount:          1  len:        168
    { skbaddr: ffff88006a54f400 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcc500 } hitcount:          1  len:        260
    { skbaddr: ffff880064505000 } hitcount:          1  len:         46
    { skbaddr: ffff8800baf24e00 } hitcount:          1  len:         32
    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:         46
    { skbaddr: ffff8800d3edff00 } hitcount:          1  len:         44
    { skbaddr: ffff88009fe0b400 } hitcount:          1  len:        168
    { skbaddr: ffff8800a1c55a00 } hitcount:          1  len:         40
    { skbaddr: ffff8800d2bcd100 } hitcount:          1  len:         40
    { skbaddr: ffff880064505f00 } hitcount:          1  len:        174
    { skbaddr: ffff8800a8bff200 } hitcount:          1  len:        160
    { skbaddr: ffff880044e3cc00 } hitcount:          1  len:         76
    { skbaddr: ffff8800a8bfe700 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcdc00 } hitcount:          1  len:         32
    { skbaddr: ffff8800a1f64800 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcde00 } hitcount:          1  len:        988
    { skbaddr: ffff88006a5dea00 } hitcount:          1  len:         46
    { skbaddr: ffff88002e37a200 } hitcount:          1  len:         44
    { skbaddr: ffff8800a1f32c00 } hitcount:          2  len:        676
    { skbaddr: ffff88000ad52600 } hitcount:          2  len:        107
    { skbaddr: ffff8800a1f91e00 } hitcount:          2  len:         92
    { skbaddr: ffff8800af5a0200 } hitcount:          2  len:        142
    { skbaddr: ffff8800d2bcc600 } hitcount:          2  len:        220
    { skbaddr: ffff8800ba36f500 } hitcount:          2  len:         92
    { skbaddr: ffff8800d021f800 } hitcount:          2  len:         92
    { skbaddr: ffff8800a1f33600 } hitcount:          2  len:        675
    { skbaddr: ffff8800a8bfff00 } hitcount:          3  len:        138
    { skbaddr: ffff8800d62a1300 } hitcount:          3  len:        138
    { skbaddr: ffff88002e37a100 } hitcount:          4  len:        184
    { skbaddr: ffff880064504400 } hitcount:          4  len:        184
    { skbaddr: ffff8800a8bfec00 } hitcount:          4  len:        184
    { skbaddr: ffff88000ad53700 } hitcount:          5  len:        230
    { skbaddr: ffff8800d2bcdb00 } hitcount:          5  len:        196
    { skbaddr: ffff8800a1f90000 } hitcount:          6  len:        276
    { skbaddr: ffff88006a54f900 } hitcount:          6  len:        276

    Totals:
        Hits: 81
        Entries: 42
        Dropped: 0
    # event histogram
    #
    # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
    #

    { skbaddr: ffff88000ad53500 } hitcount:          1  len:         46
    { skbaddr: ffff8800af5a1500 } hitcount:          1  len:         76
    { skbaddr: ffff8800d62a1900 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bccb00 } hitcount:          1  len:        468
    { skbaddr: ffff8800d3c69900 } hitcount:          1  len:         46
    { skbaddr: ffff88009ff09100 } hitcount:          1  len:         52
    { skbaddr: ffff88010f13ab00 } hitcount:          1  len:        168
    { skbaddr: ffff88006a54f400 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcc500 } hitcount:          1  len:        260
    { skbaddr: ffff880064505000 } hitcount:          1  len:         46
    { skbaddr: ffff8800baf24e00 } hitcount:          1  len:         32
    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:         46
    { skbaddr: ffff8800d3edff00 } hitcount:          1  len:         44
    { skbaddr: ffff88009fe0b400 } hitcount:          1  len:        168
    { skbaddr: ffff8800a1c55a00 } hitcount:          1  len:         40
    { skbaddr: ffff8800d2bcd100 } hitcount:          1  len:         40
    { skbaddr: ffff880064505f00 } hitcount:          1  len:        174
    { skbaddr: ffff8800a8bff200 } hitcount:          1  len:        160
    { skbaddr: ffff880044e3cc00 } hitcount:          1  len:         76
    { skbaddr: ffff8800a8bfe700 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcdc00 } hitcount:          1  len:         32
    { skbaddr: ffff8800a1f64800 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcde00 } hitcount:          1  len:        988
    { skbaddr: ffff88006a5dea00 } hitcount:          1  len:         46
    { skbaddr: ffff88002e37a200 } hitcount:          1  len:         44
    { skbaddr: ffff8800a1f32c00 } hitcount:          2  len:        676
    { skbaddr: ffff88000ad52600 } hitcount:          2  len:        107
    { skbaddr: ffff8800a1f91e00 } hitcount:          2  len:         92
    { skbaddr: ffff8800af5a0200 } hitcount:          2  len:        142
    { skbaddr: ffff8800d2bcc600 } hitcount:          2  len:        220
    { skbaddr: ffff8800ba36f500 } hitcount:          2  len:         92
    { skbaddr: ffff8800d021f800 } hitcount:          2  len:         92
    { skbaddr: ffff8800a1f33600 } hitcount:          2  len:        675
    { skbaddr: ffff8800a8bfff00 } hitcount:          3  len:        138
    { skbaddr: ffff8800d62a1300 } hitcount:          3  len:        138
    { skbaddr: ffff88002e37a100 } hitcount:          4  len:        184
    { skbaddr: ffff880064504400 } hitcount:          4  len:        184
    { skbaddr: ffff8800a8bfec00 } hitcount:          4  len:        184
    { skbaddr: ffff88000ad53700 } hitcount:          5  len:        230
    { skbaddr: ffff8800d2bcdb00 } hitcount:          5  len:        196
    { skbaddr: ffff8800a1f90000 } hitcount:          6  len:        276
    { skbaddr: ffff88006a54f900 } hitcount:          6  len:        276

    Totals:
        Hits: 81
        Entries: 42
        Dropped: 0

  And here's an example that shows how to combine histogram data from
  any two events even if they don't share any 'compatible' fields
  other than 'hitcount' and 'stacktrace'.  These commands create a
  couple of triggers named 'bar' using those fields::

    # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
    # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
          /sys/kernel/debug/tracing/events/net/netif_rx/trigger

  And displaying the output of either shows some interesting if
  somewhat confusing output::

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
    # cat /sys/kernel/debug/tracing/events/net/netif_rx/hist

    # event histogram
    #
    # trigger info: hist:name=bar:keys=stacktrace:vals=hitcount:sort=hitcount:size=2048 [active]
    #

    { stacktrace:
             kernel_clone+0x18e/0x330
             kernel_thread+0x29/0x30
             kthreadd+0x154/0x1b0
             ret_from_fork+0x3f/0x70
    } hitcount:          1
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx_ni+0x20/0x70
             dev_loopback_xmit+0xaa/0xd0
             ip_mc_output+0x126/0x240
             ip_local_out_sk+0x31/0x40
             igmp_send_report+0x1e9/0x230
             igmp_timer_expire+0xe9/0x120
             call_timer_fn+0x39/0xf0
             run_timer_softirq+0x1e1/0x290
             __do_softirq+0xfd/0x290
             irq_exit+0x98/0xb0
             smp_apic_timer_interrupt+0x4a/0x60
             apic_timer_interrupt+0x6d/0x80
             cpuidle_enter+0x17/0x20
             call_cpuidle+0x3b/0x60
             cpu_startup_entry+0x22d/0x310
    } hitcount:          1
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx_ni+0x20/0x70
             dev_loopback_xmit+0xaa/0xd0
             ip_mc_output+0x17f/0x240
             ip_local_out_sk+0x31/0x40
             ip_send_skb+0x1a/0x50
             udp_send_skb+0x13e/0x270
             udp_sendmsg+0x2bf/0x980
             inet_sendmsg+0x67/0xa0
             sock_sendmsg+0x38/0x50
             SYSC_sendto+0xef/0x170
             SyS_sendto+0xe/0x10
             entry_SYSCALL_64_fastpath+0x12/0x6a
    } hitcount:          2
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx+0x1c/0x60
             loopback_xmit+0x6c/0xb0
             dev_hard_start_xmit+0x219/0x3a0
             __dev_queue_xmit+0x415/0x4f0
             dev_queue_xmit_sk+0x13/0x20
             ip_finish_output2+0x237/0x340
             ip_finish_output+0x113/0x1d0
             ip_output+0x66/0xc0
             ip_local_out_sk+0x31/0x40
             ip_send_skb+0x1a/0x50
             udp_send_skb+0x16d/0x270
             udp_sendmsg+0x2bf/0x980
             inet_sendmsg+0x67/0xa0
             sock_sendmsg+0x38/0x50
             ___sys_sendmsg+0x14e/0x270
    } hitcount:         76
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx+0x1c/0x60
             loopback_xmit+0x6c/0xb0
             dev_hard_start_xmit+0x219/0x3a0
             __dev_queue_xmit+0x415/0x4f0
             dev_queue_xmit_sk+0x13/0x20
             ip_finish_output2+0x237/0x340
             ip_finish_output+0x113/0x1d0
             ip_output+0x66/0xc0
             ip_local_out_sk+0x31/0x40
             ip_send_skb+0x1a/0x50
             udp_send_skb+0x16d/0x270
             udp_sendmsg+0x2bf/0x980
             inet_sendmsg+0x67/0xa0
             sock_sendmsg+0x38/0x50
             ___sys_sendmsg+0x269/0x270
    } hitcount:         77
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx+0x1c/0x60
             loopback_xmit+0x6c/0xb0
             dev_hard_start_xmit+0x219/0x3a0
             __dev_queue_xmit+0x415/0x4f0
             dev_queue_xmit_sk+0x13/0x20
             ip_finish_output2+0x237/0x340
             ip_finish_output+0x113/0x1d0
             ip_output+0x66/0xc0
             ip_local_out_sk+0x31/0x40
             ip_send_skb+0x1a/0x50
             udp_send_skb+0x16d/0x270
             udp_sendmsg+0x2bf/0x980
             inet_sendmsg+0x67/0xa0
             sock_sendmsg+0x38/0x50
             SYSC_sendto+0xef/0x170
    } hitcount:         88
    { stacktrace:
             kernel_clone+0x18e/0x330
             SyS_clone+0x19/0x20
             entry_SYSCALL_64_fastpath+0x12/0x6a
    } hitcount:        244

    Totals:
        Hits: 489
        Entries: 7
        Dropped: 0

2.2 Inter-event hist triggers
-----------------------------

Inter-event hist triggers are hist triggers that combine values from
one or more other events and create a histogram using that data.  Data
from an inter-event histogram can in turn become the source for
further combined histograms, thus providing a chain of related
histograms, which is important for some applications.

The most important example of an inter-event quantity that can be used
in this manner is latency, which is simply a difference in timestamps
between two events.  Although latency is the most important
inter-event quantity, note that because the support is completely
general across the trace event subsystem, any event field can be used
in an inter-event quantity.

An example of a histogram that combines data from other histograms
into a useful chain would be a 'wakeupswitch latency' histogram that
combines a 'wakeup latency' histogram and a 'switch latency'
histogram.

Normally, a hist trigger specification consists of a (possibly
compound) key along with one or more numeric values, which are
continually updated sums associated with that key.  A histogram
specification in this case consists of individual key and value
specifications that refer to trace event fields associated with a
single event type.

The inter-event hist trigger extension allows fields from multiple
events to be referenced and combined into a multi-event histogram
specification.  In support of this overall goal, a few enabling
features have been added to the hist trigger support:

  - In order to compute an inter-event quantity, a value from one
    event needs to saved and then referenced from another event.  This
    requires the introduction of support for histogram 'variables'.

  - The computation of inter-event quantities and their combination
    require some minimal amount of support for applying simple
    expressions to variables (+ and -).

  - A histogram consisting of inter-event quantities isn't logically a
    histogram on either event (so having the 'hist' file for either
    event host the histogram output doesn't really make sense).  To
    address the idea that the histogram is associated with a
    combination of events, support is added allowing the creation of
    'synthetic' events that are events derived from other events.
    These synthetic events are full-fledged events just like any other
    and can be used as such, as for instance to create the
    'combination' histograms mentioned previously.

  - A set of 'actions' can be associated with histogram entries -
    these can be used to generate the previously mentioned synthetic
    events, but can also be used for other purposes, such as for
    example saving context when a 'max' latency has been hit.

  - Trace events don't have a 'timestamp' associated with them, but
    there is an implicit timestamp saved along with an event in the
    underlying ftrace ring buffer.  This timestamp is now exposed as a
    a synthetic field named 'common_timestamp' which can be used in
    histograms as if it were any other event field; it isn't an actual
    field in the trace format but rather is a synthesized value that
    nonetheless can be used as if it were an actual field.  By default
    it is in units of nanoseconds; appending '.usecs' to a
    common_timestamp field changes the units to microseconds.

A note on inter-event timestamps: If common_timestamp is used in a
histogram, the trace buffer is automatically switched over to using
absolute timestamps and the "global" trace clock, in order to avoid
bogus timestamp differences with other clocks that aren't coherent
across CPUs.  This can be overridden by specifying one of the other
trace clocks instead, using the "clock=XXX" hist trigger attribute,
where XXX is any of the clocks listed in the tracing/trace_clock
pseudo-file.

These features are described in more detail in the following sections.

2.2.1 Histogram Variables
-------------------------

Variables are simply named locations used for saving and retrieving
values between matching events.  A 'matching' event is defined as an
event that has a matching key - if a variable is saved for a histogram
entry corresponding to that key, any subsequent event with a matching
key can access that variable.

A variable's value is normally available to any subsequent event until
it is set to something else by a subsequent event.  The one exception
to that rule is that any variable used in an expression is essentially
'read-once' - once it's used by an expression in a subsequent event,
it's reset to its 'unset' state, which means it can't be used again
unless it's set again.  This ensures not only that an event doesn't
use an uninitialized variable in a calculation, but that that variable
is used only once and not for any unrelated subsequent match.

The basic syntax for saving a variable is to simply prefix a unique
variable name not corresponding to any keyword along with an '=' sign
to any event field.

Either keys or values can be saved and retrieved in this way.  This
creates a variable named 'ts0' for a histogram entry with the key
'next_pid'::

  # echo 'hist:keys=next_pid:vals=$ts0:ts0=common_timestamp ... >> \
	event/trigger

The ts0 variable can be accessed by any subsequent event having the
same pid as 'next_pid'.

Variable references are formed by prepending the variable name with
the '$' sign.  Thus for example, the ts0 variable above would be
referenced as '$ts0' in expressions.

Because 'vals=' is used, the common_timestamp variable value above
will also be summed as a normal histogram value would (though for a
timestamp it makes little sense).

The below shows that a key value can also be saved in the same way::

  # echo 'hist:timer_pid=common_pid:key=timer_pid ...' >> event/trigger

If a variable isn't a key variable or prefixed with 'vals=', the
associated event field will be saved in a variable but won't be summed
as a value::

  # echo 'hist:keys=next_pid:ts1=common_timestamp ...' >> event/trigger

Multiple variables can be assigned at the same time.  The below would
result in both ts0 and b being created as variables, with both
common_timestamp and field1 additionally being summed as values::

  # echo 'hist:keys=pid:vals=$ts0,$b:ts0=common_timestamp,b=field1 ...' >> \
	event/trigger

Note that variable assignments can appear either preceding or
following their use.  The command below behaves identically to the
command above::

  # echo 'hist:keys=pid:ts0=common_timestamp,b=field1:vals=$ts0,$b ...' >> \
	event/trigger

Any number of variables not bound to a 'vals=' prefix can also be
assigned by simply separating them with colons.  Below is the same
thing but without the values being summed in the histogram::

  # echo 'hist:keys=pid:ts0=common_timestamp:b=field1 ...' >> event/trigger

Variables set as above can be referenced and used in expressions on
another event.

For example, here's how a latency can be calculated::

  # echo 'hist:keys=pid,prio:ts0=common_timestamp ...' >> event1/trigger
  # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp-$ts0 ...' >> event2/trigger

In the first line above, the event's timestamp is saved into the
variable ts0.  In the next line, ts0 is subtracted from the second
event's timestamp to produce the latency, which is then assigned into
yet another variable, 'wakeup_lat'.  The hist trigger below in turn
makes use of the wakeup_lat variable to compute a combined latency
using the same key and variable from yet another event::

  # echo 'hist:key=pid:wakeupswitch_lat=$wakeup_lat+$switchtime_lat ...' >> event3/trigger

2.2.2 Synthetic Events
----------------------

Synthetic events are user-defined events generated from hist trigger
variables or fields associated with one or more other events.  Their
purpose is to provide a mechanism for displaying data spanning
multiple events consistent with the existing and already familiar
usage for normal events.

To define a synthetic event, the user writes a simple specification
consisting of the name of the new event along with one or more
variables and their types, which can be any valid field type,
separated by semicolons, to the tracing/synthetic_events file.

See synth_field_size() for available types.

If field_name contains [n], the field is considered to be a static array.

If field_names contains[] (no subscript), the field is considered to
be a dynamic array, which will only take as much space in the event as
is required to hold the array.

A string field can be specified using either the static notation:

  char name[32];

Or the dynamic:

  char name[];

The size limit for either is 256.

For instance, the following creates a new event named 'wakeup_latency'
with 3 fields: lat, pid, and prio.  Each of those fields is simply a
variable reference to a variable on another event::

  # echo 'wakeup_latency \
          u64 lat; \
          pid_t pid; \
	  int prio' >> \
	  /sys/kernel/debug/tracing/synthetic_events

Reading the tracing/synthetic_events file lists all the currently
defined synthetic events, in this case the event defined above::

  # cat /sys/kernel/debug/tracing/synthetic_events
    wakeup_latency u64 lat; pid_t pid; int prio

An existing synthetic event definition can be removed by prepending
the command that defined it with a '!'::

  # echo '!wakeup_latency u64 lat pid_t pid int prio' >> \
    /sys/kernel/debug/tracing/synthetic_events

At this point, there isn't yet an actual 'wakeup_latency' event
instantiated in the event subsystem - for this to happen, a 'hist
trigger action' needs to be instantiated and bound to actual fields
and variables defined on other events (see Section 2.2.3 below on
how that is done using hist trigger 'onmatch' action). Once that is
done, the 'wakeup_latency' synthetic event instance is created.

A histogram can now be defined for the new synthetic event::

  # echo 'hist:keys=pid,prio,lat.log2:sort=pid,lat' >> \
        /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger

The new event is created under the tracing/events/synthetic/ directory
and looks and behaves just like any other event::

  # ls /sys/kernel/debug/tracing/events/synthetic/wakeup_latency
        enable  filter  format  hist  id  trigger

Like any other event, once a histogram is enabled for the event, the
output can be displayed by reading the event's 'hist' file.

2.2.3 Hist trigger 'handlers' and 'actions'
-------------------------------------------

A hist trigger 'action' is a function that's executed (in most cases
conditionally) whenever a histogram entry is added or updated.

When a histogram entry is added or updated, a hist trigger 'handler'
is what decides whether the corresponding action is actually invoked
or not.

Hist trigger handlers and actions are paired together in the general
form:

  <handler>.<action>

To specify a handler.action pair for a given event, simply specify
that handler.action pair between colons in the hist trigger
specification.

In theory, any handler can be combined with any action, but in
practice, not every handler.action combination is currently supported;
if a given handler.action combination isn't supported, the hist
trigger will fail with -EINVAL;

The default 'handler.action' if none is explicitly specified is as it
always has been, to simply update the set of values associated with an
entry.  Some applications, however, may want to perform additional
actions at that point, such as generate another event, or compare and
save a maximum.

The supported handlers and actions are listed below, and each is
described in more detail in the following paragraphs, in the context
of descriptions of some common and useful handler.action combinations.

The available handlers are:

  - onmatch(matching.event)    - invoke action on any addition or update
  - onmax(var)                 - invoke action if var exceeds current max
  - onchange(var)              - invoke action if var changes

The available actions are:

  - trace(<synthetic_event_name>,param list)   - generate synthetic event
  - save(field,...)                            - save current event fields
  - snapshot()                                 - snapshot the trace buffer

The following commonly-used handler.action pairs are available:

  - onmatch(matching.event).trace(<synthetic_event_name>,param list)

    The 'onmatch(matching.event).trace(<synthetic_event_name>,param
    list)' hist trigger action is invoked whenever an event matches
    and the histogram entry would be added or updated.  It causes the
    named synthetic event to be generated with the values given in the
    'param list'.  The result is the generation of a synthetic event
    that consists of the values contained in those variables at the
    time the invoking event was hit.  For example, if the synthetic
    event name is 'wakeup_latency', a wakeup_latency event is
    generated using onmatch(event).trace(wakeup_latency,arg1,arg2).

    There is also an equivalent alternative form available for
    generating synthetic events.  In this form, the synthetic event
    name is used as if it were a function name.  For example, using
    the 'wakeup_latency' synthetic event name again, the
    wakeup_latency event would be generated by invoking it as if it
    were a function call, with the event field values passed in as
    arguments: onmatch(event).wakeup_latency(arg1,arg2).  The syntax
    for this form is:

      onmatch(matching.event).<synthetic_event_name>(param list)

    In either case, the 'param list' consists of one or more
    parameters which may be either variables or fields defined on
    either the 'matching.event' or the target event.  The variables or
    fields specified in the param list may be either fully-qualified
    or unqualified.  If a variable is specified as unqualified, it
    must be unique between the two events.  A field name used as a
    param can be unqualified if it refers to the target event, but
    must be fully qualified if it refers to the matching event.  A
    fully-qualified name is of the form 'system.event_name.$var_name'
    or 'system.event_name.field'.

    The 'matching.event' specification is simply the fully qualified
    event name of the event that matches the target event for the
    onmatch() functionality, in the form 'system.event_name'. Histogram
    keys of both events are compared to find if events match. In case
    multiple histogram keys are used, they all must match in the specified
    order.

    Finally, the number and type of variables/fields in the 'param
    list' must match the number and types of the fields in the
    synthetic event being generated.

    As an example the below defines a simple synthetic event and uses
    a variable defined on the sched_wakeup_new event as a parameter
    when invoking the synthetic event.  Here we define the synthetic
    event::

      # echo 'wakeup_new_test pid_t pid' >> \
             /sys/kernel/debug/tracing/synthetic_events

      # cat /sys/kernel/debug/tracing/synthetic_events
            wakeup_new_test pid_t pid

    The following hist trigger both defines the missing testpid
    variable and specifies an onmatch() action that generates a
    wakeup_new_test synthetic event whenever a sched_wakeup_new event
    occurs, which because of the 'if comm == "cyclictest"' filter only
    happens when the executable is cyclictest::

      # echo 'hist:keys=$testpid:testpid=pid:onmatch(sched.sched_wakeup_new).\
              wakeup_new_test($testpid) if comm=="cyclictest"' >> \
              /sys/kernel/debug/tracing/events/sched/sched_wakeup_new/trigger

    Or, equivalently, using the 'trace' keyword syntax:

    # echo 'hist:keys=$testpid:testpid=pid:onmatch(sched.sched_wakeup_new).\
            trace(wakeup_new_test,$testpid) if comm=="cyclictest"' >> \
            /sys/kernel/debug/tracing/events/sched/sched_wakeup_new/trigger

    Creating and displaying a histogram based on those events is now
    just a matter of using the fields and new synthetic event in the
    tracing/events/synthetic directory, as usual::

      # echo 'hist:keys=pid:sort=pid' >> \
             /sys/kernel/debug/tracing/events/synthetic/wakeup_new_test/trigger

    Running 'cyclictest' should cause wakeup_new events to generate
    wakeup_new_test synthetic events which should result in histogram
    output in the wakeup_new_test event's hist file::

      # cat /sys/kernel/debug/tracing/events/synthetic/wakeup_new_test/hist

    A more typical usage would be to use two events to calculate a
    latency.  The following example uses a set of hist triggers to
    produce a 'wakeup_latency' histogram.

    First, we define a 'wakeup_latency' synthetic event::

      # echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> \
              /sys/kernel/debug/tracing/synthetic_events

    Next, we specify that whenever we see a sched_waking event for a
    cyclictest thread, save the timestamp in a 'ts0' variable::

      # echo 'hist:keys=$saved_pid:saved_pid=pid:ts0=common_timestamp.usecs \
              if comm=="cyclictest"' >> \
	      /sys/kernel/debug/tracing/events/sched/sched_waking/trigger

    Then, when the corresponding thread is actually scheduled onto the
    CPU by a sched_switch event (saved_pid matches next_pid), calculate
    the latency and use that along with another variable and an event field
    to generate a wakeup_latency synthetic event::

      # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:\
              onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,\
	              $saved_pid,next_prio) if next_comm=="cyclictest"' >> \
	      /sys/kernel/debug/tracing/events/sched/sched_switch/trigger

    We also need to create a histogram on the wakeup_latency synthetic
    event in order to aggregate the generated synthetic event data::

      # echo 'hist:keys=pid,prio,lat:sort=pid,lat' >> \
              /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger

    Finally, once we've run cyclictest to actually generate some
    events, we can see the output by looking at the wakeup_latency
    synthetic event's hist file::

      # cat /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/hist

  - onmax(var).save(field,..	.)

    The 'onmax(var).save(field,...)' hist trigger action is invoked
    whenever the value of 'var' associated with a histogram entry
    exceeds the current maximum contained in that variable.

    The end result is that the trace event fields specified as the
    onmax.save() params will be saved if 'var' exceeds the current
    maximum for that hist trigger entry.  This allows context from the
    event that exhibited the new maximum to be saved for later
    reference.  When the histogram is displayed, additional fields
    displaying the saved values will be printed.

    As an example the below defines a couple of hist triggers, one for
    sched_waking and another for sched_switch, keyed on pid.  Whenever
    a sched_waking occurs, the timestamp is saved in the entry
    corresponding to the current pid, and when the scheduler switches
    back to that pid, the timestamp difference is calculated.  If the
    resulting latency, stored in wakeup_lat, exceeds the current
    maximum latency, the values specified in the save() fields are
    recorded::

      # echo 'hist:keys=pid:ts0=common_timestamp.usecs \
              if comm=="cyclictest"' >> \
              /sys/kernel/debug/tracing/events/sched/sched_waking/trigger

      # echo 'hist:keys=next_pid:\
              wakeup_lat=common_timestamp.usecs-$ts0:\
              onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm) \
              if next_comm=="cyclictest"' >> \
              /sys/kernel/debug/tracing/events/sched/sched_switch/trigger

    When the histogram is displayed, the max value and the saved
    values corresponding to the max are displayed following the rest
    of the fields::

      # cat /sys/kernel/debug/tracing/events/sched/sched_switch/hist
        { next_pid:       2255 } hitcount:        239
          common_timestamp-ts0:          0
          max:         27
	  next_comm: cyclictest
          prev_pid:          0  prev_prio:        120  prev_comm: swapper/1

        { next_pid:       2256 } hitcount:       2355
          common_timestamp-ts0: 0
          max:         49  next_comm: cyclictest
          prev_pid:          0  prev_prio:        120  prev_comm: swapper/0

        Totals:
            Hits: 12970
            Entries: 2
            Dropped: 0

  - onmax(var).snapshot()

    The 'onmax(var).snapshot()' hist trigger action is invoked
    whenever the value of 'var' associated with a histogram entry
    exceeds the current maximum contained in that variable.

    The end result is that a global snapshot of the trace buffer will
    be saved in the tracing/snapshot file if 'var' exceeds the current
    maximum for any hist trigger entry.

    Note that in this case the maximum is a global maximum for the
    current trace instance, which is the maximum across all buckets of
    the histogram.  The key of the specific trace event that caused
    the global maximum and the global maximum itself are displayed,
    along with a message stating that a snapshot has been taken and
    where to find it.  The user can use the key information displayed
    to locate the corresponding bucket in the histogram for even more
    detail.

    As an example the below defines a couple of hist triggers, one for
    sched_waking and another for sched_switch, keyed on pid.  Whenever
    a sched_waking event occurs, the timestamp is saved in the entry
    corresponding to the current pid, and when the scheduler switches
    back to that pid, the timestamp difference is calculated.  If the
    resulting latency, stored in wakeup_lat, exceeds the current
    maximum latency, a snapshot is taken.  As part of the setup, all
    the scheduler events are also enabled, which are the events that
    will show up in the snapshot when it is taken at some point:

    # echo 1 > /sys/kernel/debug/tracing/events/sched/enable

    # echo 'hist:keys=pid:ts0=common_timestamp.usecs \
            if comm=="cyclictest"' >> \
            /sys/kernel/debug/tracing/events/sched/sched_waking/trigger

    # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
            onmax($wakeup_lat).save(next_prio,next_comm,prev_pid,prev_prio, \
	    prev_comm):onmax($wakeup_lat).snapshot() \
	    if next_comm=="cyclictest"' >> \
	    /sys/kernel/debug/tracing/events/sched/sched_switch/trigger

    When the histogram is displayed, for each bucket the max value
    and the saved values corresponding to the max are displayed
    following the rest of the fields.

    If a snapshot was taken, there is also a message indicating that,
    along with the value and event that triggered the global maximum:

    # cat /sys/kernel/debug/tracing/events/sched/sched_switch/hist
      { next_pid:       2101 } hitcount:        200
	max:         52  next_prio:        120  next_comm: cyclictest \
        prev_pid:          0  prev_prio:        120  prev_comm: swapper/6

      { next_pid:       2103 } hitcount:       1326
	max:        572  next_prio:         19  next_comm: cyclictest \
        prev_pid:          0  prev_prio:        120  prev_comm: swapper/1

      { next_pid:       2102 } hitcount:       1982 \
	max:         74  next_prio:         19  next_comm: cyclictest \
        prev_pid:          0  prev_prio:        120  prev_comm: swapper/5

    Snapshot taken (see tracing/snapshot).  Details:
	triggering value { onmax($wakeup_lat) }:        572	\
	triggered by event with key: { next_pid:       2103 }

    Totals:
        Hits: 3508
        Entries: 3
        Dropped: 0

    In the above case, the event that triggered the global maximum has
    the key with next_pid == 2103.  If you look at the bucket that has
    2103 as the key, you'll find the additional values save()'d along
    with the local maximum for that bucket, which should be the same
    as the global maximum (since that was the same value that
    triggered the global snapshot).

    And finally, looking at the snapshot data should show at or near
    the end the event that triggered the snapshot (in this case you
    can verify the timestamps between the sched_waking and
    sched_switch events, which should match the time displayed in the
    global maximum)::

     # cat /sys/kernel/debug/tracing/snapshot

         <...>-2103  [005] d..3   309.873125: sched_switch: prev_comm=cyclictest prev_pid=2103 prev_prio=19 prev_state=D ==> next_comm=swapper/5 next_pid=0 next_prio=120
         <idle>-0     [005] d.h3   309.873611: sched_waking: comm=cyclictest pid=2102 prio=19 target_cpu=005
         <idle>-0     [005] dNh4   309.873613: sched_wakeup: comm=cyclictest pid=2102 prio=19 target_cpu=005
         <idle>-0     [005] d..3   309.873616: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2102 next_prio=19
         <...>-2102  [005] d..3   309.873625: sched_switch: prev_comm=cyclictest prev_pid=2102 prev_prio=19 prev_state=D ==> next_comm=swapper/5 next_pid=0 next_prio=120
         <idle>-0     [005] d.h3   309.874624: sched_waking: comm=cyclictest pid=2102 prio=19 target_cpu=005
         <idle>-0     [005] dNh4   309.874626: sched_wakeup: comm=cyclictest pid=2102 prio=19 target_cpu=005
         <idle>-0     [005] dNh3   309.874628: sched_waking: comm=cyclictest pid=2103 prio=19 target_cpu=005
         <idle>-0     [005] dNh4   309.874630: sched_wakeup: comm=cyclictest pid=2103 prio=19 target_cpu=005
         <idle>-0     [005] d..3   309.874633: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2102 next_prio=19
         <idle>-0     [004] d.h3   309.874757: sched_waking: comm=gnome-terminal- pid=1699 prio=120 target_cpu=004
         <idle>-0     [004] dNh4   309.874762: sched_wakeup: comm=gnome-terminal- pid=1699 prio=120 target_cpu=004
         <idle>-0     [004] d..3   309.874766: sched_switch: prev_comm=swapper/4 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=gnome-terminal- next_pid=1699 next_prio=120
     gnome-terminal--1699  [004] d.h2   309.874941: sched_stat_runtime: comm=gnome-terminal- pid=1699 runtime=180706 [ns] vruntime=1126870572 [ns]
         <idle>-0     [003] d.s4   309.874956: sched_waking: comm=rcu_sched pid=9 prio=120 target_cpu=007
         <idle>-0     [003] d.s5   309.874960: sched_wake_idle_without_ipi: cpu=7
         <idle>-0     [003] d.s5   309.874961: sched_wakeup: comm=rcu_sched pid=9 prio=120 target_cpu=007
         <idle>-0     [007] d..3   309.874963: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=rcu_sched next_pid=9 next_prio=120
      rcu_sched-9     [007] d..3   309.874973: sched_stat_runtime: comm=rcu_sched pid=9 runtime=13646 [ns] vruntime=22531430286 [ns]
      rcu_sched-9     [007] d..3   309.874978: sched_switch: prev_comm=rcu_sched prev_pid=9 prev_prio=120 prev_state=R+ ==> next_comm=swapper/7 next_pid=0 next_prio=120
          <...>-2102  [005] d..4   309.874994: sched_migrate_task: comm=cyclictest pid=2103 prio=19 orig_cpu=5 dest_cpu=1
          <...>-2102  [005] d..4   309.875185: sched_wake_idle_without_ipi: cpu=1
         <idle>-0     [001] d..3   309.875200: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2103 next_prio=19

  - onchange(var).save(field,..	.)

    The 'onchange(var).save(field,...)' hist trigger action is invoked
    whenever the value of 'var' associated with a histogram entry
    changes.

    The end result is that the trace event fields specified as the
    onchange.save() params will be saved if 'var' changes for that
    hist trigger entry.  This allows context from the event that
    changed the value to be saved for later reference.  When the
    histogram is displayed, additional fields displaying the saved
    values will be printed.

  - onchange(var).snapshot()

    The 'onchange(var).snapshot()' hist trigger action is invoked
    whenever the value of 'var' associated with a histogram entry
    changes.

    The end result is that a global snapshot of the trace buffer will
    be saved in the tracing/snapshot file if 'var' changes for any
    hist trigger entry.

    Note that in this case the changed value is a global variable
    associated with current trace instance.  The key of the specific
    trace event that caused the value to change and the global value
    itself are displayed, along with a message stating that a snapshot
    has been taken and where to find it.  The user can use the key
    information displayed to locate the corresponding bucket in the
    histogram for even more detail.

    As an example the below defines a hist trigger on the tcp_probe
    event, keyed on dport.  Whenever a tcp_probe event occurs, the
    cwnd field is checked against the current value stored in the
    $cwnd variable.  If the value has changed, a snapshot is taken.
    As part of the setup, all the scheduler and tcp events are also
    enabled, which are the events that will show up in the snapshot
    when it is taken at some point:

    # echo 1 > /sys/kernel/debug/tracing/events/sched/enable
    # echo 1 > /sys/kernel/debug/tracing/events/tcp/enable

    # echo 'hist:keys=dport:cwnd=snd_cwnd: \
            onchange($cwnd).save(snd_wnd,srtt,rcv_wnd): \
	    onchange($cwnd).snapshot()' >> \
	    /sys/kernel/debug/tracing/events/tcp/tcp_probe/trigger

    When the histogram is displayed, for each bucket the tracked value
    and the saved values corresponding to that value are displayed
    following the rest of the fields.

    If a snapshot was taken, there is also a message indicating that,
    along with the value and event that triggered the snapshot::

      # cat /sys/kernel/debug/tracing/events/tcp/tcp_probe/hist

      { dport:       1521 } hitcount:          8
	changed:         10  snd_wnd:      35456  srtt:     154262  rcv_wnd:      42112

      { dport:         80 } hitcount:         23
	changed:         10  snd_wnd:      28960  srtt:      19604  rcv_wnd:      29312

      { dport:       9001 } hitcount:        172
	changed:         10  snd_wnd:      48384  srtt:     260444  rcv_wnd:      55168

      { dport:        443 } hitcount:        211
	changed:         10  snd_wnd:      26960  srtt:      17379  rcv_wnd:      28800

    Snapshot taken (see tracing/snapshot).  Details::

        triggering value { onchange($cwnd) }:         10
        triggered by event with key: { dport:         80 }

      Totals:
          Hits: 414
          Entries: 4
          Dropped: 0

    In the above case, the event that triggered the snapshot has the
    key with dport == 80.  If you look at the bucket that has 80 as
    the key, you'll find the additional values save()'d along with the
    changed value for that bucket, which should be the same as the
    global changed value (since that was the same value that triggered
    the global snapshot).

    And finally, looking at the snapshot data should show at or near
    the end the event that triggered the snapshot::

      # cat /sys/kernel/debug/tracing/snapshot

         gnome-shell-1261  [006] dN.3    49.823113: sched_stat_runtime: comm=gnome-shell pid=1261 runtime=49347 [ns] vruntime=1835730389 [ns]
       kworker/u16:4-773   [003] d..3    49.823114: sched_switch: prev_comm=kworker/u16:4 prev_pid=773 prev_prio=120 prev_state=R+ ==> next_comm=kworker/3:2 next_pid=135 next_prio=120
         gnome-shell-1261  [006] d..3    49.823114: sched_switch: prev_comm=gnome-shell prev_pid=1261 prev_prio=120 prev_state=R+ ==> next_comm=kworker/6:2 next_pid=387 next_prio=120
         kworker/3:2-135   [003] d..3    49.823118: sched_stat_runtime: comm=kworker/3:2 pid=135 runtime=5339 [ns] vruntime=17815800388 [ns]
         kworker/6:2-387   [006] d..3    49.823120: sched_stat_runtime: comm=kworker/6:2 pid=387 runtime=9594 [ns] vruntime=14589605367 [ns]
         kworker/6:2-387   [006] d..3    49.823122: sched_switch: prev_comm=kworker/6:2 prev_pid=387 prev_prio=120 prev_state=R+ ==> next_comm=gnome-shell next_pid=1261 next_prio=120
         kworker/3:2-135   [003] d..3    49.823123: sched_switch: prev_comm=kworker/3:2 prev_pid=135 prev_prio=120 prev_state=T ==> next_comm=swapper/3 next_pid=0 next_prio=120
              <idle>-0     [004] ..s7    49.823798: tcp_probe: src=10.0.0.10:54326 dest=23.215.104.193:80 mark=0x0 length=32 snd_nxt=0xe3ae2ff5 snd_una=0xe3ae2ecd snd_cwnd=10 ssthresh=2147483647 snd_wnd=28960 srtt=19604 rcv_wnd=29312

3. User space creating a trigger
--------------------------------

Writing into /sys/kernel/tracing/trace_marker writes into the ftrace
ring buffer. This can also act like an event, by writing into the trigger
file located in /sys/kernel/tracing/events/ftrace/print/

Modifying cyclictest to write into the trace_marker file before it sleeps
and after it wakes up, something like this::

  static void traceputs(char *str)
  {
	/* tracemark_fd is the trace_marker file descriptor */
	if (tracemark_fd < 0)
		return;
	/* write the tracemark message */
	write(tracemark_fd, str, strlen(str));
  }

And later add something like::

	traceputs("start");
	clock_nanosleep(...);
	traceputs("end");

We can make a histogram from this::

 # cd /sys/kernel/tracing
 # echo 'latency u64 lat' > synthetic_events
 # echo 'hist:keys=common_pid:ts0=common_timestamp.usecs if buf == "start"' > events/ftrace/print/trigger
 # echo 'hist:keys=common_pid:lat=common_timestamp.usecs-$ts0:onmatch(ftrace.print).latency($lat) if buf == "end"' >> events/ftrace/print/trigger
 # echo 'hist:keys=lat,common_pid:sort=lat' > events/synthetic/latency/trigger

The above created a synthetic event called "latency" and two histograms
against the trace_marker, one gets triggered when "start" is written into the
trace_marker file and the other when "end" is written. If the pids match, then
it will call the "latency" synthetic event with the calculated latency as its
parameter. Finally, a histogram is added to the latency synthetic event to
record the calculated latency along with the pid.

Now running cyclictest with::

 # ./cyclictest -p80 -d0 -i250 -n -a -t --tracemark -b 1000

 -p80  : run threads at priority 80
 -d0   : have all threads run at the same interval
 -i250 : start the interval at 250 microseconds (all threads will do this)
 -n    : sleep with nanosleep
 -a    : affine all threads to a separate CPU
 -t    : one thread per available CPU
 --tracemark : enable trace mark writing
 -b 1000 : stop if any latency is greater than 1000 microseconds

Note, the -b 1000 is used just to make --tracemark available.

Then we can see the histogram created by this with::

 # cat events/synthetic/latency/hist
 # event histogram
 #
 # trigger info: hist:keys=lat,common_pid:vals=hitcount:sort=lat:size=2048 [active]
 #

 { lat:        107, common_pid:       2039 } hitcount:          1
 { lat:        122, common_pid:       2041 } hitcount:          1
 { lat:        166, common_pid:       2039 } hitcount:          1
 { lat:        174, common_pid:       2039 } hitcount:          1
 { lat:        194, common_pid:       2041 } hitcount:          1
 { lat:        196, common_pid:       2036 } hitcount:          1
 { lat:        197, common_pid:       2038 } hitcount:          1
 { lat:        198, common_pid:       2039 } hitcount:          1
 { lat:        199, common_pid:       2039 } hitcount:          1
 { lat:        200, common_pid:       2041 } hitcount:          1
 { lat:        201, common_pid:       2039 } hitcount:          2
 { lat:        202, common_pid:       2038 } hitcount:          1
 { lat:        202, common_pid:       2043 } hitcount:          1
 { lat:        203, common_pid:       2039 } hitcount:          1
 { lat:        203, common_pid:       2036 } hitcount:          1
 { lat:        203, common_pid:       2041 } hitcount:          1
 { lat:        206, common_pid:       2038 } hitcount:          2
 { lat:        207, common_pid:       2039 } hitcount:          1
 { lat:        207, common_pid:       2036 } hitcount:          1
 { lat:        208, common_pid:       2040 } hitcount:          1
 { lat:        209, common_pid:       2043 } hitcount:          1
 { lat:        210, common_pid:       2039 } hitcount:          1
 { lat:        211, common_pid:       2039 } hitcount:          4
 { lat:        212, common_pid:       2043 } hitcount:          1
 { lat:        212, common_pid:       2039 } hitcount:          2
 { lat:        213, common_pid:       2039 } hitcount:          1
 { lat:        214, common_pid:       2038 } hitcount:          1
 { lat:        214, common_pid:       2039 } hitcount:          2
 { lat:        214, common_pid:       2042 } hitcount:          1
 { lat:        215, common_pid:       2039 } hitcount:          1
 { lat:        217, common_pid:       2036 } hitcount:          1
 { lat:        217, common_pid:       2040 } hitcount:          1
 { lat:        217, common_pid:       2039 } hitcount:          1
 { lat:        218, common_pid:       2039 } hitcount:          6
 { lat:        219, common_pid:       2039 } hitcount:          9
 { lat:        220, common_pid:       2039 } hitcount:         11
 { lat:        221, common_pid:       2039 } hitcount:          5
 { lat:        221, common_pid:       2042 } hitcount:          1
 { lat:        222, common_pid:       2039 } hitcount:          7
 { lat:        223, common_pid:       2036 } hitcount:          1
 { lat:        223, common_pid:       2039 } hitcount:          3
 { lat:        224, common_pid:       2039 } hitcount:          4
 { lat:        224, common_pid:       2037 } hitcount:          1
 { lat:        224, common_pid:       2036 } hitcount:          2
 { lat:        225, common_pid:       2039 } hitcount:          5
 { lat:        225, common_pid:       2042 } hitcount:          1
 { lat:        226, common_pid:       2039 } hitcount:          7
 { lat:        226, common_pid:       2036 } hitcount:          4
 { lat:        227, common_pid:       2039 } hitcount:          6
 { lat:        227, common_pid:       2036 } hitcount:         12
 { lat:        227, common_pid:       2043 } hitcount:          1
 { lat:        228, common_pid:       2039 } hitcount:          7
 { lat:        228, common_pid:       2036 } hitcount:         14
 { lat:        229, common_pid:       2039 } hitcount:          9
 { lat:        229, common_pid:       2036 } hitcount:          8
 { lat:        229, common_pid:       2038 } hitcount:          1
 { lat:        230, common_pid:       2039 } hitcount:         11
 { lat:        230, common_pid:       2036 } hitcount:          6
 { lat:        230, common_pid:       2043 } hitcount:          1
 { lat:        230, common_pid:       2042 } hitcount:          2
 { lat:        231, common_pid:       2041 } hitcount:          1
 { lat:        231, common_pid:       2036 } hitcount:          6
 { lat:        231, common_pid:       2043 } hitcount:          1
 { lat:        231, common_pid:       2039 } hitcount:          8
 { lat:        232, common_pid:       2037 } hitcount:          1
 { lat:        232, common_pid:       2039 } hitcount:          6
 { lat:        232, common_pid:       2040 } hitcount:          2
 { lat:        232, common_pid:       2036 } hitcount:          5
 { lat:        232, common_pid:       2043 } hitcount:          1
 { lat:        233, common_pid:       2036 } hitcount:          5
 { lat:        233, common_pid:       2039 } hitcount:         11
 { lat:        234, common_pid:       2039 } hitcount:          4
 { lat:        234, common_pid:       2038 } hitcount:          2
 { lat:        234, common_pid:       2043 } hitcount:          2
 { lat:        234, common_pid:       2036 } hitcount:         11
 { lat:        234, common_pid:       2040 } hitcount:          1
 { lat:        235, common_pid:       2037 } hitcount:          2
 { lat:        235, common_pid:       2036 } hitcount:          8
 { lat:        235, common_pid:       2043 } hitcount:          2
 { lat:        235, common_pid:       2039 } hitcount:          5
 { lat:        235, common_pid:       2042 } hitcount:          2
 { lat:        235, common_pid:       2040 } hitcount:          4
 { lat:        235, common_pid:       2041 } hitcount:          1
 { lat:        236, common_pid:       2036 } hitcount:          7
 { lat:        236, common_pid:       2037 } hitcount:          1
 { lat:        236, common_pid:       2041 } hitcount:          5
 { lat:        236, common_pid:       2039 } hitcount:          3
 { lat:        236, common_pid:       2043 } hitcount:          9
 { lat:        236, common_pid:       2040 } hitcount:          7
 { lat:        237, common_pid:       2037 } hitcount:          1
 { lat:        237, common_pid:       2040 } hitcount:          1
 { lat:        237, common_pid:       2036 } hitcount:          9
 { lat:        237, common_pid:       2039 } hitcount:          3
 { lat:        237, common_pid:       2043 } hitcount:          8
 { lat:        237, common_pid:       2042 } hitcount:          2
 { lat:        237, common_pid:       2041 } hitcount:          2
 { lat:        238, common_pid:       2043 } hitcount:         10
 { lat:        238, common_pid:       2040 } hitcount:          1
 { lat:        238, common_pid:       2037 } hitcount:          9
 { lat:        238, common_pid:       2038 } hitcount:          1
 { lat:        238, common_pid:       2039 } hitcount:          1
 { lat:        238, common_pid:       2042 } hitcount:          3
 { lat:        238, common_pid:       2036 } hitcount:          7
 { lat:        239, common_pid:       2041 } hitcount:          1
 { lat:        239, common_pid:       2043 } hitcount:         11
 { lat:        239, common_pid:       2037 } hitcount:         11
 { lat:        239, common_pid:       2038 } hitcount:          6
 { lat:        239, common_pid:       2036 } hitcount:          7
 { lat:        239, common_pid:       2040 } hitcount:          1
 { lat:        239, common_pid:       2042 } hitcount:          9
 { lat:        240, common_pid:       2037 } hitcount:         29
 { lat:        240, common_pid:       2043 } hitcount:         15
 { lat:        240, common_pid:       2040 } hitcount:         44
 { lat:        240, common_pid:       2039 } hitcount:          1
 { lat:        240, common_pid:       2041 } hitcount:          2
 { lat:        240, common_pid:       2038 } hitcount:          1
 { lat:        240, common_pid:       2036 } hitcount:         10
 { lat:        240, common_pid:       2042 } hitcount:         13
 { lat:        241, common_pid:       2036 } hitcount:         21
 { lat:        241, common_pid:       2041 } hitcount:         36
 { lat:        241, common_pid:       2037 } hitcount:         34
 { lat:        241, common_pid:       2042 } hitcount:         14
 { lat:        241, common_pid:       2040 } hitcount:         94
 { lat:        241, common_pid:       2039 } hitcount:         12
 { lat:        241, common_pid:       2038 } hitcount:          2
 { lat:        241, common_pid:       2043 } hitcount:         28
 { lat:        242, common_pid:       2040 } hitcount:        109
 { lat:        242, common_pid:       2041 } hitcount:        506
 { lat:        242, common_pid:       2039 } hitcount:        155
 { lat:        242, common_pid:       2042 } hitcount:         21
 { lat:        242, common_pid:       2037 } hitcount:         52
 { lat:        242, common_pid:       2043 } hitcount:         21
 { lat:        242, common_pid:       2036 } hitcount:         16
 { lat:        242, common_pid:       2038 } hitcount:        156
 { lat:        243, common_pid:       2037 } hitcount:         46
 { lat:        243, common_pid:       2039 } hitcount:         40
 { lat:        243, common_pid:       2042 } hitcount:        119
 { lat:        243, common_pid:       2041 } hitcount:        611
 { lat:        243, common_pid:       2036 } hitcount:         69
 { lat:        243, common_pid:       2038 } hitcount:        784
 { lat:        243, common_pid:       2040 } hitcount:        323
 { lat:        243, common_pid:       2043 } hitcount:         14
 { lat:        244, common_pid:       2043 } hitcount:         35
 { lat:        244, common_pid:       2042 } hitcount:        305
 { lat:        244, common_pid:       2039 } hitcount:          8
 { lat:        244, common_pid:       2040 } hitcount:       4515
 { lat:        244, common_pid:       2038 } hitcount:        371
 { lat:        244, common_pid:       2037 } hitcount:         31
 { lat:        244, common_pid:       2036 } hitcount:        114
 { lat:        244, common_pid:       2041 } hitcount:       3396
 { lat:        245, common_pid:       2036 } hitcount:        700
 { lat:        245, common_pid:       2041 } hitcount:       2772
 { lat:        245, common_pid:       2037 } hitcount:        268
 { lat:        245, common_pid:       2039 } hitcount:        472
 { lat:        245, common_pid:       2038 } hitcount:       2758
 { lat:        245, common_pid:       2042 } hitcount:       3833
 { lat:        245, common_pid:       2040 } hitcount:       3105
 { lat:        245, common_pid:       2043 } hitcount:        645
 { lat:        246, common_pid:       2038 } hitcount:       3451
 { lat:        246, common_pid:       2041 } hitcount:        142
 { lat:        246, common_pid:       2037 } hitcount:       5101
 { lat:        246, common_pid:       2040 } hitcount:         68
 { lat:        246, common_pid:       2043 } hitcount:       5099
 { lat:        246, common_pid:       2039 } hitcount:       5608
 { lat:        246, common_pid:       2042 } hitcount:       3723
 { lat:        246, common_pid:       2036 } hitcount:       4738
 { lat:        247, common_pid:       2042 } hitcount:        312
 { lat:        247, common_pid:       2043 } hitcount:       2385
 { lat:        247, common_pid:       2041 } hitcount:        452
 { lat:        247, common_pid:       2038 } hitcount:        792
 { lat:        247, common_pid:       2040 } hitcount:         78
 { lat:        247, common_pid:       2036 } hitcount:       2375
 { lat:        247, common_pid:       2039 } hitcount:       1834
 { lat:        247, common_pid:       2037 } hitcount:       2655
 { lat:        248, common_pid:       2037 } hitcount:         36
 { lat:        248, common_pid:       2042 } hitcount:         11
 { lat:        248, common_pid:       2038 } hitcount:        122
 { lat:        248, common_pid:       2036 } hitcount:        135
 { lat:        248, common_pid:       2039 } hitcount:         26
 { lat:        248, common_pid:       2041 } hitcount:        503
 { lat:        248, common_pid:       2043 } hitcount:         66
 { lat:        248, common_pid:       2040 } hitcount:         46
 { lat:        249, common_pid:       2037 } hitcount:         29
 { lat:        249, common_pid:       2038 } hitcount:          1
 { lat:        249, common_pid:       2043 } hitcount:         29
 { lat:        249, common_pid:       2039 } hitcount:          8
 { lat:        249, common_pid:       2042 } hitcount:         56
 { lat:        249, common_pid:       2040 } hitcount:         27
 { lat:        249, common_pid:       2041 } hitcount:         11
 { lat:        249, common_pid:       2036 } hitcount:         27
 { lat:        250, common_pid:       2038 } hitcount:          1
 { lat:        250, common_pid:       2036 } hitcount:         30
 { lat:        250, common_pid:       2040 } hitcount:         19
 { lat:        250, common_pid:       2043 } hitcount:         22
 { lat:        250, common_pid:       2042 } hitcount:         20
 { lat:        250, common_pid:       2041 } hitcount:          1
 { lat:        250, common_pid:       2039 } hitcount:          6
 { lat:        250, common_pid:       2037 } hitcount:         48
 { lat:        251, common_pid:       2037 } hitcount:         43
 { lat:        251, common_pid:       2039 } hitcount:          1
 { lat:        251, common_pid:       2036 } hitcount:         12
 { lat:        251, common_pid:       2042 } hitcount:          2
 { lat:        251, common_pid:       2041 } hitcount:          1
 { lat:        251, common_pid:       2043 } hitcount:         15
 { lat:        251, common_pid:       2040 } hitcount:          3
 { lat:        252, common_pid:       2040 } hitcount:          1
 { lat:        252, common_pid:       2036 } hitcount:         12
 { lat:        252, common_pid:       2037 } hitcount:         21
 { lat:        252, common_pid:       2043 } hitcount:         14
 { lat:        253, common_pid:       2037 } hitcount:         21
 { lat:        253, common_pid:       2039 } hitcount:          2
 { lat:        253, common_pid:       2036 } hitcount:          9
 { lat:        253, common_pid:       2043 } hitcount:          6
 { lat:        253, common_pid:       2040 } hitcount:          1
 { lat:        254, common_pid:       2036 } hitcount:          8
 { lat:        254, common_pid:       2043 } hitcount:          3
 { lat:        254, common_pid:       2041 } hitcount:          1
 { lat:        254, common_pid:       2042 } hitcount:          1
 { lat:        254, common_pid:       2039 } hitcount:          1
 { lat:        254, common_pid:       2037 } hitcount:         12
 { lat:        255, common_pid:       2043 } hitcount:          1
 { lat:        255, common_pid:       2037 } hitcount:          2
 { lat:        255, common_pid:       2036 } hitcount:          2
 { lat:        255, common_pid:       2039 } hitcount:          8
 { lat:        256, common_pid:       2043 } hitcount:          1
 { lat:        256, common_pid:       2036 } hitcount:          4
 { lat:        256, common_pid:       2039 } hitcount:          6
 { lat:        257, common_pid:       2039 } hitcount:          5
 { lat:        257, common_pid:       2036 } hitcount:          4
 { lat:        258, common_pid:       2039 } hitcount:          5
 { lat:        258, common_pid:       2036 } hitcount:          2
 { lat:        259, common_pid:       2036 } hitcount:          7
 { lat:        259, common_pid:       2039 } hitcount:          7
 { lat:        260, common_pid:       2036 } hitcount:          8
 { lat:        260, common_pid:       2039 } hitcount:          6
 { lat:        261, common_pid:       2036 } hitcount:          5
 { lat:        261, common_pid:       2039 } hitcount:          7
 { lat:        262, common_pid:       2039 } hitcount:          5
 { lat:        262, common_pid:       2036 } hitcount:          5
 { lat:        263, common_pid:       2039 } hitcount:          7
 { lat:        263, common_pid:       2036 } hitcount:          7
 { lat:        264, common_pid:       2039 } hitcount:          9
 { lat:        264, common_pid:       2036 } hitcount:          9
 { lat:        265, common_pid:       2036 } hitcount:          5
 { lat:        265, common_pid:       2039 } hitcount:          1
 { lat:        266, common_pid:       2036 } hitcount:          1
 { lat:        266, common_pid:       2039 } hitcount:          3
 { lat:        267, common_pid:       2036 } hitcount:          1
 { lat:        267, common_pid:       2039 } hitcount:          3
 { lat:        268, common_pid:       2036 } hitcount:          1
 { lat:        268, common_pid:       2039 } hitcount:          6
 { lat:        269, common_pid:       2036 } hitcount:          1
 { lat:        269, common_pid:       2043 } hitcount:          1
 { lat:        269, common_pid:       2039 } hitcount:          2
 { lat:        270, common_pid:       2040 } hitcount:          1
 { lat:        270, common_pid:       2039 } hitcount:          6
 { lat:        271, common_pid:       2041 } hitcount:          1
 { lat:        271, common_pid:       2039 } hitcount:          5
 { lat:        272, common_pid:       2039 } hitcount:         10
 { lat:        273, common_pid:       2039 } hitcount:          8
 { lat:        274, common_pid:       2039 } hitcount:          2
 { lat:        275, common_pid:       2039 } hitcount:          1
 { lat:        276, common_pid:       2039 } hitcount:          2
 { lat:        276, common_pid:       2037 } hitcount:          1
 { lat:        276, common_pid:       2038 } hitcount:          1
 { lat:        277, common_pid:       2039 } hitcount:          1
 { lat:        277, common_pid:       2042 } hitcount:          1
 { lat:        278, common_pid:       2039 } hitcount:          1
 { lat:        279, common_pid:       2039 } hitcount:          4
 { lat:        279, common_pid:       2043 } hitcount:          1
 { lat:        280, common_pid:       2039 } hitcount:          3
 { lat:        283, common_pid:       2036 } hitcount:          2
 { lat:        284, common_pid:       2039 } hitcount:          1
 { lat:        284, common_pid:       2043 } hitcount:          1
 { lat:        288, common_pid:       2039 } hitcount:          1
 { lat:        289, common_pid:       2039 } hitcount:          1
 { lat:        300, common_pid:       2039 } hitcount:          1
 { lat:        384, common_pid:       2039 } hitcount:          1

 Totals:
     Hits: 67625
     Entries: 278
     Dropped: 0

Note, the writes are around the sleep, so ideally they will all be of 250
microseconds. If you are wondering how there are several that are under
250 microseconds, that is because the way cyclictest works, is if one
iteration comes in late, the next one will set the timer to wake up less that
250. That is, if an iteration came in 50 microseconds late, the next wake up
will be at 200 microseconds.

But this could easily be done in userspace. To make this even more
interesting, we can mix the histogram between events that happened in the
kernel with trace_marker::

 # cd /sys/kernel/tracing
 # echo 'latency u64 lat' > synthetic_events
 # echo 'hist:keys=pid:ts0=common_timestamp.usecs' > events/sched/sched_waking/trigger
 # echo 'hist:keys=common_pid:lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).latency($lat) if buf == "end"' > events/ftrace/print/trigger
 # echo 'hist:keys=lat,common_pid:sort=lat' > events/synthetic/latency/trigger

The difference this time is that instead of using the trace_marker to start
the latency, the sched_waking event is used, matching the common_pid for the
trace_marker write with the pid that is being woken by sched_waking.

After running cyclictest again with the same parameters, we now have::

 # cat events/synthetic/latency/hist
 # event histogram
 #
 # trigger info: hist:keys=lat,common_pid:vals=hitcount:sort=lat:size=2048 [active]
 #

 { lat:          7, common_pid:       2302 } hitcount:        640
 { lat:          7, common_pid:       2299 } hitcount:         42
 { lat:          7, common_pid:       2303 } hitcount:         18
 { lat:          7, common_pid:       2305 } hitcount:        166
 { lat:          7, common_pid:       2306 } hitcount:          1
 { lat:          7, common_pid:       2301 } hitcount:         91
 { lat:          7, common_pid:       2300 } hitcount:         17
 { lat:          8, common_pid:       2303 } hitcount:       8296
 { lat:          8, common_pid:       2304 } hitcount:       6864
 { lat:          8, common_pid:       2305 } hitcount:       9464
 { lat:          8, common_pid:       2301 } hitcount:       9213
 { lat:          8, common_pid:       2306 } hitcount:       6246
 { lat:          8, common_pid:       2302 } hitcount:       8797
 { lat:          8, common_pid:       2299 } hitcount:       8771
 { lat:          8, common_pid:       2300 } hitcount:       8119
 { lat:          9, common_pid:       2305 } hitcount:       1519
 { lat:          9, common_pid:       2299 } hitcount:       2346
 { lat:          9, common_pid:       2303 } hitcount:       2841
 { lat:          9, common_pid:       2301 } hitcount:       1846
 { lat:          9, common_pid:       2304 } hitcount:       3861
 { lat:          9, common_pid:       2302 } hitcount:       1210
 { lat:          9, common_pid:       2300 } hitcount:       2762
 { lat:          9, common_pid:       2306 } hitcount:       4247
 { lat:         10, common_pid:       2299 } hitcount:         16
 { lat:         10, common_pid:       2306 } hitcount:        333
 { lat:         10, common_pid:       2303 } hitcount:         16
 { lat:         10, common_pid:       2304 } hitcount:        168
 { lat:         10, common_pid:       2302 } hitcount:        240
 { lat:         10, common_pid:       2301 } hitcount:         28
 { lat:         10, common_pid:       2300 } hitcount:         95
 { lat:         10, common_pid:       2305 } hitcount:         18
 { lat:         11, common_pid:       2303 } hitcount:          5
 { lat:         11, common_pid:       2305 } hitcount:          8
 { lat:         11, common_pid:       2306 } hitcount:        221
 { lat:         11, common_pid:       2302 } hitcount:         76
 { lat:         11, common_pid:       2304 } hitcount:         26
 { lat:         11, common_pid:       2300 } hitcount:        125
 { lat:         11, common_pid:       2299 } hitcount:          2
 { lat:         12, common_pid:       2305 } hitcount:          3
 { lat:         12, common_pid:       2300 } hitcount:          6
 { lat:         12, common_pid:       2306 } hitcount:         90
 { lat:         12, common_pid:       2302 } hitcount:          4
 { lat:         12, common_pid:       2303 } hitcount:          1
 { lat:         12, common_pid:       2304 } hitcount:        122
 { lat:         13, common_pid:       2300 } hitcount:         12
 { lat:         13, common_pid:       2301 } hitcount:          1
 { lat:         13, common_pid:       2306 } hitcount:         32
 { lat:         13, common_pid:       2302 } hitcount:          5
 { lat:         13, common_pid:       2305 } hitcount:          1
 { lat:         13, common_pid:       2303 } hitcount:          1
 { lat:         13, common_pid:       2304 } hitcount:         61
 { lat:         14, common_pid:       2303 } hitcount:          4
 { lat:         14, common_pid:       2306 } hitcount:          5
 { lat:         14, common_pid:       2305 } hitcount:          4
 { lat:         14, common_pid:       2304 } hitcount:         62
 { lat:         14, common_pid:       2302 } hitcount:         19
 { lat:         14, common_pid:       2300 } hitcount:         33
 { lat:         14, common_pid:       2299 } hitcount:          1
 { lat:         14, common_pid:       2301 } hitcount:          4
 { lat:         15, common_pid:       2305 } hitcount:          1
 { lat:         15, common_pid:       2302 } hitcount:         25
 { lat:         15, common_pid:       2300 } hitcount:         11
 { lat:         15, common_pid:       2299 } hitcount:          5
 { lat:         15, common_pid:       2301 } hitcount:          1
 { lat:         15, common_pid:       2304 } hitcount:          8
 { lat:         15, common_pid:       2303 } hitcount:          1
 { lat:         15, common_pid:       2306 } hitcount:          6
 { lat:         16, common_pid:       2302 } hitcount:         31
 { lat:         16, common_pid:       2306 } hitcount:          3
 { lat:         16, common_pid:       2300 } hitcount:          5
 { lat:         17, common_pid:       2302 } hitcount:          6
 { lat:         17, common_pid:       2303 } hitcount:          1
 { lat:         18, common_pid:       2304 } hitcount:          1
 { lat:         18, common_pid:       2302 } hitcount:          8
 { lat:         18, common_pid:       2299 } hitcount:          1
 { lat:         18, common_pid:       2301 } hitcount:          1
 { lat:         19, common_pid:       2303 } hitcount:          4
 { lat:         19, common_pid:       2304 } hitcount:          5
 { lat:         19, common_pid:       2302 } hitcount:          4
 { lat:         19, common_pid:       2299 } hitcount:          3
 { lat:         19, common_pid:       2306 } hitcount:          1
 { lat:         19, common_pid:       2300 } hitcount:          4
 { lat:         19, common_pid:       2305 } hitcount:          5
 { lat:         20, common_pid:       2299 } hitcount:          2
 { lat:         20, common_pid:       2302 } hitcount:          3
 { lat:         20, common_pid:       2305 } hitcount:          1
 { lat:         20, common_pid:       2300 } hitcount:          2
 { lat:         20, common_pid:       2301 } hitcount:          2
 { lat:         20, common_pid:       2303 } hitcount:          3
 { lat:         21, common_pid:       2305 } hitcount:          1
 { lat:         21, common_pid:       2299 } hitcount:          5
 { lat:         21, common_pid:       2303 } hitcount:          4
 { lat:         21, common_pid:       2302 } hitcount:          7
 { lat:         21, common_pid:       2300 } hitcount:          1
 { lat:         21, common_pid:       2301 } hitcount:          5
 { lat:         21, common_pid:       2304 } hitcount:          2
 { lat:         22, common_pid:       2302 } hitcount:          5
 { lat:         22, common_pid:       2303 } hitcount:          1
 { lat:         22, common_pid:       2306 } hitcount:          3
 { lat:         22, common_pid:       2301 } hitcount:          2
 { lat:         22, common_pid:       2300 } hitcount:          1
 { lat:         22, common_pid:       2299 } hitcount:          1
 { lat:         22, common_pid:       2305 } hitcount:          1
 { lat:         22, common_pid:       2304 } hitcount:          1
 { lat:         23, common_pid:       2299 } hitcount:          1
 { lat:         23, common_pid:       2306 } hitcount:          2
 { lat:         23, common_pid:       2302 } hitcount:          6
 { lat:         24, common_pid:       2302 } hitcount:          3
 { lat:         24, common_pid:       2300 } hitcount:          1
 { lat:         24, common_pid:       2306 } hitcount:          2
 { lat:         24, common_pid:       2305 } hitcount:          1
 { lat:         24, common_pid:       2299 } hitcount:          1
 { lat:         25, common_pid:       2300 } hitcount:          1
 { lat:         25, common_pid:       2302 } hitcount:          4
 { lat:         26, common_pid:       2302 } hitcount:          2
 { lat:         27, common_pid:       2305 } hitcount:          1
 { lat:         27, common_pid:       2300 } hitcount:          1
 { lat:         27, common_pid:       2302 } hitcount:          3
 { lat:         28, common_pid:       2306 } hitcount:          1
 { lat:         28, common_pid:       2302 } hitcount:          4
 { lat:         29, common_pid:       2302 } hitcount:          1
 { lat:         29, common_pid:       2300 } hitcount:          2
 { lat:         29, common_pid:       2306 } hitcount:          1
 { lat:         29, common_pid:       2304 } hitcount:          1
 { lat:         30, common_pid:       2302 } hitcount:          4
 { lat:         31, common_pid:       2302 } hitcount:          6
 { lat:         32, common_pid:       2302 } hitcount:          1
 { lat:         33, common_pid:       2299 } hitcount:          1
 { lat:         33, common_pid:       2302 } hitcount:          3
 { lat:         34, common_pid:       2302 } hitcount:          2
 { lat:         35, common_pid:       2302 } hitcount:          1
 { lat:         35, common_pid:       2304 } hitcount:          1
 { lat:         36, common_pid:       2302 } hitcount:          4
 { lat:         37, common_pid:       2302 } hitcount:          6
 { lat:         38, common_pid:       2302 } hitcount:          2
 { lat:         39, common_pid:       2302 } hitcount:          2
 { lat:         39, common_pid:       2304 } hitcount:          1
 { lat:         40, common_pid:       2304 } hitcount:          2
 { lat:         40, common_pid:       2302 } hitcount:          5
 { lat:         41, common_pid:       2304 } hitcount:          1
 { lat:         41, common_pid:       2302 } hitcount:          8
 { lat:         42, common_pid:       2302 } hitcount:          6
 { lat:         42, common_pid:       2304 } hitcount:          1
 { lat:         43, common_pid:       2302 } hitcount:          3
 { lat:         43, common_pid:       2304 } hitcount:          4
 { lat:         44, common_pid:       2302 } hitcount:          6
 { lat:         45, common_pid:       2302 } hitcount:          5
 { lat:         46, common_pid:       2302 } hitcount:          5
 { lat:         47, common_pid:       2302 } hitcount:          7
 { lat:         48, common_pid:       2301 } hitcount:          1
 { lat:         48, common_pid:       2302 } hitcount:          9
 { lat:         49, common_pid:       2302 } hitcount:          3
 { lat:         50, common_pid:       2302 } hitcount:          1
 { lat:         50, common_pid:       2301 } hitcount:          1
 { lat:         51, common_pid:       2302 } hitcount:          2
 { lat:         51, common_pid:       2301 } hitcount:          1
 { lat:         61, common_pid:       2302 } hitcount:          1
 { lat:        110, common_pid:       2302 } hitcount:          1

 Totals:
     Hits: 89565
     Entries: 158
     Dropped: 0

This doesn't tell us any information about how late cyclictest may have
woken up, but it does show us a nice histogram of how long it took from
the time that cyclictest was woken to the time it made it into user space.