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.h>
18 #include <linux/sched/loadavg.h>
19 #include <linux/sched/stat.h>
20 #include <linux/math64.h>
21
22 #define BUCKETS 12
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 3 decision factors for picking a C
33 * state:
34 * 1) Energy break even point
35 * 2) Performance impact
36 * 3) Latency tolerance (from pmqos infrastructure)
37 * These these three factors are treated independently.
38 *
39 * Energy break even point
40 * -----------------------
41 * C state entry and exit have an energy cost, and a certain amount of time in
42 * the C state is required to actually break even on this cost. CPUIDLE
43 * provides us this duration in the "target_residency" field. So all that we
44 * need is a good prediction of how long we'll be idle. Like the traditional
45 * menu governor, we start with the actual known "next timer event" time.
46 *
47 * Since there are other source of wakeups (interrupts for example) than
48 * the next timer event, this estimation is rather optimistic. To get a
49 * more realistic estimate, a correction factor is applied to the estimate,
50 * that is based on historic behavior. For example, if in the past the actual
51 * duration always was 50% of the next timer tick, the correction factor will
52 * be 0.5.
53 *
54 * menu uses a running average for this correction factor, however it uses a
55 * set of factors, not just a single factor. This stems from the realization
56 * that the ratio is dependent on the order of magnitude of the expected
57 * duration; if we expect 500 milliseconds of idle time the likelihood of
58 * getting an interrupt very early is much higher than if we expect 50 micro
59 * seconds of idle time. A second independent factor that has big impact on
60 * the actual factor is if there is (disk) IO outstanding or not.
61 * (as a special twist, we consider every sleep longer than 50 milliseconds
62 * as perfect; there are no power gains for sleeping longer than this)
63 *
64 * For these two reasons we keep an array of 12 independent factors, that gets
65 * indexed based on the magnitude of the expected duration as well as the
66 * "is IO outstanding" property.
67 *
68 * Repeatable-interval-detector
69 * ----------------------------
70 * There are some cases where "next timer" is a completely unusable predictor:
71 * Those cases where the interval is fixed, for example due to hardware
72 * interrupt mitigation, but also due to fixed transfer rate devices such as
73 * mice.
74 * For this, we use a different predictor: We track the duration of the last 8
75 * intervals and if the stand deviation of these 8 intervals is below a
76 * threshold value, we use the average of these intervals as prediction.
77 *
78 * Limiting Performance Impact
79 * ---------------------------
80 * C states, especially those with large exit latencies, can have a real
81 * noticeable impact on workloads, which is not acceptable for most sysadmins,
82 * and in addition, less performance has a power price of its own.
83 *
84 * As a general rule of thumb, menu assumes that the following heuristic
85 * holds:
86 * The busier the system, the less impact of C states is acceptable
87 *
88 * This rule-of-thumb is implemented using a performance-multiplier:
89 * If the exit latency times the performance multiplier is longer than
90 * the predicted duration, the C state is not considered a candidate
91 * for selection due to a too high performance impact. So the higher
92 * this multiplier is, the longer we need to be idle to pick a deep C
93 * state, and thus the less likely a busy CPU will hit such a deep
94 * C state.
95 *
96 * Two factors are used in determing this multiplier:
97 * a value of 10 is added for each point of "per cpu load average" we have.
98 * a value of 5 points is added for each process that is waiting for
99 * IO on this CPU.
100 * (these values are experimentally determined)
101 *
102 * The load average factor gives a longer term (few seconds) input to the
103 * decision, while the iowait value gives a cpu local instantanious input.
104 * The iowait factor may look low, but realize that this is also already
105 * represented in the system load average.
106 *
107 */
108
109 struct menu_device {
110 int needs_update;
111 int tick_wakeup;
112
113 u64 next_timer_ns;
114 unsigned int bucket;
115 unsigned int correction_factor[BUCKETS];
116 unsigned int intervals[INTERVALS];
117 int interval_ptr;
118 };
119
which_bucket(u64 duration_ns,unsigned int nr_iowaiters)120 static inline int which_bucket(u64 duration_ns, unsigned int nr_iowaiters)
121 {
122 int bucket = 0;
123
124 /*
125 * We keep two groups of stats; one with no
126 * IO pending, one without.
127 * This allows us to calculate
128 * E(duration)|iowait
129 */
130 if (nr_iowaiters)
131 bucket = BUCKETS/2;
132
133 if (duration_ns < 10ULL * NSEC_PER_USEC)
134 return bucket;
135 if (duration_ns < 100ULL * NSEC_PER_USEC)
136 return bucket + 1;
137 if (duration_ns < 1000ULL * NSEC_PER_USEC)
138 return bucket + 2;
139 if (duration_ns < 10000ULL * NSEC_PER_USEC)
140 return bucket + 3;
141 if (duration_ns < 100000ULL * NSEC_PER_USEC)
142 return bucket + 4;
143 return bucket + 5;
144 }
145
146 /*
147 * Return a multiplier for the exit latency that is intended
148 * to take performance requirements into account.
149 * The more performance critical we estimate the system
150 * to be, the higher this multiplier, and thus the higher
151 * the barrier to go to an expensive C state.
152 */
performance_multiplier(unsigned int nr_iowaiters)153 static inline int performance_multiplier(unsigned int nr_iowaiters)
154 {
155 /* for IO wait tasks (per cpu!) we add 10x each */
156 return 1 + 10 * nr_iowaiters;
157 }
158
159 static DEFINE_PER_CPU(struct menu_device, menu_devices);
160
161 static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev);
162
163 /*
164 * Try detecting repeating patterns by keeping track of the last 8
165 * intervals, and checking if the standard deviation of that set
166 * of points is below a threshold. If it is... then use the
167 * average of these 8 points as the estimated value.
168 */
get_typical_interval(struct menu_device * data,unsigned int predicted_us)169 static unsigned int get_typical_interval(struct menu_device *data,
170 unsigned int predicted_us)
171 {
172 int i, divisor;
173 unsigned int min, max, thresh, avg;
174 uint64_t sum, variance;
175
176 thresh = INT_MAX; /* Discard outliers above this value */
177
178 again:
179
180 /* First calculate the average of past intervals */
181 min = UINT_MAX;
182 max = 0;
183 sum = 0;
184 divisor = 0;
185 for (i = 0; i < INTERVALS; i++) {
186 unsigned int value = data->intervals[i];
187 if (value <= thresh) {
188 sum += value;
189 divisor++;
190 if (value > max)
191 max = value;
192
193 if (value < min)
194 min = value;
195 }
196 }
197
198 /*
199 * If the result of the computation is going to be discarded anyway,
200 * avoid the computation altogether.
201 */
202 if (min >= predicted_us)
203 return UINT_MAX;
204
205 if (divisor == INTERVALS)
206 avg = sum >> INTERVAL_SHIFT;
207 else
208 avg = div_u64(sum, divisor);
209
210 /* Then try to determine variance */
211 variance = 0;
212 for (i = 0; i < INTERVALS; i++) {
213 unsigned int value = data->intervals[i];
214 if (value <= thresh) {
215 int64_t diff = (int64_t)value - avg;
216 variance += diff * diff;
217 }
218 }
219 if (divisor == INTERVALS)
220 variance >>= INTERVAL_SHIFT;
221 else
222 do_div(variance, divisor);
223
224 /*
225 * The typical interval is obtained when standard deviation is
226 * small (stddev <= 20 us, variance <= 400 us^2) or standard
227 * deviation is small compared to the average interval (avg >
228 * 6*stddev, avg^2 > 36*variance). The average is smaller than
229 * UINT_MAX aka U32_MAX, so computing its square does not
230 * overflow a u64. We simply reject this candidate average if
231 * the standard deviation is greater than 715 s (which is
232 * rather unlikely).
233 *
234 * Use this result only if there is no timer to wake us up sooner.
235 */
236 if (likely(variance <= U64_MAX/36)) {
237 if ((((u64)avg*avg > variance*36) && (divisor * 4 >= INTERVALS * 3))
238 || variance <= 400) {
239 return avg;
240 }
241 }
242
243 /*
244 * If we have outliers to the upside in our distribution, discard
245 * those by setting the threshold to exclude these outliers, then
246 * calculate the average and standard deviation again. Once we get
247 * down to the bottom 3/4 of our samples, stop excluding samples.
248 *
249 * This can deal with workloads that have long pauses interspersed
250 * with sporadic activity with a bunch of short pauses.
251 */
252 if ((divisor * 4) <= INTERVALS * 3)
253 return UINT_MAX;
254
255 thresh = max - 1;
256 goto again;
257 }
258
259 /**
260 * menu_select - selects the next idle state to enter
261 * @drv: cpuidle driver containing state data
262 * @dev: the CPU
263 * @stop_tick: indication on whether or not to stop the tick
264 */
menu_select(struct cpuidle_driver * drv,struct cpuidle_device * dev,bool * stop_tick)265 static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
266 bool *stop_tick)
267 {
268 struct menu_device *data = this_cpu_ptr(&menu_devices);
269 s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
270 unsigned int predicted_us;
271 u64 predicted_ns;
272 u64 interactivity_req;
273 unsigned int nr_iowaiters;
274 ktime_t delta, delta_tick;
275 int i, idx;
276
277 if (data->needs_update) {
278 menu_update(drv, dev);
279 data->needs_update = 0;
280 }
281
282 /* determine the expected residency time, round up */
283 delta = tick_nohz_get_sleep_length(&delta_tick);
284 if (unlikely(delta < 0)) {
285 delta = 0;
286 delta_tick = 0;
287 }
288 data->next_timer_ns = delta;
289
290 nr_iowaiters = nr_iowait_cpu(dev->cpu);
291 data->bucket = which_bucket(data->next_timer_ns, nr_iowaiters);
292
293 if (unlikely(drv->state_count <= 1 || latency_req == 0) ||
294 ((data->next_timer_ns < drv->states[1].target_residency_ns ||
295 latency_req < drv->states[1].exit_latency_ns) &&
296 !dev->states_usage[0].disable)) {
297 /*
298 * In this case state[0] will be used no matter what, so return
299 * it right away and keep the tick running if state[0] is a
300 * polling one.
301 */
302 *stop_tick = !(drv->states[0].flags & CPUIDLE_FLAG_POLLING);
303 return 0;
304 }
305
306 /* Round up the result for half microseconds. */
307 predicted_us = div_u64(data->next_timer_ns *
308 data->correction_factor[data->bucket] +
309 (RESOLUTION * DECAY * NSEC_PER_USEC) / 2,
310 RESOLUTION * DECAY * NSEC_PER_USEC);
311 /* Use the lowest expected idle interval to pick the idle state. */
312 predicted_ns = (u64)min(predicted_us,
313 get_typical_interval(data, predicted_us)) *
314 NSEC_PER_USEC;
315
316 if (tick_nohz_tick_stopped()) {
317 /*
318 * If the tick is already stopped, the cost of possible short
319 * idle duration misprediction is much higher, because the CPU
320 * may be stuck in a shallow idle state for a long time as a
321 * result of it. In that case say we might mispredict and use
322 * the known time till the closest timer event for the idle
323 * state selection.
324 */
325 if (predicted_ns < TICK_NSEC)
326 predicted_ns = data->next_timer_ns;
327 } else {
328 /*
329 * Use the performance multiplier and the user-configurable
330 * latency_req to determine the maximum exit latency.
331 */
332 interactivity_req = div64_u64(predicted_ns,
333 performance_multiplier(nr_iowaiters));
334 if (latency_req > interactivity_req)
335 latency_req = interactivity_req;
336 }
337
338 /*
339 * Find the idle state with the lowest power while satisfying
340 * our constraints.
341 */
342 idx = -1;
343 for (i = 0; i < drv->state_count; i++) {
344 struct cpuidle_state *s = &drv->states[i];
345
346 if (dev->states_usage[i].disable)
347 continue;
348
349 if (idx == -1)
350 idx = i; /* first enabled state */
351
352 if (s->target_residency_ns > predicted_ns) {
353 /*
354 * Use a physical idle state, not busy polling, unless
355 * a timer is going to trigger soon enough.
356 */
357 if ((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) &&
358 s->exit_latency_ns <= latency_req &&
359 s->target_residency_ns <= data->next_timer_ns) {
360 predicted_ns = s->target_residency_ns;
361 idx = i;
362 break;
363 }
364 if (predicted_ns < TICK_NSEC)
365 break;
366
367 if (!tick_nohz_tick_stopped()) {
368 /*
369 * If the state selected so far is shallow,
370 * waking up early won't hurt, so retain the
371 * tick in that case and let the governor run
372 * again in the next iteration of the loop.
373 */
374 predicted_ns = drv->states[idx].target_residency_ns;
375 break;
376 }
377
378 /*
379 * If the state selected so far is shallow and this
380 * state's target residency matches the time till the
381 * closest timer event, select this one to avoid getting
382 * stuck in the shallow one for too long.
383 */
384 if (drv->states[idx].target_residency_ns < TICK_NSEC &&
385 s->target_residency_ns <= delta_tick)
386 idx = i;
387
388 return idx;
389 }
390 if (s->exit_latency_ns > latency_req)
391 break;
392
393 idx = i;
394 }
395
396 if (idx == -1)
397 idx = 0; /* No states enabled. Must use 0. */
398
399 /*
400 * Don't stop the tick if the selected state is a polling one or if the
401 * expected idle duration is shorter than the tick period length.
402 */
403 if (((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) ||
404 predicted_ns < TICK_NSEC) && !tick_nohz_tick_stopped()) {
405 *stop_tick = false;
406
407 if (idx > 0 && drv->states[idx].target_residency_ns > delta_tick) {
408 /*
409 * The tick is not going to be stopped and the target
410 * residency of the state to be returned is not within
411 * the time until the next timer event including the
412 * tick, so try to correct that.
413 */
414 for (i = idx - 1; i >= 0; i--) {
415 if (dev->states_usage[i].disable)
416 continue;
417
418 idx = i;
419 if (drv->states[i].target_residency_ns <= delta_tick)
420 break;
421 }
422 }
423 }
424
425 return idx;
426 }
427
428 /**
429 * menu_reflect - records that data structures need update
430 * @dev: the CPU
431 * @index: the index of actual entered state
432 *
433 * NOTE: it's important to be fast here because this operation will add to
434 * the overall exit latency.
435 */
menu_reflect(struct cpuidle_device * dev,int index)436 static void menu_reflect(struct cpuidle_device *dev, int index)
437 {
438 struct menu_device *data = this_cpu_ptr(&menu_devices);
439
440 dev->last_state_idx = index;
441 data->needs_update = 1;
442 data->tick_wakeup = tick_nohz_idle_got_tick();
443 }
444
445 /**
446 * menu_update - attempts to guess what happened after entry
447 * @drv: cpuidle driver containing state data
448 * @dev: the CPU
449 */
menu_update(struct cpuidle_driver * drv,struct cpuidle_device * dev)450 static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
451 {
452 struct menu_device *data = this_cpu_ptr(&menu_devices);
453 int last_idx = dev->last_state_idx;
454 struct cpuidle_state *target = &drv->states[last_idx];
455 u64 measured_ns;
456 unsigned int new_factor;
457
458 /*
459 * Try to figure out how much time passed between entry to low
460 * power state and occurrence of the wakeup event.
461 *
462 * If the entered idle state didn't support residency measurements,
463 * we use them anyway if they are short, and if long,
464 * truncate to the whole expected time.
465 *
466 * Any measured amount of time will include the exit latency.
467 * Since we are interested in when the wakeup begun, not when it
468 * was completed, we must subtract the exit latency. However, if
469 * the measured amount of time is less than the exit latency,
470 * assume the state was never reached and the exit latency is 0.
471 */
472
473 if (data->tick_wakeup && data->next_timer_ns > TICK_NSEC) {
474 /*
475 * The nohz code said that there wouldn't be any events within
476 * the tick boundary (if the tick was stopped), but the idle
477 * duration predictor had a differing opinion. Since the CPU
478 * was woken up by a tick (that wasn't stopped after all), the
479 * predictor was not quite right, so assume that the CPU could
480 * have been idle long (but not forever) to help the idle
481 * duration predictor do a better job next time.
482 */
483 measured_ns = 9 * MAX_INTERESTING / 10;
484 } else if ((drv->states[last_idx].flags & CPUIDLE_FLAG_POLLING) &&
485 dev->poll_time_limit) {
486 /*
487 * The CPU exited the "polling" state due to a time limit, so
488 * the idle duration prediction leading to the selection of that
489 * state was inaccurate. If a better prediction had been made,
490 * the CPU might have been woken up from idle by the next timer.
491 * Assume that to be the case.
492 */
493 measured_ns = data->next_timer_ns;
494 } else {
495 /* measured value */
496 measured_ns = dev->last_residency_ns;
497
498 /* Deduct exit latency */
499 if (measured_ns > 2 * target->exit_latency_ns)
500 measured_ns -= target->exit_latency_ns;
501 else
502 measured_ns /= 2;
503 }
504
505 /* Make sure our coefficients do not exceed unity */
506 if (measured_ns > data->next_timer_ns)
507 measured_ns = data->next_timer_ns;
508
509 /* Update our correction ratio */
510 new_factor = data->correction_factor[data->bucket];
511 new_factor -= new_factor / DECAY;
512
513 if (data->next_timer_ns > 0 && measured_ns < MAX_INTERESTING)
514 new_factor += div64_u64(RESOLUTION * measured_ns,
515 data->next_timer_ns);
516 else
517 /*
518 * we were idle so long that we count it as a perfect
519 * prediction
520 */
521 new_factor += RESOLUTION;
522
523 /*
524 * We don't want 0 as factor; we always want at least
525 * a tiny bit of estimated time. Fortunately, due to rounding,
526 * new_factor will stay nonzero regardless of measured_us values
527 * and the compiler can eliminate this test as long as DECAY > 1.
528 */
529 if (DECAY == 1 && unlikely(new_factor == 0))
530 new_factor = 1;
531
532 data->correction_factor[data->bucket] = new_factor;
533
534 /* update the repeating-pattern data */
535 data->intervals[data->interval_ptr++] = ktime_to_us(measured_ns);
536 if (data->interval_ptr >= INTERVALS)
537 data->interval_ptr = 0;
538 }
539
540 /**
541 * menu_enable_device - scans a CPU's states and does setup
542 * @drv: cpuidle driver
543 * @dev: the CPU
544 */
menu_enable_device(struct cpuidle_driver * drv,struct cpuidle_device * dev)545 static int menu_enable_device(struct cpuidle_driver *drv,
546 struct cpuidle_device *dev)
547 {
548 struct menu_device *data = &per_cpu(menu_devices, dev->cpu);
549 int i;
550
551 memset(data, 0, sizeof(struct menu_device));
552
553 /*
554 * if the correction factor is 0 (eg first time init or cpu hotplug
555 * etc), we actually want to start out with a unity factor.
556 */
557 for(i = 0; i < BUCKETS; i++)
558 data->correction_factor[i] = RESOLUTION * DECAY;
559
560 return 0;
561 }
562
563 static struct cpuidle_governor menu_governor = {
564 .name = "menu",
565 .rating = 20,
566 .enable = menu_enable_device,
567 .select = menu_select,
568 .reflect = menu_reflect,
569 };
570
571 /**
572 * init_menu - initializes the governor
573 */
init_menu(void)574 static int __init init_menu(void)
575 {
576 return cpuidle_register_governor(&menu_governor);
577 }
578
579 postcore_initcall(init_menu);
580