menu.c

  1// SPDX-License-Identifier: GPL-2.0-only
  2/*
  3 * menu.c - the menu idle governor
  4 *
  5 * Copyright (C) 2006-2007 Adam Belay <abelay@novell.com>
  6 * Copyright (C) 2009 Intel Corporation
  7 * Author:
  8 *        Arjan van de Ven <arjan@linux.intel.com>
  9 */
 10
 11#include <linux/kernel.h>
 12#include <linux/cpuidle.h>
 13#include <linux/time.h>
 14#include <linux/ktime.h>
 15#include <linux/hrtimer.h>
 16#include <linux/tick.h>
 17#include <linux/sched/stat.h>
 18#include <linux/math64.h>
 19
 20#include "gov.h"
 21
 22#define BUCKETS 6
 23#define INTERVAL_SHIFT 3
 24#define INTERVALS (1UL << INTERVAL_SHIFT)
 25#define RESOLUTION 1024
 26#define DECAY 8
 27#define MAX_INTERESTING (50000 * NSEC_PER_USEC)
 28
 29/*
 30 * Concepts and ideas behind the menu governor
 31 *
 32 * For the menu governor, there are 2 decision factors for picking a C
 33 * state:
 34 * 1) Energy break even point
 35 * 2) Latency tolerance (from pmqos infrastructure)
 36 * These two factors are treated independently.
 37 *
 38 * Energy break even point
 39 * -----------------------
 40 * C state entry and exit have an energy cost, and a certain amount of time in
 41 * the  C state is required to actually break even on this cost. CPUIDLE
 42 * provides us this duration in the "target_residency" field. So all that we
 43 * need is a good prediction of how long we'll be idle. Like the traditional
 44 * menu governor, we take the actual known "next timer event" time.
 45 *
 46 * Since there are other source of wakeups (interrupts for example) than
 47 * the next timer event, this estimation is rather optimistic. To get a
 48 * more realistic estimate, a correction factor is applied to the estimate,
 49 * that is based on historic behavior. For example, if in the past the actual
 50 * duration always was 50% of the next timer tick, the correction factor will
 51 * be 0.5.
 52 *
 53 * menu uses a running average for this correction factor, but it uses a set of
 54 * factors, not just a single factor. This stems from the realization that the
 55 * ratio is dependent on the order of magnitude of the expected duration; if we
 56 * expect 500 milliseconds of idle time the likelihood of getting an interrupt
 57 * very early is much higher than if we expect 50 micro seconds of idle time.
 58 * For this reason, menu keeps an array of 6 independent factors, that gets
 59 * indexed based on the magnitude of the expected duration.
 60 *
 61 * Repeatable-interval-detector
 62 * ----------------------------
 63 * There are some cases where "next timer" is a completely unusable predictor:
 64 * Those cases where the interval is fixed, for example due to hardware
 65 * interrupt mitigation, but also due to fixed transfer rate devices like mice.
 66 * For this, we use a different predictor: We track the duration of the last 8
 67 * intervals and use them to estimate the duration of the next one.
 68 */
 69
 70struct menu_device {
 71	int             needs_update;
 72	int             tick_wakeup;
 73
 74	u64		next_timer_ns;
 75	unsigned int	bucket;
 76	unsigned int	correction_factor[BUCKETS];
 77	unsigned int	intervals[INTERVALS];
 78	int		interval_ptr;
 79};
 80
 81static inline int which_bucket(u64 duration_ns)
 82{
 83	int bucket = 0;
 84
 85	if (duration_ns < 10ULL * NSEC_PER_USEC)
 86		return bucket;
 87	if (duration_ns < 100ULL * NSEC_PER_USEC)
 88		return bucket + 1;
 89	if (duration_ns < 1000ULL * NSEC_PER_USEC)
 90		return bucket + 2;
 91	if (duration_ns < 10000ULL * NSEC_PER_USEC)
 92		return bucket + 3;
 93	if (duration_ns < 100000ULL * NSEC_PER_USEC)
 94		return bucket + 4;
 95	return bucket + 5;
 96}
 97
 98static DEFINE_PER_CPU(struct menu_device, menu_devices);
 99
100static void menu_update_intervals(struct menu_device *data, unsigned int interval_us)
101{
102	/* Update the repeating-pattern data. */
103	data->intervals[data->interval_ptr++] = interval_us;
104	if (data->interval_ptr >= INTERVALS)
105		data->interval_ptr = 0;
106}
107
108static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev);
109
110/*
111 * Try detecting repeating patterns by keeping track of the last 8
112 * intervals, and checking if the standard deviation of that set
113 * of points is below a threshold. If it is... then use the
114 * average of these 8 points as the estimated value.
115 */
116static unsigned int get_typical_interval(struct menu_device *data)
117{
118	s64 value, min_thresh = -1, max_thresh = UINT_MAX;
119	unsigned int max, min, divisor;
120	u64 avg, variance, avg_sq;
121	int i;
122
123again:
124	/* Compute the average and variance of past intervals. */
125	max = 0;
126	min = UINT_MAX;
127	avg = 0;
128	variance = 0;
129	divisor = 0;
130	for (i = 0; i < INTERVALS; i++) {
131		value = data->intervals[i];
132		/*
133		 * Discard the samples outside the interval between the min and
134		 * max thresholds.
135		 */
136		if (value <= min_thresh || value >= max_thresh)
137			continue;
138
139		divisor++;
140
141		avg += value;
142		variance += value * value;
143
144		if (value > max)
145			max = value;
146
147		if (value < min)
148			min = value;
149	}
150
151	if (!max)
152		return UINT_MAX;
153
154	if (divisor == INTERVALS) {
155		avg >>= INTERVAL_SHIFT;
156		variance >>= INTERVAL_SHIFT;
157	} else {
158		do_div(avg, divisor);
159		do_div(variance, divisor);
160	}
161
162	avg_sq = avg * avg;
163	variance -= avg_sq;
164
165	/*
166	 * The typical interval is obtained when standard deviation is
167	 * small (stddev <= 20 us, variance <= 400 us^2) or standard
168	 * deviation is small compared to the average interval (avg >
169	 * 6*stddev, avg^2 > 36*variance). The average is smaller than
170	 * UINT_MAX aka U32_MAX, so computing its square does not
171	 * overflow a u64. We simply reject this candidate average if
172	 * the standard deviation is greater than 715 s (which is
173	 * rather unlikely).
174	 *
175	 * Use this result only if there is no timer to wake us up sooner.
176	 */
177	if (likely(variance <= U64_MAX/36)) {
178		if ((avg_sq > variance * 36 && divisor * 4 >= INTERVALS * 3) ||
179		    variance <= 400)
180			return avg;
181	}
182
183	/*
184	 * If there are outliers, discard them by setting thresholds to exclude
185	 * data points at a large enough distance from the average, then
186	 * calculate the average and standard deviation again. Once we get
187	 * down to the last 3/4 of our samples, stop excluding samples.
188	 *
189	 * This can deal with workloads that have long pauses interspersed
190	 * with sporadic activity with a bunch of short pauses.
191	 *
192	 * However, if the number of remaining samples is too small to exclude
193	 * any more outliers, allow the deepest available idle state to be
194	 * selected because there are systems where the time spent by CPUs in
195	 * deep idle states is correlated to the maximum frequency the CPUs
196	 * can get to.  On those systems, shallow idle states should be avoided
197	 * unless there is a clear indication that the given CPU is most likley
198	 * going to be woken up shortly.
199	 */
200	if (divisor * 4 <= INTERVALS * 3)
201		return UINT_MAX;
202
203	/* Update the thresholds for the next round. */
204	if (avg - min > max - avg)
205		min_thresh = min;
206	else
207		max_thresh = max;
208
209	goto again;
210}
211
212/**
213 * menu_select - selects the next idle state to enter
214 * @drv: cpuidle driver containing state data
215 * @dev: the CPU
216 * @stop_tick: indication on whether or not to stop the tick
217 */
218static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
219		       bool *stop_tick)
220{
221	struct menu_device *data = this_cpu_ptr(&menu_devices);
222	s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
223	u64 predicted_ns;
224	ktime_t delta, delta_tick;
225	int i, idx;
226
227	if (data->needs_update) {
228		menu_update(drv, dev);
229		data->needs_update = 0;
230	} else if (!dev->last_residency_ns) {
231		/*
232		 * This happens when the driver rejects the previously selected
233		 * idle state and returns an error, so update the recent
234		 * intervals table to prevent invalid information from being
235		 * used going forward.
236		 */
237		menu_update_intervals(data, UINT_MAX);
238	}
239
240	/* Find the shortest expected idle interval. */
241	predicted_ns = get_typical_interval(data) * NSEC_PER_USEC;
242	if (predicted_ns > RESIDENCY_THRESHOLD_NS || tick_nohz_tick_stopped()) {
243		unsigned int timer_us;
244
245		/* Determine the time till the closest timer. */
246		delta = tick_nohz_get_sleep_length(&delta_tick);
247		if (unlikely(delta < 0)) {
248			delta = 0;
249			delta_tick = 0;
250		}
251
252		data->next_timer_ns = delta;
253		data->bucket = which_bucket(data->next_timer_ns);
254
255		/* Round up the result for half microseconds. */
256		timer_us = div_u64((RESOLUTION * DECAY * NSEC_PER_USEC) / 2 +
257					data->next_timer_ns *
258						data->correction_factor[data->bucket],
259				   RESOLUTION * DECAY * NSEC_PER_USEC);
260		/* Use the lowest expected idle interval to pick the idle state. */
261		predicted_ns = min((u64)timer_us * NSEC_PER_USEC, predicted_ns);
262		/*
263		 * If the tick is already stopped, the cost of possible short
264		 * idle duration misprediction is much higher, because the CPU
265		 * may be stuck in a shallow idle state for a long time as a
266		 * result of it.  In that case, say we might mispredict and use
267		 * the known time till the closest timer event for the idle
268		 * state selection.
269		 */
270		if (tick_nohz_tick_stopped() && predicted_ns < TICK_NSEC)
271			predicted_ns = data->next_timer_ns;
272	} else {
273		/*
274		 * Because the next timer event is not going to be determined
275		 * in this case, assume that without the tick the closest timer
276		 * will be in distant future and that the closest tick will occur
277		 * after 1/2 of the tick period.
278		 */
279		data->next_timer_ns = KTIME_MAX;
280		delta_tick = TICK_NSEC / 2;
281		data->bucket = BUCKETS - 1;
282	}
283
284	if (unlikely(drv->state_count <= 1 || latency_req == 0) ||
285	    ((data->next_timer_ns < drv->states[1].target_residency_ns ||
286	      latency_req < drv->states[1].exit_latency_ns) &&
287	     !dev->states_usage[0].disable)) {
288		/*
289		 * In this case state[0] will be used no matter what, so return
290		 * it right away and keep the tick running if state[0] is a
291		 * polling one.
292		 */
293		*stop_tick = !(drv->states[0].flags & CPUIDLE_FLAG_POLLING);
294		return 0;
295	}
296
297	/*
298	 * Find the idle state with the lowest power while satisfying
299	 * our constraints.
300	 */
301	idx = -1;
302	for (i = 0; i < drv->state_count; i++) {
303		struct cpuidle_state *s = &drv->states[i];
304
305		if (dev->states_usage[i].disable)
306			continue;
307
308		if (idx == -1)
309			idx = i; /* first enabled state */
310
311		if (s->exit_latency_ns > latency_req)
312			break;
313
314		if (s->target_residency_ns <= predicted_ns) {
315			idx = i;
316			continue;
317		}
318
319		/*
320		 * Use a physical idle state instead of busy polling so long as
321		 * its target residency is below the residency threshold, its
322		 * exit latency is not greater than the predicted idle duration,
323		 * and the next timer doesn't expire soon.
324		 */
325		if ((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) &&
326		    s->target_residency_ns < RESIDENCY_THRESHOLD_NS &&
327		    s->target_residency_ns <= data->next_timer_ns &&
328		    s->exit_latency_ns <= predicted_ns) {
329			predicted_ns = s->target_residency_ns;
330			idx = i;
331			break;
332		}
333
334		if (predicted_ns < TICK_NSEC)
335			break;
336
337		if (!tick_nohz_tick_stopped()) {
338			/*
339			 * If the state selected so far is shallow, waking up
340			 * early won't hurt, so retain the tick in that case and
341			 * let the governor run again in the next iteration of
342			 * the idle loop.
343			 */
344			predicted_ns = drv->states[idx].target_residency_ns;
345			break;
346		}
347
348		/*
349		 * If the state selected so far is shallow and this state's
350		 * target residency matches the time till the closest timer
351		 * event, select this one to avoid getting stuck in the shallow
352		 * one for too long.
353		 */
354		if (drv->states[idx].target_residency_ns < TICK_NSEC &&
355		    s->target_residency_ns <= delta_tick)
356			idx = i;
357
358		return idx;
359	}
360
361	if (idx == -1)
362		idx = 0; /* No states enabled. Must use 0. */
363
364	/*
365	 * Don't stop the tick if the selected state is a polling one or if the
366	 * expected idle duration is shorter than the tick period length.
367	 */
368	if (((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) ||
369	     predicted_ns < TICK_NSEC) && !tick_nohz_tick_stopped()) {
370		*stop_tick = false;
371
372		if (idx > 0 && drv->states[idx].target_residency_ns > delta_tick) {
373			/*
374			 * The tick is not going to be stopped and the target
375			 * residency of the state to be returned is not within
376			 * the time until the next timer event including the
377			 * tick, so try to correct that.
378			 */
379			for (i = idx - 1; i >= 0; i--) {
380				if (dev->states_usage[i].disable)
381					continue;
382
383				idx = i;
384				if (drv->states[i].target_residency_ns <= delta_tick)
385					break;
386			}
387		}
388	}
389
390	return idx;
391}
392
393/**
394 * menu_reflect - records that data structures need update
395 * @dev: the CPU
396 * @index: the index of actual entered state
397 *
398 * NOTE: it's important to be fast here because this operation will add to
399 *       the overall exit latency.
400 */
401static void menu_reflect(struct cpuidle_device *dev, int index)
402{
403	struct menu_device *data = this_cpu_ptr(&menu_devices);
404
405	dev->last_state_idx = index;
406	data->needs_update = 1;
407	data->tick_wakeup = tick_nohz_idle_got_tick();
408}
409
410/**
411 * menu_update - attempts to guess what happened after entry
412 * @drv: cpuidle driver containing state data
413 * @dev: the CPU
414 */
415static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
416{
417	struct menu_device *data = this_cpu_ptr(&menu_devices);
418	int last_idx = dev->last_state_idx;
419	struct cpuidle_state *target = &drv->states[last_idx];
420	u64 measured_ns;
421	unsigned int new_factor;
422
423	/*
424	 * Try to figure out how much time passed between entry to low
425	 * power state and occurrence of the wakeup event.
426	 *
427	 * If the entered idle state didn't support residency measurements,
428	 * we use them anyway if they are short, and if long,
429	 * truncate to the whole expected time.
430	 *
431	 * Any measured amount of time will include the exit latency.
432	 * Since we are interested in when the wakeup begun, not when it
433	 * was completed, we must subtract the exit latency. However, if
434	 * the measured amount of time is less than the exit latency,
435	 * assume the state was never reached and the exit latency is 0.
436	 */
437
438	if (data->tick_wakeup && data->next_timer_ns > TICK_NSEC) {
439		/*
440		 * The nohz code said that there wouldn't be any events within
441		 * the tick boundary (if the tick was stopped), but the idle
442		 * duration predictor had a differing opinion.  Since the CPU
443		 * was woken up by a tick (that wasn't stopped after all), the
444		 * predictor was not quite right, so assume that the CPU could
445		 * have been idle long (but not forever) to help the idle
446		 * duration predictor do a better job next time.
447		 */
448		measured_ns = 9 * MAX_INTERESTING / 10;
449	} else if ((drv->states[last_idx].flags & CPUIDLE_FLAG_POLLING) &&
450		   dev->poll_time_limit) {
451		/*
452		 * The CPU exited the "polling" state due to a time limit, so
453		 * the idle duration prediction leading to the selection of that
454		 * state was inaccurate.  If a better prediction had been made,
455		 * the CPU might have been woken up from idle by the next timer.
456		 * Assume that to be the case.
457		 */
458		measured_ns = data->next_timer_ns;
459	} else {
460		/* measured value */
461		measured_ns = dev->last_residency_ns;
462
463		/* Deduct exit latency */
464		if (measured_ns > 2 * target->exit_latency_ns)
465			measured_ns -= target->exit_latency_ns;
466		else
467			measured_ns /= 2;
468	}
469
470	/* Make sure our coefficients do not exceed unity */
471	if (measured_ns > data->next_timer_ns)
472		measured_ns = data->next_timer_ns;
473
474	/* Update our correction ratio */
475	new_factor = data->correction_factor[data->bucket];
476	new_factor -= new_factor / DECAY;
477
478	if (data->next_timer_ns > 0 && measured_ns < MAX_INTERESTING)
479		new_factor += div64_u64(RESOLUTION * measured_ns,
480					data->next_timer_ns);
481	else
482		/*
483		 * we were idle so long that we count it as a perfect
484		 * prediction
485		 */
486		new_factor += RESOLUTION;
487
488	/*
489	 * We don't want 0 as factor; we always want at least
490	 * a tiny bit of estimated time. Fortunately, due to rounding,
491	 * new_factor will stay nonzero regardless of measured_us values
492	 * and the compiler can eliminate this test as long as DECAY > 1.
493	 */
494	if (DECAY == 1 && unlikely(new_factor == 0))
495		new_factor = 1;
496
497	data->correction_factor[data->bucket] = new_factor;
498
499	menu_update_intervals(data, ktime_to_us(measured_ns));
500}
501
502/**
503 * menu_enable_device - scans a CPU's states and does setup
504 * @drv: cpuidle driver
505 * @dev: the CPU
506 */
507static int menu_enable_device(struct cpuidle_driver *drv,
508				struct cpuidle_device *dev)
509{
510	struct menu_device *data = &per_cpu(menu_devices, dev->cpu);
511	int i;
512
513	memset(data, 0, sizeof(struct menu_device));
514
515	/*
516	 * if the correction factor is 0 (eg first time init or cpu hotplug
517	 * etc), we actually want to start out with a unity factor.
518	 */
519	for(i = 0; i < BUCKETS; i++)
520		data->correction_factor[i] = RESOLUTION * DECAY;
521
522	return 0;
523}
524
525static struct cpuidle_governor menu_governor = {
526	.name =		"menu",
527	.rating =	20,
528	.enable =	menu_enable_device,
529	.select =	menu_select,
530	.reflect =	menu_reflect,
531};
532
533/**
534 * init_menu - initializes the governor
535 */
536static int __init init_menu(void)
537{
538	return cpuidle_register_governor(&menu_governor);
539}
540
541postcore_initcall(init_menu);