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 }; 119 120 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 */ 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 */ 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 */ 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 */ 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 */ 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 */ 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 */ 574 static int __init init_menu(void) 575 { 576 return cpuidle_register_governor(&menu_governor); 577 } 578 579 postcore_initcall(init_menu); 580