1 /*
2 * menu.c - the menu idle governor
3 *
4 * Copyright (C) 2006-2007 Adam Belay <abelay@novell.com>
5 * Copyright (C) 2009 Intel Corporation
6 * Author:
7 * Arjan van de Ven <arjan@linux.intel.com>
8 *
9 * This code is licenced under the GPL version 2 as described
10 * in the COPYING file that acompanies the Linux Kernel.
11 */
12
13 #include <linux/kernel.h>
14 #include <linux/cpuidle.h>
15 #include <linux/pm_qos.h>
16 #include <linux/time.h>
17 #include <linux/ktime.h>
18 #include <linux/hrtimer.h>
19 #include <linux/tick.h>
20 #include <linux/sched.h>
21 #include <linux/math64.h>
22 #include <linux/module.h>
23
24 /*
25 * Please note when changing the tuning values:
26 * If (MAX_INTERESTING-1) * RESOLUTION > UINT_MAX, the result of
27 * a scaling operation multiplication may overflow on 32 bit platforms.
28 * In that case, #define RESOLUTION as ULL to get 64 bit result:
29 * #define RESOLUTION 1024ULL
30 *
31 * The default values do not overflow.
32 */
33 #define BUCKETS 12
34 #define INTERVAL_SHIFT 3
35 #define INTERVALS (1UL << INTERVAL_SHIFT)
36 #define RESOLUTION 1024
37 #define DECAY 8
38 #define MAX_INTERESTING 50000
39
40
41 /*
42 * Concepts and ideas behind the menu governor
43 *
44 * For the menu governor, there are 3 decision factors for picking a C
45 * state:
46 * 1) Energy break even point
47 * 2) Performance impact
48 * 3) Latency tolerance (from pmqos infrastructure)
49 * These these three factors are treated independently.
50 *
51 * Energy break even point
52 * -----------------------
53 * C state entry and exit have an energy cost, and a certain amount of time in
54 * the C state is required to actually break even on this cost. CPUIDLE
55 * provides us this duration in the "target_residency" field. So all that we
56 * need is a good prediction of how long we'll be idle. Like the traditional
57 * menu governor, we start with the actual known "next timer event" time.
58 *
59 * Since there are other source of wakeups (interrupts for example) than
60 * the next timer event, this estimation is rather optimistic. To get a
61 * more realistic estimate, a correction factor is applied to the estimate,
62 * that is based on historic behavior. For example, if in the past the actual
63 * duration always was 50% of the next timer tick, the correction factor will
64 * be 0.5.
65 *
66 * menu uses a running average for this correction factor, however it uses a
67 * set of factors, not just a single factor. This stems from the realization
68 * that the ratio is dependent on the order of magnitude of the expected
69 * duration; if we expect 500 milliseconds of idle time the likelihood of
70 * getting an interrupt very early is much higher than if we expect 50 micro
71 * seconds of idle time. A second independent factor that has big impact on
72 * the actual factor is if there is (disk) IO outstanding or not.
73 * (as a special twist, we consider every sleep longer than 50 milliseconds
74 * as perfect; there are no power gains for sleeping longer than this)
75 *
76 * For these two reasons we keep an array of 12 independent factors, that gets
77 * indexed based on the magnitude of the expected duration as well as the
78 * "is IO outstanding" property.
79 *
80 * Repeatable-interval-detector
81 * ----------------------------
82 * There are some cases where "next timer" is a completely unusable predictor:
83 * Those cases where the interval is fixed, for example due to hardware
84 * interrupt mitigation, but also due to fixed transfer rate devices such as
85 * mice.
86 * For this, we use a different predictor: We track the duration of the last 8
87 * intervals and if the stand deviation of these 8 intervals is below a
88 * threshold value, we use the average of these intervals as prediction.
89 *
90 * Limiting Performance Impact
91 * ---------------------------
92 * C states, especially those with large exit latencies, can have a real
93 * noticeable impact on workloads, which is not acceptable for most sysadmins,
94 * and in addition, less performance has a power price of its own.
95 *
96 * As a general rule of thumb, menu assumes that the following heuristic
97 * holds:
98 * The busier the system, the less impact of C states is acceptable
99 *
100 * This rule-of-thumb is implemented using a performance-multiplier:
101 * If the exit latency times the performance multiplier is longer than
102 * the predicted duration, the C state is not considered a candidate
103 * for selection due to a too high performance impact. So the higher
104 * this multiplier is, the longer we need to be idle to pick a deep C
105 * state, and thus the less likely a busy CPU will hit such a deep
106 * C state.
107 *
108 * Two factors are used in determing this multiplier:
109 * a value of 10 is added for each point of "per cpu load average" we have.
110 * a value of 5 points is added for each process that is waiting for
111 * IO on this CPU.
112 * (these values are experimentally determined)
113 *
114 * The load average factor gives a longer term (few seconds) input to the
115 * decision, while the iowait value gives a cpu local instantanious input.
116 * The iowait factor may look low, but realize that this is also already
117 * represented in the system load average.
118 *
119 */
120
121 struct menu_device {
122 int last_state_idx;
123 int needs_update;
124
125 unsigned int next_timer_us;
126 unsigned int predicted_us;
127 unsigned int bucket;
128 unsigned int correction_factor[BUCKETS];
129 unsigned int intervals[INTERVALS];
130 int interval_ptr;
131 };
132
133
134 #define LOAD_INT(x) ((x) >> FSHIFT)
135 #define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100)
136
get_loadavg(unsigned long load)137 static inline int get_loadavg(unsigned long load)
138 {
139 return LOAD_INT(load) * 10 + LOAD_FRAC(load) / 10;
140 }
141
which_bucket(unsigned int duration,unsigned long nr_iowaiters)142 static inline int which_bucket(unsigned int duration, unsigned long nr_iowaiters)
143 {
144 int bucket = 0;
145
146 /*
147 * We keep two groups of stats; one with no
148 * IO pending, one without.
149 * This allows us to calculate
150 * E(duration)|iowait
151 */
152 if (nr_iowaiters)
153 bucket = BUCKETS/2;
154
155 if (duration < 10)
156 return bucket;
157 if (duration < 100)
158 return bucket + 1;
159 if (duration < 1000)
160 return bucket + 2;
161 if (duration < 10000)
162 return bucket + 3;
163 if (duration < 100000)
164 return bucket + 4;
165 return bucket + 5;
166 }
167
168 /*
169 * Return a multiplier for the exit latency that is intended
170 * to take performance requirements into account.
171 * The more performance critical we estimate the system
172 * to be, the higher this multiplier, and thus the higher
173 * the barrier to go to an expensive C state.
174 */
performance_multiplier(unsigned long nr_iowaiters,unsigned long load)175 static inline int performance_multiplier(unsigned long nr_iowaiters, unsigned long load)
176 {
177 int mult = 1;
178
179 /* for higher loadavg, we are more reluctant */
180
181 /*
182 * this doesn't work as intended - it is almost always 0, but can
183 * sometimes, depending on workload, spike very high into the hundreds
184 * even when the average cpu load is under 10%.
185 */
186 /* mult += 2 * get_loadavg(); */
187
188 /* for IO wait tasks (per cpu!) we add 5x each */
189 mult += 10 * nr_iowaiters;
190
191 return mult;
192 }
193
194 static DEFINE_PER_CPU(struct menu_device, menu_devices);
195
196 static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev);
197
198 /*
199 * Try detecting repeating patterns by keeping track of the last 8
200 * intervals, and checking if the standard deviation of that set
201 * of points is below a threshold. If it is... then use the
202 * average of these 8 points as the estimated value.
203 */
get_typical_interval(struct menu_device * data)204 static void get_typical_interval(struct menu_device *data)
205 {
206 int i, divisor;
207 unsigned int max, thresh;
208 uint64_t avg, stddev;
209
210 thresh = UINT_MAX; /* Discard outliers above this value */
211
212 again:
213
214 /* First calculate the average of past intervals */
215 max = 0;
216 avg = 0;
217 divisor = 0;
218 for (i = 0; i < INTERVALS; i++) {
219 unsigned int value = data->intervals[i];
220 if (value <= thresh) {
221 avg += value;
222 divisor++;
223 if (value > max)
224 max = value;
225 }
226 }
227 if (divisor == INTERVALS)
228 avg >>= INTERVAL_SHIFT;
229 else
230 do_div(avg, divisor);
231
232 /* Then try to determine standard deviation */
233 stddev = 0;
234 for (i = 0; i < INTERVALS; i++) {
235 unsigned int value = data->intervals[i];
236 if (value <= thresh) {
237 int64_t diff = value - avg;
238 stddev += diff * diff;
239 }
240 }
241 if (divisor == INTERVALS)
242 stddev >>= INTERVAL_SHIFT;
243 else
244 do_div(stddev, divisor);
245
246 /*
247 * The typical interval is obtained when standard deviation is small
248 * or standard deviation is small compared to the average interval.
249 *
250 * int_sqrt() formal parameter type is unsigned long. When the
251 * greatest difference to an outlier exceeds ~65 ms * sqrt(divisor)
252 * the resulting squared standard deviation exceeds the input domain
253 * of int_sqrt on platforms where unsigned long is 32 bits in size.
254 * In such case reject the candidate average.
255 *
256 * Use this result only if there is no timer to wake us up sooner.
257 */
258 if (likely(stddev <= ULONG_MAX)) {
259 stddev = int_sqrt(stddev);
260 if (((avg > stddev * 6) && (divisor * 4 >= INTERVALS * 3))
261 || stddev <= 20) {
262 if (data->next_timer_us > avg)
263 data->predicted_us = avg;
264 return;
265 }
266 }
267
268 /*
269 * If we have outliers to the upside in our distribution, discard
270 * those by setting the threshold to exclude these outliers, then
271 * calculate the average and standard deviation again. Once we get
272 * down to the bottom 3/4 of our samples, stop excluding samples.
273 *
274 * This can deal with workloads that have long pauses interspersed
275 * with sporadic activity with a bunch of short pauses.
276 */
277 if ((divisor * 4) <= INTERVALS * 3)
278 return;
279
280 thresh = max - 1;
281 goto again;
282 }
283
284 /**
285 * menu_select - selects the next idle state to enter
286 * @drv: cpuidle driver containing state data
287 * @dev: the CPU
288 */
menu_select(struct cpuidle_driver * drv,struct cpuidle_device * dev)289 static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
290 {
291 struct menu_device *data = this_cpu_ptr(&menu_devices);
292 int latency_req = pm_qos_request(PM_QOS_CPU_DMA_LATENCY);
293 int i;
294 unsigned int interactivity_req;
295 unsigned long nr_iowaiters, cpu_load;
296
297 if (data->needs_update) {
298 menu_update(drv, dev);
299 data->needs_update = 0;
300 }
301
302 data->last_state_idx = CPUIDLE_DRIVER_STATE_START - 1;
303
304 /* Special case when user has set very strict latency requirement */
305 if (unlikely(latency_req == 0))
306 return 0;
307
308 /* determine the expected residency time, round up */
309 data->next_timer_us = ktime_to_us(tick_nohz_get_sleep_length());
310
311 get_iowait_load(&nr_iowaiters, &cpu_load);
312 data->bucket = which_bucket(data->next_timer_us, nr_iowaiters);
313
314 /*
315 * Force the result of multiplication to be 64 bits even if both
316 * operands are 32 bits.
317 * Make sure to round up for half microseconds.
318 */
319 data->predicted_us = DIV_ROUND_CLOSEST_ULL((uint64_t)data->next_timer_us *
320 data->correction_factor[data->bucket],
321 RESOLUTION * DECAY);
322
323 get_typical_interval(data);
324
325 /*
326 * Performance multiplier defines a minimum predicted idle
327 * duration / latency ratio. Adjust the latency limit if
328 * necessary.
329 */
330 interactivity_req = data->predicted_us / performance_multiplier(nr_iowaiters, cpu_load);
331 if (latency_req > interactivity_req)
332 latency_req = interactivity_req;
333
334 /*
335 * We want to default to C1 (hlt), not to busy polling
336 * unless the timer is happening really really soon.
337 */
338 if (data->next_timer_us > 5 &&
339 !drv->states[CPUIDLE_DRIVER_STATE_START].disabled &&
340 dev->states_usage[CPUIDLE_DRIVER_STATE_START].disable == 0)
341 data->last_state_idx = CPUIDLE_DRIVER_STATE_START;
342
343 /*
344 * Find the idle state with the lowest power while satisfying
345 * our constraints.
346 */
347 for (i = CPUIDLE_DRIVER_STATE_START; i < drv->state_count; i++) {
348 struct cpuidle_state *s = &drv->states[i];
349 struct cpuidle_state_usage *su = &dev->states_usage[i];
350
351 if (s->disabled || su->disable)
352 continue;
353 if (s->target_residency > data->predicted_us)
354 continue;
355 if (s->exit_latency > latency_req)
356 continue;
357
358 data->last_state_idx = i;
359 }
360
361 return data->last_state_idx;
362 }
363
364 /**
365 * menu_reflect - records that data structures need update
366 * @dev: the CPU
367 * @index: the index of actual entered state
368 *
369 * NOTE: it's important to be fast here because this operation will add to
370 * the overall exit latency.
371 */
menu_reflect(struct cpuidle_device * dev,int index)372 static void menu_reflect(struct cpuidle_device *dev, int index)
373 {
374 struct menu_device *data = this_cpu_ptr(&menu_devices);
375
376 data->last_state_idx = index;
377 data->needs_update = 1;
378 }
379
380 /**
381 * menu_update - attempts to guess what happened after entry
382 * @drv: cpuidle driver containing state data
383 * @dev: the CPU
384 */
menu_update(struct cpuidle_driver * drv,struct cpuidle_device * dev)385 static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
386 {
387 struct menu_device *data = this_cpu_ptr(&menu_devices);
388 int last_idx = data->last_state_idx;
389 struct cpuidle_state *target = &drv->states[last_idx];
390 unsigned int measured_us;
391 unsigned int new_factor;
392
393 /*
394 * Try to figure out how much time passed between entry to low
395 * power state and occurrence of the wakeup event.
396 *
397 * If the entered idle state didn't support residency measurements,
398 * we use them anyway if they are short, and if long,
399 * truncate to the whole expected time.
400 *
401 * Any measured amount of time will include the exit latency.
402 * Since we are interested in when the wakeup begun, not when it
403 * was completed, we must subtract the exit latency. However, if
404 * the measured amount of time is less than the exit latency,
405 * assume the state was never reached and the exit latency is 0.
406 */
407
408 /* measured value */
409 measured_us = cpuidle_get_last_residency(dev);
410
411 /* Deduct exit latency */
412 if (measured_us > target->exit_latency)
413 measured_us -= target->exit_latency;
414
415 /* Make sure our coefficients do not exceed unity */
416 if (measured_us > data->next_timer_us)
417 measured_us = data->next_timer_us;
418
419 /* Update our correction ratio */
420 new_factor = data->correction_factor[data->bucket];
421 new_factor -= new_factor / DECAY;
422
423 if (data->next_timer_us > 0 && measured_us < MAX_INTERESTING)
424 new_factor += RESOLUTION * measured_us / data->next_timer_us;
425 else
426 /*
427 * we were idle so long that we count it as a perfect
428 * prediction
429 */
430 new_factor += RESOLUTION;
431
432 /*
433 * We don't want 0 as factor; we always want at least
434 * a tiny bit of estimated time. Fortunately, due to rounding,
435 * new_factor will stay nonzero regardless of measured_us values
436 * and the compiler can eliminate this test as long as DECAY > 1.
437 */
438 if (DECAY == 1 && unlikely(new_factor == 0))
439 new_factor = 1;
440
441 data->correction_factor[data->bucket] = new_factor;
442
443 /* update the repeating-pattern data */
444 data->intervals[data->interval_ptr++] = measured_us;
445 if (data->interval_ptr >= INTERVALS)
446 data->interval_ptr = 0;
447 }
448
449 /**
450 * menu_enable_device - scans a CPU's states and does setup
451 * @drv: cpuidle driver
452 * @dev: the CPU
453 */
menu_enable_device(struct cpuidle_driver * drv,struct cpuidle_device * dev)454 static int menu_enable_device(struct cpuidle_driver *drv,
455 struct cpuidle_device *dev)
456 {
457 struct menu_device *data = &per_cpu(menu_devices, dev->cpu);
458 int i;
459
460 memset(data, 0, sizeof(struct menu_device));
461
462 /*
463 * if the correction factor is 0 (eg first time init or cpu hotplug
464 * etc), we actually want to start out with a unity factor.
465 */
466 for(i = 0; i < BUCKETS; i++)
467 data->correction_factor[i] = RESOLUTION * DECAY;
468
469 return 0;
470 }
471
472 static struct cpuidle_governor menu_governor = {
473 .name = "menu",
474 .rating = 20,
475 .enable = menu_enable_device,
476 .select = menu_select,
477 .reflect = menu_reflect,
478 .owner = THIS_MODULE,
479 };
480
481 /**
482 * init_menu - initializes the governor
483 */
init_menu(void)484 static int __init init_menu(void)
485 {
486 return cpuidle_register_governor(&menu_governor);
487 }
488
489 postcore_initcall(init_menu);
490