summaryrefslogtreecommitdiffstats
path: root/Documentation/ftrace.txt
blob: 753f4de4b1752cd9c0fe7c84d4cbd6cf665e50cd (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
		ftrace - Function Tracer
		========================

Copyright 2008 Red Hat Inc.
   Author:   Steven Rostedt <srostedt@redhat.com>
  License:   The GNU Free Documentation License, Version 1.2
               (dual licensed under the GPL v2)
Reviewers:   Elias Oltmanns, Randy Dunlap, Andrew Morton,
	     John Kacur, and David Teigland.

Written for: 2.6.28-rc2

Introduction
------------

Ftrace is an internal tracer designed to help out developers and
designers of systems to find what is going on inside the kernel.
It can be used for debugging or analyzing latencies and performance
issues that take place outside of user-space.

Although ftrace is the function tracer, it also includes an
infrastructure that allows for other types of tracing. Some of the
tracers that are currently in ftrace include a tracer to trace
context switches, the time it takes for a high priority task to
run after it was woken up, the time interrupts are disabled, and
more (ftrace allows for tracer plugins, which means that the list of
tracers can always grow).


The File System
---------------

Ftrace uses the debugfs file system to hold the control files as well
as the files to display output.

To mount the debugfs system:

  # mkdir /debug
  # mount -t debugfs nodev /debug

(Note: it is more common to mount at /sys/kernel/debug, but for simplicity
 this document will use /debug)

That's it! (assuming that you have ftrace configured into your kernel)

After mounting the debugfs, you can see a directory called
"tracing".  This directory contains the control and output files
of ftrace. Here is a list of some of the key files:


 Note: all time values are in microseconds.

  current_tracer: This is used to set or display the current tracer
		that is configured.

  available_tracers: This holds the different types of tracers that
		have been compiled into the kernel. The tracers
		listed here can be configured by echoing their name
		into current_tracer.

  tracing_enabled: This sets or displays whether the current_tracer
		is activated and tracing or not. Echo 0 into this
		file to disable the tracer or 1 to enable it.

  trace: This file holds the output of the trace in a human readable
		format (described below).

  latency_trace: This file shows the same trace but the information
		is organized more to display possible latencies
		in the system (described below).

  trace_pipe: The output is the same as the "trace" file but this
		file is meant to be streamed with live tracing.
		Reads from this file will block until new data
		is retrieved. Unlike the "trace" and "latency_trace"
		files, this file is a consumer. This means reading
		from this file causes sequential reads to display
		more current data. Once data is read from this
		file, it is consumed, and will not be read
		again with a sequential read. The "trace" and
		"latency_trace" files are static, and if the
		tracer is not adding more data, they will display
		the same information every time they are read.

  trace_options: This file lets the user control the amount of data
		that is displayed in one of the above output
		files.

  trace_max_latency: Some of the tracers record the max latency.
		For example, the time interrupts are disabled.
		This time is saved in this file. The max trace
		will also be stored, and displayed by either
		"trace" or "latency_trace".  A new max trace will
		only be recorded if the latency is greater than
		the value in this file. (in microseconds)

  buffer_size_kb: This sets or displays the number of kilobytes each CPU
		buffer can hold. The tracer buffers are the same size
		for each CPU. The displayed number is the size of the
		CPU buffer and not total size of all buffers. The
		trace buffers are allocated in pages (blocks of memory
		that the kernel uses for allocation, usually 4 KB in size).
		If the last page allocated has room for more bytes
		than requested, the rest of the page will be used,
		making the actual allocation bigger than requested.
		(Note, the size may not be a multiple of the page size due
		to buffer managment overhead.)

		This can only be updated when the current_tracer
		is set to "nop".

  tracing_cpumask: This is a mask that lets the user only trace
		on specified CPUS. The format is a hex string
		representing the CPUS.

  set_ftrace_filter: When dynamic ftrace is configured in (see the
		section below "dynamic ftrace"), the code is dynamically
		modified (code text rewrite) to disable calling of the
		function profiler (mcount). This lets tracing be configured
		in with practically no overhead in performance.  This also
		has a side effect of enabling or disabling specific functions
		to be traced. Echoing names of functions into this file
		will limit the trace to only those functions.

  set_ftrace_notrace: This has an effect opposite to that of
		set_ftrace_filter. Any function that is added here will not
		be traced. If a function exists in both set_ftrace_filter
		and set_ftrace_notrace,	the function will _not_ be traced.

  available_filter_functions: This lists the functions that ftrace
		has processed and can trace. These are the function
		names that you can pass to "set_ftrace_filter" or
		"set_ftrace_notrace". (See the section "dynamic ftrace"
		below for more details.)


The Tracers
-----------

Here is the list of current tracers that may be configured.

  function - function tracer that uses mcount to trace all functions.

  sched_switch - traces the context switches between tasks.

  irqsoff - traces the areas that disable interrupts and saves
  		the trace with the longest max latency.
		See tracing_max_latency.  When a new max is recorded,
		it replaces the old trace. It is best to view this
		trace via the latency_trace file.

  preemptoff - Similar to irqsoff but traces and records the amount of
		time for which preemption is disabled.

  preemptirqsoff - Similar to irqsoff and preemptoff, but traces and
		 records the largest time for which irqs and/or preemption
		 is disabled.

  wakeup - Traces and records the max latency that it takes for
		the highest priority task to get scheduled after
		it has been woken up.

  nop - This is not a tracer. To remove all tracers from tracing
		simply echo "nop" into current_tracer.


Examples of using the tracer
----------------------------

Here are typical examples of using the tracers when controlling them only
with the debugfs interface (without using any user-land utilities).

Output format:
--------------

Here is an example of the output format of the file "trace"

                             --------
# tracer: function
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |
            bash-4251  [01] 10152.583854: path_put <-path_walk
            bash-4251  [01] 10152.583855: dput <-path_put
            bash-4251  [01] 10152.583855: _atomic_dec_and_lock <-dput
                             --------

A header is printed with the tracer name that is represented by the trace.
In this case the tracer is "function". Then a header showing the format. Task
name "bash", the task PID "4251", the CPU that it was running on
"01", the timestamp in <secs>.<usecs> format, the function name that was
traced "path_put" and the parent function that called this function
"path_walk". The timestamp is the time at which the function was
entered.

The sched_switch tracer also includes tracing of task wakeups and
context switches.

     ksoftirqd/1-7     [01]  1453.070013:      7:115:R   +  2916:115:S
     ksoftirqd/1-7     [01]  1453.070013:      7:115:R   +    10:115:S
     ksoftirqd/1-7     [01]  1453.070013:      7:115:R ==>    10:115:R
        events/1-10    [01]  1453.070013:     10:115:S ==>  2916:115:R
     kondemand/1-2916  [01]  1453.070013:   2916:115:S ==>     7:115:R
     ksoftirqd/1-7     [01]  1453.070013:      7:115:S ==>     0:140:R

Wake ups are represented by a "+" and the context switches are shown as
"==>".  The format is:

 Context switches:

       Previous task              Next Task

  <pid>:<prio>:<state>  ==>  <pid>:<prio>:<state>

 Wake ups:

       Current task               Task waking up

  <pid>:<prio>:<state>    +  <pid>:<prio>:<state>

The prio is the internal kernel priority, which is the inverse of the
priority that is usually displayed by user-space tools. Zero represents
the highest priority (99). Prio 100 starts the "nice" priorities with
100 being equal to nice -20 and 139 being nice 19. The prio "140" is
reserved for the idle task which is the lowest priority thread (pid 0).


Latency trace format
--------------------

For traces that display latency times, the latency_trace file gives
somewhat more information to see why a latency happened. Here is a typical
trace.

# tracer: irqsoff
#
irqsoff latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: apic_timer_interrupt
 => ended at:   do_softirq

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
  <idle>-0     0d..1    0us+: trace_hardirqs_off_thunk (apic_timer_interrupt)
  <idle>-0     0d.s.   97us : __do_softirq (do_softirq)
  <idle>-0     0d.s1   98us : trace_hardirqs_on (do_softirq)



This shows that the current tracer is "irqsoff" tracing the time for which
interrupts were disabled. It gives the trace version and the version
of the kernel upon which this was executed on (2.6.26-rc8). Then it displays
the max latency in microsecs (97 us). The number of trace entries displayed
and the total number recorded (both are three: #3/3). The type of
preemption that was used (PREEMPT). VP, KP, SP, and HP are always zero
and are reserved for later use. #P is the number of online CPUS (#P:2).

The task is the process that was running when the latency occurred.
(swapper pid: 0).

The start and stop (the functions in which the interrupts were disabled and
enabled respectively) that caused the latencies:

  apic_timer_interrupt is where the interrupts were disabled.
  do_softirq is where they were enabled again.

The next lines after the header are the trace itself. The header
explains which is which.

  cmd: The name of the process in the trace.

  pid: The PID of that process.

  CPU#: The CPU which the process was running on.

  irqs-off: 'd' interrupts are disabled. '.' otherwise.
	    Note: If the architecture does not support a way to
		  read the irq flags variable, an 'X' will always
		  be printed here.

  need-resched: 'N' task need_resched is set, '.' otherwise.

  hardirq/softirq:
	'H' - hard irq occurred inside a softirq.
	'h' - hard irq is running
	's' - soft irq is running
	'.' - normal context.

  preempt-depth: The level of preempt_disabled

The above is mostly meaningful for kernel developers.

  time: This differs from the trace file output. The trace file output
	includes an absolute timestamp. The timestamp used by the
	latency_trace file is relative to the start of the trace.

  delay: This is just to help catch your eye a bit better. And
	needs to be fixed to be only relative to the same CPU.
	The marks are determined by the difference between this
	current trace and the next trace.
	 '!' - greater than preempt_mark_thresh (default 100)
	 '+' - greater than 1 microsecond
	 ' ' - less than or equal to 1 microsecond.

  The rest is the same as the 'trace' file.


trace_options
-------------

The trace_options file is used to control what gets printed in the trace
output. To see what is available, simply cat the file:

  cat /debug/tracing/trace_options
  print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \
 noblock nostacktrace nosched-tree

To disable one of the options, echo in the option prepended with "no".

  echo noprint-parent > /debug/tracing/trace_options

To enable an option, leave off the "no".

  echo sym-offset > /debug/tracing/trace_options

Here are the available options:

  print-parent - On function traces, display the calling function
		as well as the function being traced.

  print-parent:
   bash-4000  [01]  1477.606694: simple_strtoul <-strict_strtoul

  noprint-parent:
   bash-4000  [01]  1477.606694: simple_strtoul


  sym-offset - Display not only the function name, but also the offset
		in the function. For example, instead of seeing just
		"ktime_get", you will see "ktime_get+0xb/0x20".

  sym-offset:
   bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0

  sym-addr - this will also display the function address as well as
		the function name.

  sym-addr:
   bash-4000  [01]  1477.606694: simple_strtoul <c0339346>

  verbose - This deals with the latency_trace file.

    bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
    (+0.000ms): simple_strtoul (strict_strtoul)

  raw - This will display raw numbers. This option is best for use with
	user applications that can translate the raw numbers better than
	having it done in the kernel.

  hex - Similar to raw, but the numbers will be in a hexadecimal format.

  bin - This will print out the formats in raw binary.

  block - TBD (needs update)

  stacktrace - This is one of the options that changes the trace itself.
		When a trace is recorded, so is the stack of functions.
		This allows for back traces of trace sites.

  sched-tree - TBD (any users??)


sched_switch
------------

This tracer simply records schedule switches. Here is an example
of how to use it.

 # echo sched_switch > /debug/tracing/current_tracer
 # echo 1 > /debug/tracing/tracing_enabled
 # sleep 1
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/trace

# tracer: sched_switch
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |
            bash-3997  [01]   240.132281:   3997:120:R   +  4055:120:R
            bash-3997  [01]   240.132284:   3997:120:R ==>  4055:120:R
           sleep-4055  [01]   240.132371:   4055:120:S ==>  3997:120:R
            bash-3997  [01]   240.132454:   3997:120:R   +  4055:120:S
            bash-3997  [01]   240.132457:   3997:120:R ==>  4055:120:R
           sleep-4055  [01]   240.132460:   4055:120:D ==>  3997:120:R
            bash-3997  [01]   240.132463:   3997:120:R   +  4055:120:D
            bash-3997  [01]   240.132465:   3997:120:R ==>  4055:120:R
          <idle>-0     [00]   240.132589:      0:140:R   +     4:115:S
          <idle>-0     [00]   240.132591:      0:140:R ==>     4:115:R
     ksoftirqd/0-4     [00]   240.132595:      4:115:S ==>     0:140:R
          <idle>-0     [00]   240.132598:      0:140:R   +     4:115:S
          <idle>-0     [00]   240.132599:      0:140:R ==>     4:115:R
     ksoftirqd/0-4     [00]   240.132603:      4:115:S ==>     0:140:R
           sleep-4055  [01]   240.133058:   4055:120:S ==>  3997:120:R
 [...]


As we have discussed previously about this format, the header shows
the name of the trace and points to the options. The "FUNCTION"
is a misnomer since here it represents the wake ups and context
switches.

The sched_switch file only lists the wake ups (represented with '+')
and context switches ('==>') with the previous task or current task
first followed by the next task or task waking up. The format for both
of these is PID:KERNEL-PRIO:TASK-STATE. Remember that the KERNEL-PRIO
is the inverse of the actual priority with zero (0) being the highest
priority and the nice values starting at 100 (nice -20). Below is
a quick chart to map the kernel priority to user land priorities.

  Kernel priority: 0 to 99    ==> user RT priority 99 to 0
  Kernel priority: 100 to 139 ==> user nice -20 to 19
  Kernel priority: 140        ==> idle task priority

The task states are:

 R - running : wants to run, may not actually be running
 S - sleep   : process is waiting to be woken up (handles signals)
 D - disk sleep (uninterruptible sleep) : process must be woken up
					(ignores signals)
 T - stopped : process suspended
 t - traced  : process is being traced (with something like gdb)
 Z - zombie  : process waiting to be cleaned up
 X - unknown


ftrace_enabled
--------------

The following tracers (listed below) give different output depending
on whether or not the sysctl ftrace_enabled is set. To set ftrace_enabled,
one can either use the sysctl function or set it via the proc
file system interface.

  sysctl kernel.ftrace_enabled=1

 or

  echo 1 > /proc/sys/kernel/ftrace_enabled

To disable ftrace_enabled simply replace the '1' with '0' in
the above commands.

When ftrace_enabled is set the tracers will also record the functions
that are within the trace. The descriptions of the tracers
will also show an example with ftrace enabled.


irqsoff
-------

When interrupts are disabled, the CPU can not react to any other
external event (besides NMIs and SMIs). This prevents the timer
interrupt from triggering or the mouse interrupt from letting the
kernel know of a new mouse event. The result is a latency with the
reaction time.

The irqsoff tracer tracks the time for which interrupts are disabled.
When a new maximum latency is hit, the tracer saves the trace leading up
to that latency point so that every time a new maximum is reached, the old
saved trace is discarded and the new trace is saved.

To reset the maximum, echo 0 into tracing_max_latency. Here is an
example:

 # echo irqsoff > /debug/tracing/current_tracer
 # echo 0 > /debug/tracing/tracing_max_latency
 # echo 1 > /debug/tracing/tracing_enabled
 # ls -ltr
 [...]
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/latency_trace
# tracer: irqsoff
#
irqsoff latency trace v1.1.5 on 2.6.26
--------------------------------------------------------------------
 latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: sys_setpgid
 => ended at:   sys_setpgid

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
    bash-3730  1d...    0us : _write_lock_irq (sys_setpgid)
    bash-3730  1d..1    1us+: _write_unlock_irq (sys_setpgid)
    bash-3730  1d..2   14us : trace_hardirqs_on (sys_setpgid)


Here we see that that we had a latency of 12 microsecs (which is
very good). The _write_lock_irq in sys_setpgid disabled interrupts.
The difference between the 12 and the displayed timestamp 14us occurred
because the clock was incremented between the time of recording the max
latency and the time of recording the function that had that latency.

Note the above example had ftrace_enabled not set. If we set the
ftrace_enabled, we get a much larger output:

# tracer: irqsoff
#
irqsoff latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: __alloc_pages_internal
 => ended at:   __alloc_pages_internal

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
      ls-4339  0...1    0us+: get_page_from_freelist (__alloc_pages_internal)
      ls-4339  0d..1    3us : rmqueue_bulk (get_page_from_freelist)
      ls-4339  0d..1    3us : _spin_lock (rmqueue_bulk)
      ls-4339  0d..1    4us : add_preempt_count (_spin_lock)
      ls-4339  0d..2    4us : __rmqueue (rmqueue_bulk)
      ls-4339  0d..2    5us : __rmqueue_smallest (__rmqueue)
      ls-4339  0d..2    5us : __mod_zone_page_state (__rmqueue_smallest)
      ls-4339  0d..2    6us : __rmqueue (rmqueue_bulk)
      ls-4339  0d..2    6us : __rmqueue_smallest (__rmqueue)
      ls-4339  0d..2    7us : __mod_zone_page_state (__rmqueue_smallest)
      ls-4339  0d..2    7us : __rmqueue (rmqueue_bulk)
      ls-4339  0d..2    8us : __rmqueue_smallest (__rmqueue)
[...]
      ls-4339  0d..2   46us : __rmqueue_smallest (__rmqueue)
      ls-4339  0d..2   47us : __mod_zone_page_state (__rmqueue_smallest)
      ls-4339  0d..2   47us : __rmqueue (rmqueue_bulk)
      ls-4339  0d..2   48us : __rmqueue_smallest (__rmqueue)
      ls-4339  0d..2   48us : __mod_zone_page_state (__rmqueue_smallest)
      ls-4339  0d..2   49us : _spin_unlock (rmqueue_bulk)
      ls-4339  0d..2   49us : sub_preempt_count (_spin_unlock)
      ls-4339  0d..1   50us : get_page_from_freelist (__alloc_pages_internal)
      ls-4339  0d..2   51us : trace_hardirqs_on (__alloc_pages_internal)



Here we traced a 50 microsecond latency. But we also see all the
functions that were called during that time. Note that by enabling
function tracing, we incur an added overhead. This overhead may
extend the latency times. But nevertheless, this trace has provided
some very helpful debugging information.


preemptoff
----------

When preemption is disabled, we may be able to receive interrupts but
the task cannot be preempted and a higher priority task must wait
for preemption to be enabled again before it can preempt a lower
priority task.

The preemptoff tracer traces the places that disable preemption.
Like the irqsoff tracer, it records the maximum latency for which preemption
was disabled. The control of preemptoff tracer is much like the irqsoff
tracer.

 # echo preemptoff > /debug/tracing/current_tracer
 # echo 0 > /debug/tracing/tracing_max_latency
 # echo 1 > /debug/tracing/tracing_enabled
 # ls -ltr
 [...]
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/latency_trace
# tracer: preemptoff
#
preemptoff latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: do_IRQ
 => ended at:   __do_softirq

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
    sshd-4261  0d.h.    0us+: irq_enter (do_IRQ)
    sshd-4261  0d.s.   29us : _local_bh_enable (__do_softirq)
    sshd-4261  0d.s1   30us : trace_preempt_on (__do_softirq)


This has some more changes. Preemption was disabled when an interrupt
came in (notice the 'h'), and was enabled while doing a softirq.
(notice the 's'). But we also see that interrupts have been disabled
when entering the preempt off section and leaving it (the 'd').
We do not know if interrupts were enabled in the mean time.

# tracer: preemptoff
#
preemptoff latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: remove_wait_queue
 => ended at:   __do_softirq

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
    sshd-4261  0d..1    0us : _spin_lock_irqsave (remove_wait_queue)
    sshd-4261  0d..1    1us : _spin_unlock_irqrestore (remove_wait_queue)
    sshd-4261  0d..1    2us : do_IRQ (common_interrupt)
    sshd-4261  0d..1    2us : irq_enter (do_IRQ)
    sshd-4261  0d..1    2us : idle_cpu (irq_enter)
    sshd-4261  0d..1    3us : add_preempt_count (irq_enter)
    sshd-4261  0d.h1    3us : idle_cpu (irq_enter)
    sshd-4261  0d.h.    4us : handle_fasteoi_irq (do_IRQ)
[...]
    sshd-4261  0d.h.   12us : add_preempt_count (_spin_lock)
    sshd-4261  0d.h1   12us : ack_ioapic_quirk_irq (handle_fasteoi_irq)
    sshd-4261  0d.h1   13us : move_native_irq (ack_ioapic_quirk_irq)
    sshd-4261  0d.h1   13us : _spin_unlock (handle_fasteoi_irq)
    sshd-4261  0d.h1   14us : sub_preempt_count (_spin_unlock)
    sshd-4261  0d.h1   14us : irq_exit (do_IRQ)
    sshd-4261  0d.h1   15us : sub_preempt_count (irq_exit)
    sshd-4261  0d..2   15us : do_softirq (irq_exit)
    sshd-4261  0d...   15us : __do_softirq (do_softirq)
    sshd-4261  0d...   16us : __local_bh_disable (__do_softirq)
    sshd-4261  0d...   16us+: add_preempt_count (__local_bh_disable)
    sshd-4261  0d.s4   20us : add_preempt_count (__local_bh_disable)
    sshd-4261  0d.s4   21us : sub_preempt_count (local_bh_enable)
    sshd-4261  0d.s5   21us : sub_preempt_count (local_bh_enable)
[...]
    sshd-4261  0d.s6   41us : add_preempt_count (__local_bh_disable)
    sshd-4261  0d.s6   42us : sub_preempt_count (local_bh_enable)
    sshd-4261  0d.s7   42us : sub_preempt_count (local_bh_enable)
    sshd-4261  0d.s5   43us : add_preempt_count (__local_bh_disable)
    sshd-4261  0d.s5   43us : sub_preempt_count (local_bh_enable_ip)
    sshd-4261  0d.s6   44us : sub_preempt_count (local_bh_enable_ip)
    sshd-4261  0d.s5   44us : add_preempt_count (__local_bh_disable)
    sshd-4261  0d.s5   45us : sub_preempt_count (local_bh_enable)
[...]
    sshd-4261  0d.s.   63us : _local_bh_enable (__do_softirq)
    sshd-4261  0d.s1   64us : trace_preempt_on (__do_softirq)


The above is an example of the preemptoff trace with ftrace_enabled
set. Here we see that interrupts were disabled the entire time.
The irq_enter code lets us know that we entered an interrupt 'h'.
Before that, the functions being traced still show that it is not
in an interrupt, but we can see from the functions themselves that
this is not the case.

Notice that __do_softirq when called does not have a preempt_count.
It may seem that we missed a preempt enabling. What really happened
is that the preempt count is held on the thread's stack and we
switched to the softirq stack (4K stacks in effect). The code
does not copy the preempt count, but because interrupts are disabled,
we do not need to worry about it. Having a tracer like this is good
for letting people know what really happens inside the kernel.


preemptirqsoff
--------------

Knowing the locations that have interrupts disabled or preemption
disabled for the longest times is helpful. But sometimes we would
like to know when either preemption and/or interrupts are disabled.

Consider the following code:

    local_irq_disable();
    call_function_with_irqs_off();
    preempt_disable();
    call_function_with_irqs_and_preemption_off();
    local_irq_enable();
    call_function_with_preemption_off();
    preempt_enable();

The irqsoff tracer will record the total length of
call_function_with_irqs_off() and
call_function_with_irqs_and_preemption_off().

The preemptoff tracer will record the total length of
call_function_with_irqs_and_preemption_off() and
call_function_with_preemption_off().

But neither will trace the time that interrupts and/or preemption
is disabled. This total time is the time that we can not schedule.
To record this time, use the preemptirqsoff tracer.

Again, using this trace is much like the irqsoff and preemptoff tracers.

 # echo preemptirqsoff > /debug/tracing/current_tracer
 # echo 0 > /debug/tracing/tracing_max_latency
 # echo 1 > /debug/tracing/tracing_enabled
 # ls -ltr
 [...]
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/latency_trace
# tracer: preemptirqsoff
#
preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: apic_timer_interrupt
 => ended at:   __do_softirq

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
      ls-4860  0d...    0us!: trace_hardirqs_off_thunk (apic_timer_interrupt)
      ls-4860  0d.s.  294us : _local_bh_enable (__do_softirq)
      ls-4860  0d.s1  294us : trace_preempt_on (__do_softirq)



The trace_hardirqs_off_thunk is called from assembly on x86 when
interrupts are disabled in the assembly code. Without the function
tracing, we do not know if interrupts were enabled within the preemption
points. We do see that it started with preemption enabled.

Here is a trace with ftrace_enabled set:


# tracer: preemptirqsoff
#
preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: write_chan
 => ended at:   __do_softirq

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
      ls-4473  0.N..    0us : preempt_schedule (write_chan)
      ls-4473  0dN.1    1us : _spin_lock (schedule)
      ls-4473  0dN.1    2us : add_preempt_count (_spin_lock)
      ls-4473  0d..2    2us : put_prev_task_fair (schedule)
[...]
      ls-4473  0d..2   13us : set_normalized_timespec (ktime_get_ts)
      ls-4473  0d..2   13us : __switch_to (schedule)
    sshd-4261  0d..2   14us : finish_task_switch (schedule)
    sshd-4261  0d..2   14us : _spin_unlock_irq (finish_task_switch)
    sshd-4261  0d..1   15us : add_preempt_count (_spin_lock_irqsave)
    sshd-4261  0d..2   16us : _spin_unlock_irqrestore (hrtick_set)
    sshd-4261  0d..2   16us : do_IRQ (common_interrupt)
    sshd-4261  0d..2   17us : irq_enter (do_IRQ)
    sshd-4261  0d..2   17us : idle_cpu (irq_enter)
    sshd-4261  0d..2   18us : add_preempt_count (irq_enter)
    sshd-4261  0d.h2   18us : idle_cpu (irq_enter)
    sshd-4261  0d.h.   18us : handle_fasteoi_irq (do_IRQ)
    sshd-4261  0d.h.   19us : _spin_lock (handle_fasteoi_irq)
    sshd-4261  0d.h.   19us : add_preempt_count (_spin_lock)
    sshd-4261  0d.h1   20us : _spin_unlock (handle_fasteoi_irq)
    sshd-4261  0d.h1   20us : sub_preempt_count (_spin_unlock)
[...]
    sshd-4261  0d.h1   28us : _spin_unlock (handle_fasteoi_irq)
    sshd-4261  0d.h1   29us : sub_preempt_count (_spin_unlock)
    sshd-4261  0d.h2   29us : irq_exit (do_IRQ)
    sshd-4261  0d.h2   29us : sub_preempt_count (irq_exit)
    sshd-4261  0d..3   30us : do_softirq (irq_exit)
    sshd-4261  0d...   30us : __do_softirq (do_softirq)
    sshd-4261  0d...   31us : __local_bh_disable (__do_softirq)
    sshd-4261  0d...   31us+: add_preempt_count (__local_bh_disable)
    sshd-4261  0d.s4   34us : add_preempt_count (__local_bh_disable)
[...]
    sshd-4261  0d.s3   43us : sub_preempt_count (local_bh_enable_ip)
    sshd-4261  0d.s4   44us : sub_preempt_count (local_bh_enable_ip)
    sshd-4261  0d.s3   44us : smp_apic_timer_interrupt (apic_timer_interrupt)
    sshd-4261  0d.s3   45us : irq_enter (smp_apic_timer_interrupt)
    sshd-4261  0d.s3   45us : idle_cpu (irq_enter)
    sshd-4261  0d.s3   46us : add_preempt_count (irq_enter)
    sshd-4261  0d.H3   46us : idle_cpu (irq_enter)
    sshd-4261  0d.H3   47us : hrtimer_interrupt (smp_apic_timer_interrupt)
    sshd-4261  0d.H3   47us : ktime_get (hrtimer_interrupt)
[...]
    sshd-4261  0d.H3   81us : tick_program_event (hrtimer_interrupt)
    sshd-4261  0d.H3   82us : ktime_get (tick_program_event)
    sshd-4261  0d.H3   82us : ktime_get_ts (ktime_get)
    sshd-4261  0d.H3   83us : getnstimeofday (ktime_get_ts)
    sshd-4261  0d.H3   83us : set_normalized_timespec (ktime_get_ts)
    sshd-4261  0d.H3   84us : clockevents_program_event (tick_program_event)
    sshd-4261  0d.H3   84us : lapic_next_event (clockevents_program_event)
    sshd-4261  0d.H3   85us : irq_exit (smp_apic_timer_interrupt)
    sshd-4261  0d.H3   85us : sub_preempt_count (irq_exit)
    sshd-4261  0d.s4   86us : sub_preempt_count (irq_exit)
    sshd-4261  0d.s3   86us : add_preempt_count (__local_bh_disable)
[...]
    sshd-4261  0d.s1   98us : sub_preempt_count (net_rx_action)
    sshd-4261  0d.s.   99us : add_preempt_count (_spin_lock_irq)
    sshd-4261  0d.s1   99us+: _spin_unlock_irq (run_timer_softirq)
    sshd-4261  0d.s.  104us : _local_bh_enable (__do_softirq)
    sshd-4261  0d.s.  104us : sub_preempt_count (_local_bh_enable)
    sshd-4261  0d.s.  105us : _local_bh_enable (__do_softirq)
    sshd-4261  0d.s1  105us : trace_preempt_on (__do_softirq)


This is a very interesting trace. It started with the preemption of
the ls task. We see that the task had the "need_resched" bit set
via the 'N' in the trace.  Interrupts were disabled before the spin_lock
at the beginning of the trace. We see that a schedule took place to run
sshd.  When the interrupts were enabled, we took an interrupt.
On return from the interrupt handler, the softirq ran. We took another
interrupt while running the softirq as we see from the capital 'H'.


wakeup
------

In a Real-Time environment it is very important to know the wakeup
time it takes for the highest priority task that is woken up to the
time that it executes. This is also known as "schedule latency".
I stress the point that this is about RT tasks. It is also important
to know the scheduling latency of non-RT tasks, but the average
schedule latency is better for non-RT tasks. Tools like
LatencyTop are more appropriate for such measurements.

Real-Time environments are interested in the worst case latency.
That is the longest latency it takes for something to happen, and
not the average. We can have a very fast scheduler that may only
have a large latency once in a while, but that would not work well
with Real-Time tasks.  The wakeup tracer was designed to record
the worst case wakeups of RT tasks. Non-RT tasks are not recorded
because the tracer only records one worst case and tracing non-RT
tasks that are unpredictable will overwrite the worst case latency
of RT tasks.

Since this tracer only deals with RT tasks, we will run this slightly
differently than we did with the previous tracers. Instead of performing
an 'ls', we will run 'sleep 1' under 'chrt' which changes the
priority of the task.

 # echo wakeup > /debug/tracing/current_tracer
 # echo 0 > /debug/tracing/tracing_max_latency
 # echo 1 > /debug/tracing/tracing_enabled
 # chrt -f 5 sleep 1
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/latency_trace
# tracer: wakeup
#
wakeup latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: sleep-4901 (uid:0 nice:0 policy:1 rt_prio:5)
    -----------------

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
  <idle>-0     1d.h4    0us+: try_to_wake_up (wake_up_process)
  <idle>-0     1d..4    4us : schedule (cpu_idle)



Running this on an idle system, we see that it only took 4 microseconds
to perform the task switch.  Note, since the trace marker in the
schedule is before the actual "switch", we stop the tracing when
the recorded task is about to schedule in. This may change if
we add a new marker at the end of the scheduler.

Notice that the recorded task is 'sleep' with the PID of 4901 and it
has an rt_prio of 5. This priority is user-space priority and not
the internal kernel priority. The policy is 1 for SCHED_FIFO and 2
for SCHED_RR.

Doing the same with chrt -r 5 and ftrace_enabled set.

# tracer: wakeup
#
wakeup latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5)
    -----------------

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
ksoftirq-7     1d.H3    0us : try_to_wake_up (wake_up_process)
ksoftirq-7     1d.H4    1us : sub_preempt_count (marker_probe_cb)
ksoftirq-7     1d.H3    2us : check_preempt_wakeup (try_to_wake_up)
ksoftirq-7     1d.H3    3us : update_curr (check_preempt_wakeup)
ksoftirq-7     1d.H3    4us : calc_delta_mine (update_curr)
ksoftirq-7     1d.H3    5us : __resched_task (check_preempt_wakeup)
ksoftirq-7     1d.H3    6us : task_wake_up_rt (try_to_wake_up)
ksoftirq-7     1d.H3    7us : _spin_unlock_irqrestore (try_to_wake_up)
[...]
ksoftirq-7     1d.H2   17us : irq_exit (smp_apic_timer_interrupt)
ksoftirq-7     1d.H2   18us : sub_preempt_count (irq_exit)
ksoftirq-7     1d.s3   19us : sub_preempt_count (irq_exit)
ksoftirq-7     1..s2   20us : rcu_process_callbacks (__do_softirq)
[...]
ksoftirq-7     1..s2   26us : __rcu_process_callbacks (rcu_process_callbacks)
ksoftirq-7     1d.s2   27us : _local_bh_enable (__do_softirq)
ksoftirq-7     1d.s2   28us : sub_preempt_count (_local_bh_enable)
ksoftirq-7     1.N.3   29us : sub_preempt_count (ksoftirqd)
ksoftirq-7     1.N.2   30us : _cond_resched (ksoftirqd)
ksoftirq-7     1.N.2   31us : __cond_resched (_cond_resched)
ksoftirq-7     1.N.2   32us : add_preempt_count (__cond_resched)
ksoftirq-7     1.N.2   33us : schedule (__cond_resched)
ksoftirq-7     1.N.2   33us : add_preempt_count (schedule)
ksoftirq-7     1.N.3   34us : hrtick_clear (schedule)
ksoftirq-7     1dN.3   35us : _spin_lock (schedule)
ksoftirq-7     1dN.3   36us : add_preempt_count (_spin_lock)
ksoftirq-7     1d..4   37us : put_prev_task_fair (schedule)
ksoftirq-7     1d..4   38us : update_curr (put_prev_task_fair)
[...]
ksoftirq-7     1d..5   47us : _spin_trylock (tracing_record_cmdline)
ksoftirq-7     1d..5   48us : add_preempt_count (_spin_trylock)
ksoftirq-7     1d..6   49us : _spin_unlock (tracing_record_cmdline)
ksoftirq-7     1d..6   49us : sub_preempt_count (_spin_unlock)
ksoftirq-7     1d..4   50us : schedule (__cond_resched)

The interrupt went off while running ksoftirqd. This task runs at
SCHED_OTHER. Why did not we see the 'N' set early? This may be
a harmless bug with x86_32 and 4K stacks. On x86_32 with 4K stacks
configured, the interrupt and softirq run with their own stack.
Some information is held on the top of the task's stack (need_resched
and preempt_count are both stored there). The setting of the NEED_RESCHED
bit is done directly to the task's stack, but the reading of the
NEED_RESCHED is done by looking at the current stack, which in this case
is the stack for the hard interrupt. This hides the fact that NEED_RESCHED
has been set. We do not see the 'N' until we switch back to the task's
assigned stack.

function
--------

This tracer is the function tracer. Enabling the function tracer
can be done from the debug file system. Make sure the ftrace_enabled is
set; otherwise this tracer is a nop.

 # sysctl kernel.ftrace_enabled=1
 # echo function > /debug/tracing/current_tracer
 # echo 1 > /debug/tracing/tracing_enabled
 # usleep 1
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/trace
# tracer: function
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |
            bash-4003  [00]   123.638713: finish_task_switch <-schedule
            bash-4003  [00]   123.638714: _spin_unlock_irq <-finish_task_switch
            bash-4003  [00]   123.638714: sub_preempt_count <-_spin_unlock_irq
            bash-4003  [00]   123.638715: hrtick_set <-schedule
            bash-4003  [00]   123.638715: _spin_lock_irqsave <-hrtick_set
            bash-4003  [00]   123.638716: add_preempt_count <-_spin_lock_irqsave
            bash-4003  [00]   123.638716: _spin_unlock_irqrestore <-hrtick_set
            bash-4003  [00]   123.638717: sub_preempt_count <-_spin_unlock_irqrestore
            bash-4003  [00]   123.638717: hrtick_clear <-hrtick_set
            bash-4003  [00]   123.638718: sub_preempt_count <-schedule
            bash-4003  [00]   123.638718: sub_preempt_count <-preempt_schedule
            bash-4003  [00]   123.638719: wait_for_completion <-__stop_machine_run
            bash-4003  [00]   123.638719: wait_for_common <-wait_for_completion
            bash-4003  [00]   123.638720: _spin_lock_irq <-wait_for_common
            bash-4003  [00]   123.638720: add_preempt_count <-_spin_lock_irq
[...]


Note: function tracer uses ring buffers to store the above entries.
The newest data may overwrite the oldest data. Sometimes using echo to
stop the trace is not sufficient because the tracing could have overwritten
the data that you wanted to record. For this reason, it is sometimes better to
disable tracing directly from a program. This allows you to stop the
tracing at the point that you hit the part that you are interested in.
To disable the tracing directly from a C program, something like following
code snippet can be used:

int trace_fd;
[...]
int main(int argc, char *argv[]) {
	[...]
	trace_fd = open("/debug/tracing/tracing_enabled", O_WRONLY);
	[...]
	if (condition_hit()) {
		write(trace_fd, "0", 1);
	}
	[...]
}

Note: Here we hard coded the path name. The debugfs mount is not
guaranteed to be at /debug (and is more commonly at /sys/kernel/debug).
For simple one time traces, the above is sufficent. For anything else,
a search through /proc/mounts may be needed to find where the debugfs
file-system is mounted.

dynamic ftrace
--------------

If CONFIG_DYNAMIC_FTRACE is set, the system will run with
virtually no overhead when function tracing is disabled. The way
this works is the mcount function call (placed at the start of
every kernel function, produced by the -pg switch in gcc), starts
of pointing to a simple return. (Enabling FTRACE will include the
-pg switch in the compiling of the kernel.)

At compile time every C file object is run through the
recordmcount.pl script (located in the scripts directory). This
script will process the C object using objdump to find all the
locations in the .text section that call mcount. (Note, only
the .text section is processed, since processing other sections
like .init.text may cause races due to those sections being freed).

A new section called "__mcount_loc" is created that holds references
to all the mcount call sites in the .text section. This section is
compiled back into the original object. The final linker will add
all these references into a single table.

On boot up, before SMP is initialized, the dynamic ftrace code
scans this table and updates all the locations into nops. It also
records the locations, which are added to the available_filter_functions
list.  Modules are processed as they are loaded and before they are
executed.  When a module is unloaded, it also removes its functions from
the ftrace function list. This is automatic in the module unload
code, and the module author does not need to worry about it.

When tracing is enabled, kstop_machine is called to prevent races
with the CPUS executing code being modified (which can cause the
CPU to do undesireable things), and the nops are patched back
to calls. But this time, they do not call mcount (which is just
a function stub). They now call into the ftrace infrastructure.

One special side-effect to the recording of the functions being
traced is that we can now selectively choose which functions we
wish to trace and which ones we want the mcount calls to remain as
nops.

Two files are used, one for enabling and one for disabling the tracing
of specified functions. They are:

  set_ftrace_filter

and

  set_ftrace_notrace

A list of available functions that you can add to these files is listed
in:

   available_filter_functions

 # cat /debug/tracing/available_filter_functions
put_prev_task_idle
kmem_cache_create
pick_next_task_rt
get_online_cpus
pick_next_task_fair
mutex_lock
[...]

If I am only interested in sys_nanosleep and hrtimer_interrupt:

 # echo sys_nanosleep hrtimer_interrupt \
		> /debug/tracing/set_ftrace_filter
 # echo ftrace > /debug/tracing/current_tracer
 # echo 1 > /debug/tracing/tracing_enabled
 # usleep 1
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/trace
# tracer: ftrace
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |
          usleep-4134  [00]  1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt
          usleep-4134  [00]  1317.070111: sys_nanosleep <-syscall_call
          <idle>-0     [00]  1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt

To see which functions are being traced, you can cat the file:

 # cat /debug/tracing/set_ftrace_filter
hrtimer_interrupt
sys_nanosleep


Perhaps this is not enough. The filters also allow simple wild cards.
Only the following are currently available

  <match>*  - will match functions that begin with <match>
  *<match>  - will match functions that end with <match>
  *<match>* - will match functions that have <match> in it

These are the only wild cards which are supported.

  <match>*<match> will not work.

 # echo hrtimer_* > /debug/tracing/set_ftrace_filter

Produces:

# tracer: ftrace
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |
            bash-4003  [00]  1480.611794: hrtimer_init <-copy_process
            bash-4003  [00]  1480.611941: hrtimer_start <-hrtick_set
            bash-4003  [00]  1480.611956: hrtimer_cancel <-hrtick_clear
            bash-4003  [00]  1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel
          <idle>-0     [00]  1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt
          <idle>-0     [00]  1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt
          <idle>-0     [00]  1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt
          <idle>-0     [00]  1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt
          <idle>-0     [00]  1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt


Notice that we lost the sys_nanosleep.

 # cat /debug/tracing/set_ftrace_filter
hrtimer_run_queues
hrtimer_run_pending
hrtimer_init
hrtimer_cancel
hrtimer_try_to_cancel
hrtimer_forward
hrtimer_start
hrtimer_reprogram
hrtimer_force_reprogram
hrtimer_get_next_event
hrtimer_interrupt
hrtimer_nanosleep
hrtimer_wakeup
hrtimer_get_remaining
hrtimer_get_res
hrtimer_init_sleeper


This is because the '>' and '>>' act just like they do in bash.
To rewrite the filters, use '>'
To append to the filters, use '>>'

To clear out a filter so that all functions will be recorded again:

 # echo > /debug/tracing/set_ftrace_filter
 # cat /debug/tracing/set_ftrace_filter
 #

Again, now we want to append.

 # echo sys_nanosleep > /debug/tracing/set_ftrace_filter
 # cat /debug/tracing/set_ftrace_filter
sys_nanosleep
 # echo hrtimer_* >> /debug/tracing/set_ftrace_filter
 # cat /debug/tracing/set_ftrace_filter
hrtimer_run_queues
hrtimer_run_pending
hrtimer_init
hrtimer_cancel
hrtimer_try_to_cancel
hrtimer_forward
hrtimer_start
hrtimer_reprogram
hrtimer_force_reprogram
hrtimer_get_next_event
hrtimer_interrupt
sys_nanosleep
hrtimer_nanosleep
hrtimer_wakeup
hrtimer_get_remaining
hrtimer_get_res
hrtimer_init_sleeper


The set_ftrace_notrace prevents those functions from being traced.

 # echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace

Produces:

# tracer: ftrace
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |
            bash-4043  [01]   115.281644: finish_task_switch <-schedule
            bash-4043  [01]   115.281645: hrtick_set <-schedule
            bash-4043  [01]   115.281645: hrtick_clear <-hrtick_set
            bash-4043  [01]   115.281646: wait_for_completion <-__stop_machine_run
            bash-4043  [01]   115.281647: wait_for_common <-wait_for_completion
            bash-4043  [01]   115.281647: kthread_stop <-stop_machine_run
            bash-4043  [01]   115.281648: init_waitqueue_head <-kthread_stop
            bash-4043  [01]   115.281648: wake_up_process <-kthread_stop
            bash-4043  [01]   115.281649: try_to_wake_up <-wake_up_process

We can see that there's no more lock or preempt tracing.

trace_pipe
----------

The trace_pipe outputs the same content as the trace file, but the effect
on the tracing is different. Every read from trace_pipe is consumed.
This means that subsequent reads will be different. The trace
is live.

 # echo function > /debug/tracing/current_tracer
 # cat /debug/tracing/trace_pipe > /tmp/trace.out &
[1] 4153
 # echo 1 > /debug/tracing/tracing_enabled
 # usleep 1
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/trace
# tracer: function
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |

 #
 # cat /tmp/trace.out
            bash-4043  [00] 41.267106: finish_task_switch <-schedule
            bash-4043  [00] 41.267106: hrtick_set <-schedule
            bash-4043  [00] 41.267107: hrtick_clear <-hrtick_set
            bash-4043  [00] 41.267108: wait_for_completion <-__stop_machine_run
            bash-4043  [00] 41.267108: wait_for_common <-wait_for_completion
            bash-4043  [00] 41.267109: kthread_stop <-stop_machine_run
            bash-4043  [00] 41.267109: init_waitqueue_head <-kthread_stop
            bash-4043  [00] 41.267110: wake_up_process <-kthread_stop
            bash-4043  [00] 41.267110: try_to_wake_up <-wake_up_process
            bash-4043  [00] 41.267111: select_task_rq_rt <-try_to_wake_up


Note, reading the trace_pipe file will block until more input is added.
By changing the tracer, trace_pipe will issue an EOF. We needed
to set the function tracer _before_ we "cat" the trace_pipe file.


trace entries
-------------

Having too much or not enough data can be troublesome in diagnosing
an issue in the kernel. The file buffer_size_kb is used to modify
the size of the internal trace buffers. The number listed
is the number of entries that can be recorded per CPU. To know
the full size, multiply the number of possible CPUS with the
number of entries.

 # cat /debug/tracing/buffer_size_kb
1408 (units kilobytes)

Note, to modify this, you must have tracing completely disabled. To do that,
echo "nop" into the current_tracer. If the current_tracer is not set
to "nop", an EINVAL error will be returned.

 # echo nop > /debug/tracing/current_tracer
 # echo 10000 > /debug/tracing/buffer_size_kb
 # cat /debug/tracing/buffer_size_kb
10000 (units kilobytes)

The number of pages which will be allocated is limited to a percentage
of available memory. Allocating too much will produce an error.

 # echo 1000000000000 > /debug/tracing/buffer_size_kb
-bash: echo: write error: Cannot allocate memory
 # cat /debug/tracing/buffer_size_kb
85