summaryrefslogtreecommitdiffstats
path: root/drivers/cpuidle/governors/teo.c
blob: d9262db79cae53c0b1d277513d014d3e91a0f879 (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
// SPDX-License-Identifier: GPL-2.0
/*
 * Timer events oriented CPU idle governor
 *
 * Copyright (C) 2018 - 2021 Intel Corporation
 * Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
 */

/**
 * DOC: teo-description
 *
 * The idea of this governor is based on the observation that on many systems
 * timer events are two or more orders of magnitude more frequent than any
 * other interrupts, so they are likely to be the most significant cause of CPU
 * wakeups from idle states.  Moreover, information about what happened in the
 * (relatively recent) past can be used to estimate whether or not the deepest
 * idle state with target residency within the (known) time till the closest
 * timer event, referred to as the sleep length, is likely to be suitable for
 * the upcoming CPU idle period and, if not, then which of the shallower idle
 * states to choose instead of it.
 *
 * Of course, non-timer wakeup sources are more important in some use cases
 * which can be covered by taking a few most recent idle time intervals of the
 * CPU into account.  However, even in that context it is not necessary to
 * consider idle duration values greater than the sleep length, because the
 * closest timer will ultimately wake up the CPU anyway unless it is woken up
 * earlier.
 *
 * Thus this governor estimates whether or not the prospective idle duration of
 * a CPU is likely to be significantly shorter than the sleep length and selects
 * an idle state for it accordingly.
 *
 * The computations carried out by this governor are based on using bins whose
 * boundaries are aligned with the target residency parameter values of the CPU
 * idle states provided by the %CPUIdle driver in the ascending order.  That is,
 * the first bin spans from 0 up to, but not including, the target residency of
 * the second idle state (idle state 1), the second bin spans from the target
 * residency of idle state 1 up to, but not including, the target residency of
 * idle state 2, the third bin spans from the target residency of idle state 2
 * up to, but not including, the target residency of idle state 3 and so on.
 * The last bin spans from the target residency of the deepest idle state
 * supplied by the driver to infinity.
 *
 * Two metrics called "hits" and "intercepts" are associated with each bin.
 * They are updated every time before selecting an idle state for the given CPU
 * in accordance with what happened last time.
 *
 * The "hits" metric reflects the relative frequency of situations in which the
 * sleep length and the idle duration measured after CPU wakeup fall into the
 * same bin (that is, the CPU appears to wake up "on time" relative to the sleep
 * length).  In turn, the "intercepts" metric reflects the relative frequency of
 * situations in which the measured idle duration is so much shorter than the
 * sleep length that the bin it falls into corresponds to an idle state
 * shallower than the one whose bin is fallen into by the sleep length (these
 * situations are referred to as "intercepts" below).
 *
 * In addition to the metrics described above, the governor counts recent
 * intercepts (that is, intercepts that have occurred during the last
 * %NR_RECENT invocations of it for the given CPU) for each bin.
 *
 * In order to select an idle state for a CPU, the governor takes the following
 * steps (modulo the possible latency constraint that must be taken into account
 * too):
 *
 * 1. Find the deepest CPU idle state whose target residency does not exceed
 *    the current sleep length (the candidate idle state) and compute 3 sums as
 *    follows:
 *
 *    - The sum of the "hits" and "intercepts" metrics for the candidate state
 *      and all of the deeper idle states (it represents the cases in which the
 *      CPU was idle long enough to avoid being intercepted if the sleep length
 *      had been equal to the current one).
 *
 *    - The sum of the "intercepts" metrics for all of the idle states shallower
 *      than the candidate one (it represents the cases in which the CPU was not
 *      idle long enough to avoid being intercepted if the sleep length had been
 *      equal to the current one).
 *
 *    - The sum of the numbers of recent intercepts for all of the idle states
 *      shallower than the candidate one.
 *
 * 2. If the second sum is greater than the first one or the third sum is
 *    greater than %NR_RECENT / 2, the CPU is likely to wake up early, so look
 *    for an alternative idle state to select.
 *
 *    - Traverse the idle states shallower than the candidate one in the
 *      descending order.
 *
 *    - For each of them compute the sum of the "intercepts" metrics and the sum
 *      of the numbers of recent intercepts over all of the idle states between
 *      it and the candidate one (including the former and excluding the
 *      latter).
 *
 *    - If each of these sums that needs to be taken into account (because the
 *      check related to it has indicated that the CPU is likely to wake up
 *      early) is greater than a half of the corresponding sum computed in step
 *      1 (which means that the target residency of the state in question had
 *      not exceeded the idle duration in over a half of the relevant cases),
 *      select the given idle state instead of the candidate one.
 *
 * 3. By default, select the candidate state.
 */

#include <linux/cpuidle.h>
#include <linux/jiffies.h>
#include <linux/kernel.h>
#include <linux/sched/clock.h>
#include <linux/tick.h>

/*
 * The PULSE value is added to metrics when they grow and the DECAY_SHIFT value
 * is used for decreasing metrics on a regular basis.
 */
#define PULSE		1024
#define DECAY_SHIFT	3

/*
 * Number of the most recent idle duration values to take into consideration for
 * the detection of recent early wakeup patterns.
 */
#define NR_RECENT	9

/**
 * struct teo_bin - Metrics used by the TEO cpuidle governor.
 * @intercepts: The "intercepts" metric.
 * @hits: The "hits" metric.
 * @recent: The number of recent "intercepts".
 */
struct teo_bin {
	unsigned int intercepts;
	unsigned int hits;
	unsigned int recent;
};

/**
 * struct teo_cpu - CPU data used by the TEO cpuidle governor.
 * @time_span_ns: Time between idle state selection and post-wakeup update.
 * @sleep_length_ns: Time till the closest timer event (at the selection time).
 * @state_bins: Idle state data bins for this CPU.
 * @total: Grand total of the "intercepts" and "hits" mertics for all bins.
 * @next_recent_idx: Index of the next @recent_idx entry to update.
 * @recent_idx: Indices of bins corresponding to recent "intercepts".
 */
struct teo_cpu {
	s64 time_span_ns;
	s64 sleep_length_ns;
	struct teo_bin state_bins[CPUIDLE_STATE_MAX];
	unsigned int total;
	int next_recent_idx;
	int recent_idx[NR_RECENT];
};

static DEFINE_PER_CPU(struct teo_cpu, teo_cpus);

/**
 * teo_update - Update CPU metrics after wakeup.
 * @drv: cpuidle driver containing state data.
 * @dev: Target CPU.
 */
static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
{
	struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
	int i, idx_timer = 0, idx_duration = 0;
	u64 measured_ns;

	if (cpu_data->time_span_ns >= cpu_data->sleep_length_ns) {
		/*
		 * One of the safety nets has triggered or the wakeup was close
		 * enough to the closest timer event expected at the idle state
		 * selection time to be discarded.
		 */
		measured_ns = U64_MAX;
	} else {
		u64 lat_ns = drv->states[dev->last_state_idx].exit_latency_ns;

		/*
		 * The computations below are to determine whether or not the
		 * (saved) time till the next timer event and the measured idle
		 * duration fall into the same "bin", so use last_residency_ns
		 * for that instead of time_span_ns which includes the cpuidle
		 * overhead.
		 */
		measured_ns = dev->last_residency_ns;
		/*
		 * The delay between the wakeup and the first instruction
		 * executed by the CPU is not likely to be worst-case every
		 * time, so take 1/2 of the exit latency as a very rough
		 * approximation of the average of it.
		 */
		if (measured_ns >= lat_ns)
			measured_ns -= lat_ns / 2;
		else
			measured_ns /= 2;
	}

	cpu_data->total = 0;

	/*
	 * Decay the "hits" and "intercepts" metrics for all of the bins and
	 * find the bins that the sleep length and the measured idle duration
	 * fall into.
	 */
	for (i = 0; i < drv->state_count; i++) {
		s64 target_residency_ns = drv->states[i].target_residency_ns;
		struct teo_bin *bin = &cpu_data->state_bins[i];

		bin->hits -= bin->hits >> DECAY_SHIFT;
		bin->intercepts -= bin->intercepts >> DECAY_SHIFT;

		cpu_data->total += bin->hits + bin->intercepts;

		if (target_residency_ns <= cpu_data->sleep_length_ns) {
			idx_timer = i;
			if (target_residency_ns <= measured_ns)
				idx_duration = i;
		}
	}

	i = cpu_data->next_recent_idx++;
	if (cpu_data->next_recent_idx >= NR_RECENT)
		cpu_data->next_recent_idx = 0;

	if (cpu_data->recent_idx[i] >= 0)
		cpu_data->state_bins[cpu_data->recent_idx[i]].recent--;

	/*
	 * If the measured idle duration falls into the same bin as the sleep
	 * length, this is a "hit", so update the "hits" metric for that bin.
	 * Otherwise, update the "intercepts" metric for the bin fallen into by
	 * the measured idle duration.
	 */
	if (idx_timer == idx_duration) {
		cpu_data->state_bins[idx_timer].hits += PULSE;
		cpu_data->recent_idx[i] = -1;
	} else {
		cpu_data->state_bins[idx_duration].intercepts += PULSE;
		cpu_data->state_bins[idx_duration].recent++;
		cpu_data->recent_idx[i] = idx_duration;
	}

	cpu_data->total += PULSE;
}

static bool teo_time_ok(u64 interval_ns)
{
	return !tick_nohz_tick_stopped() || interval_ns >= TICK_NSEC;
}

static s64 teo_middle_of_bin(int idx, struct cpuidle_driver *drv)
{
	return (drv->states[idx].target_residency_ns +
		drv->states[idx+1].target_residency_ns) / 2;
}

/**
 * teo_find_shallower_state - Find shallower idle state matching given duration.
 * @drv: cpuidle driver containing state data.
 * @dev: Target CPU.
 * @state_idx: Index of the capping idle state.
 * @duration_ns: Idle duration value to match.
 */
static int teo_find_shallower_state(struct cpuidle_driver *drv,
				    struct cpuidle_device *dev, int state_idx,
				    s64 duration_ns)
{
	int i;

	for (i = state_idx - 1; i >= 0; i--) {
		if (dev->states_usage[i].disable)
			continue;

		state_idx = i;
		if (drv->states[i].target_residency_ns <= duration_ns)
			break;
	}
	return state_idx;
}

/**
 * teo_select - Selects the next idle state to enter.
 * @drv: cpuidle driver containing state data.
 * @dev: Target CPU.
 * @stop_tick: Indication on whether or not to stop the scheduler tick.
 */
static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
		      bool *stop_tick)
{
	struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
	s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
	unsigned int idx_intercept_sum = 0;
	unsigned int intercept_sum = 0;
	unsigned int idx_recent_sum = 0;
	unsigned int recent_sum = 0;
	unsigned int idx_hit_sum = 0;
	unsigned int hit_sum = 0;
	int constraint_idx = 0;
	int idx0 = 0, idx = -1;
	bool alt_intercepts, alt_recent;
	ktime_t delta_tick;
	s64 duration_ns;
	int i;

	if (dev->last_state_idx >= 0) {
		teo_update(drv, dev);
		dev->last_state_idx = -1;
	}

	cpu_data->time_span_ns = local_clock();

	duration_ns = tick_nohz_get_sleep_length(&delta_tick);
	cpu_data->sleep_length_ns = duration_ns;

	/* Check if there is any choice in the first place. */
	if (drv->state_count < 2) {
		idx = 0;
		goto end;
	}
	if (!dev->states_usage[0].disable) {
		idx = 0;
		if (drv->states[1].target_residency_ns > duration_ns)
			goto end;
	}

	/*
	 * Find the deepest idle state whose target residency does not exceed
	 * the current sleep length and the deepest idle state not deeper than
	 * the former whose exit latency does not exceed the current latency
	 * constraint.  Compute the sums of metrics for early wakeup pattern
	 * detection.
	 */
	for (i = 1; i < drv->state_count; i++) {
		struct teo_bin *prev_bin = &cpu_data->state_bins[i-1];
		struct cpuidle_state *s = &drv->states[i];

		/*
		 * Update the sums of idle state mertics for all of the states
		 * shallower than the current one.
		 */
		intercept_sum += prev_bin->intercepts;
		hit_sum += prev_bin->hits;
		recent_sum += prev_bin->recent;

		if (dev->states_usage[i].disable)
			continue;

		if (idx < 0) {
			idx = i; /* first enabled state */
			idx0 = i;
		}

		if (s->target_residency_ns > duration_ns)
			break;

		idx = i;

		if (s->exit_latency_ns <= latency_req)
			constraint_idx = i;

		idx_intercept_sum = intercept_sum;
		idx_hit_sum = hit_sum;
		idx_recent_sum = recent_sum;
	}

	/* Avoid unnecessary overhead. */
	if (idx < 0) {
		idx = 0; /* No states enabled, must use 0. */
		goto end;
	} else if (idx == idx0) {
		goto end;
	}

	/*
	 * If the sum of the intercepts metric for all of the idle states
	 * shallower than the current candidate one (idx) is greater than the
	 * sum of the intercepts and hits metrics for the candidate state and
	 * all of the deeper states, or the sum of the numbers of recent
	 * intercepts over all of the states shallower than the candidate one
	 * is greater than a half of the number of recent events taken into
	 * account, the CPU is likely to wake up early, so find an alternative
	 * idle state to select.
	 */
	alt_intercepts = 2 * idx_intercept_sum > cpu_data->total - idx_hit_sum;
	alt_recent = idx_recent_sum > NR_RECENT / 2;
	if (alt_recent || alt_intercepts) {
		s64 first_suitable_span_ns = duration_ns;
		int first_suitable_idx = idx;

		/*
		 * Look for the deepest idle state whose target residency had
		 * not exceeded the idle duration in over a half of the relevant
		 * cases (both with respect to intercepts overall and with
		 * respect to the recent intercepts only) in the past.
		 *
		 * Take the possible latency constraint and duration limitation
		 * present if the tick has been stopped already into account.
		 */
		intercept_sum = 0;
		recent_sum = 0;

		for (i = idx - 1; i >= 0; i--) {
			struct teo_bin *bin = &cpu_data->state_bins[i];
			s64 span_ns;

			intercept_sum += bin->intercepts;
			recent_sum += bin->recent;

			span_ns = teo_middle_of_bin(i, drv);

			if ((!alt_recent || 2 * recent_sum > idx_recent_sum) &&
			    (!alt_intercepts ||
			     2 * intercept_sum > idx_intercept_sum)) {
				if (teo_time_ok(span_ns) &&
				    !dev->states_usage[i].disable) {
					idx = i;
					duration_ns = span_ns;
				} else {
					/*
					 * The current state is too shallow or
					 * disabled, so take the first enabled
					 * deeper state with suitable time span.
					 */
					idx = first_suitable_idx;
					duration_ns = first_suitable_span_ns;
				}
				break;
			}

			if (dev->states_usage[i].disable)
				continue;

			if (!teo_time_ok(span_ns)) {
				/*
				 * The current state is too shallow, but if an
				 * alternative candidate state has been found,
				 * it may still turn out to be a better choice.
				 */
				if (first_suitable_idx != idx)
					continue;

				break;
			}

			first_suitable_span_ns = span_ns;
			first_suitable_idx = i;
		}
	}

	/*
	 * If there is a latency constraint, it may be necessary to select an
	 * idle state shallower than the current candidate one.
	 */
	if (idx > constraint_idx)
		idx = constraint_idx;

end:
	/*
	 * Don't stop the tick if the selected state is a polling one or if the
	 * expected idle duration is shorter than the tick period length.
	 */
	if (((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) ||
	    duration_ns < TICK_NSEC) && !tick_nohz_tick_stopped()) {
		*stop_tick = false;

		/*
		 * The tick is not going to be stopped, so if the target
		 * residency of the state to be returned is not within the time
		 * till the closest timer including the tick, try to correct
		 * that.
		 */
		if (idx > idx0 &&
		    drv->states[idx].target_residency_ns > delta_tick)
			idx = teo_find_shallower_state(drv, dev, idx, delta_tick);
	}

	return idx;
}

/**
 * teo_reflect - Note that governor data for the CPU need to be updated.
 * @dev: Target CPU.
 * @state: Entered state.
 */
static void teo_reflect(struct cpuidle_device *dev, int state)
{
	struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);

	dev->last_state_idx = state;
	/*
	 * If the wakeup was not "natural", but triggered by one of the safety
	 * nets, assume that the CPU might have been idle for the entire sleep
	 * length time.
	 */
	if (dev->poll_time_limit ||
	    (tick_nohz_idle_got_tick() && cpu_data->sleep_length_ns > TICK_NSEC)) {
		dev->poll_time_limit = false;
		cpu_data->time_span_ns = cpu_data->sleep_length_ns;
	} else {
		cpu_data->time_span_ns = local_clock() - cpu_data->time_span_ns;
	}
}

/**
 * teo_enable_device - Initialize the governor's data for the target CPU.
 * @drv: cpuidle driver (not used).
 * @dev: Target CPU.
 */
static int teo_enable_device(struct cpuidle_driver *drv,
			     struct cpuidle_device *dev)
{
	struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
	int i;

	memset(cpu_data, 0, sizeof(*cpu_data));

	for (i = 0; i < NR_RECENT; i++)
		cpu_data->recent_idx[i] = -1;

	return 0;
}

static struct cpuidle_governor teo_governor = {
	.name =		"teo",
	.rating =	19,
	.enable =	teo_enable_device,
	.select =	teo_select,
	.reflect =	teo_reflect,
};

static int __init teo_governor_init(void)
{
	return cpuidle_register_governor(&teo_governor);
}

postcore_initcall(teo_governor_init);