// SPDX-License-Identifier: GPL-2.0-only /* * menu.c - the menu idle governor * * Copyright (C) 2006-2007 Adam Belay <abelay@novell.com> * Copyright (C) 2009 Intel Corporation * Author: * Arjan van de Ven <arjan@linux.intel.com> */ #include <linux/kernel.h> #include <linux/cpuidle.h> #include <linux/time.h> #include <linux/ktime.h> #include <linux/hrtimer.h> #include <linux/tick.h> #include <linux/sched/stat.h> #include <linux/math64.h> #include "gov.h" #define BUCKETS 6 #define INTERVAL_SHIFT 3 #define INTERVALS (1UL << INTERVAL_SHIFT) #define RESOLUTION 1024 #define DECAY 8 #define MAX_INTERESTING (50000 * NSEC_PER_USEC) /* * Concepts and ideas behind the menu governor * * For the menu governor, there are 2 decision factors for picking a C * state: * 1) Energy break even point * 2) Latency tolerance (from pmqos infrastructure) * These two factors are treated independently. * * Energy break even point * ----------------------- * C state entry and exit have an energy cost, and a certain amount of time in * the C state is required to actually break even on this cost. CPUIDLE * provides us this duration in the "target_residency" field. So all that we * need is a good prediction of how long we'll be idle. Like the traditional * menu governor, we take the actual known "next timer event" time. * * Since there are other source of wakeups (interrupts for example) than * the next timer event, this estimation is rather optimistic. To get a * more realistic estimate, a correction factor is applied to the estimate, * that is based on historic behavior. For example, if in the past the actual * duration always was 50% of the next timer tick, the correction factor will * be 0.5. * * menu uses a running average for this correction factor, but it uses a set of * factors, not just a single factor. This stems from the realization that the * ratio is dependent on the order of magnitude of the expected duration; if we * expect 500 milliseconds of idle time the likelihood of getting an interrupt * very early is much higher than if we expect 50 micro seconds of idle time. * For this reason, menu keeps an array of 6 independent factors, that gets * indexed based on the magnitude of the expected duration. * * Repeatable-interval-detector * ---------------------------- * There are some cases where "next timer" is a completely unusable predictor: * Those cases where the interval is fixed, for example due to hardware * interrupt mitigation, but also due to fixed transfer rate devices like mice. * For this, we use a different predictor: We track the duration of the last 8 * intervals and use them to estimate the duration of the next one. */ struct menu_device { int needs_update; int tick_wakeup; u64 next_timer_ns; unsigned int bucket; unsigned int correction_factor[BUCKETS]; unsigned int intervals[INTERVALS]; int interval_ptr; }; static inline int which_bucket(u64 duration_ns) { int bucket = 0; if (duration_ns < 10ULL * NSEC_PER_USEC) return bucket; if (duration_ns < 100ULL * NSEC_PER_USEC) return bucket + 1; if (duration_ns < 1000ULL * NSEC_PER_USEC) return bucket + 2; if (duration_ns < 10000ULL * NSEC_PER_USEC) return bucket + 3; if (duration_ns < 100000ULL * NSEC_PER_USEC) return bucket + 4; return bucket + 5; } static DEFINE_PER_CPU(struct menu_device, menu_devices); static void menu_update_intervals(struct menu_device *data, unsigned int interval_us) { /* Update the repeating-pattern data. */ data->intervals[data->interval_ptr++] = interval_us; if (data->interval_ptr >= INTERVALS) data->interval_ptr = 0; } static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev); /* * Try detecting repeating patterns by keeping track of the last 8 * intervals, and checking if the standard deviation of that set * of points is below a threshold. If it is... then use the * average of these 8 points as the estimated value. */ static unsigned int get_typical_interval(struct menu_device *data) { s64 value, min_thresh = -1, max_thresh = UINT_MAX; unsigned int max, min, divisor; u64 avg, variance, avg_sq; int i; again: /* Compute the average and variance of past intervals. */ max = 0; min = UINT_MAX; avg = 0; variance = 0; divisor = 0; for (i = 0; i < INTERVALS; i++) { value = data->intervals[i]; /* * Discard the samples outside the interval between the min and * max thresholds. */ if (value <= min_thresh || value >= max_thresh) continue; divisor++; avg += value; variance += value * value; if (value > max) max = value; if (value < min) min = value; } if (!max) return UINT_MAX; if (divisor == INTERVALS) { avg >>= INTERVAL_SHIFT; variance >>= INTERVAL_SHIFT; } else { do_div(avg, divisor); do_div(variance, divisor); } avg_sq = avg * avg; variance -= avg_sq; /* * The typical interval is obtained when standard deviation is * small (stddev <= 20 us, variance <= 400 us^2) or standard * deviation is small compared to the average interval (avg > * 6*stddev, avg^2 > 36*variance). The average is smaller than * UINT_MAX aka U32_MAX, so computing its square does not * overflow a u64. We simply reject this candidate average if * the standard deviation is greater than 715 s (which is * rather unlikely). * * Use this result only if there is no timer to wake us up sooner. */ if (likely(variance <= U64_MAX/36)) { if ((avg_sq > variance * 36 && divisor * 4 >= INTERVALS * 3) || variance <= 400) return avg; } /* * If there are outliers, discard them by setting thresholds to exclude * data points at a large enough distance from the average, then * calculate the average and standard deviation again. Once we get * down to the last 3/4 of our samples, stop excluding samples. * * This can deal with workloads that have long pauses interspersed * with sporadic activity with a bunch of short pauses. * * However, if the number of remaining samples is too small to exclude * any more outliers, allow the deepest available idle state to be * selected because there are systems where the time spent by CPUs in * deep idle states is correlated to the maximum frequency the CPUs * can get to. On those systems, shallow idle states should be avoided * unless there is a clear indication that the given CPU is most likley * going to be woken up shortly. */ if (divisor * 4 <= INTERVALS * 3) return UINT_MAX; /* Update the thresholds for the next round. */ if (avg - min > max - avg) min_thresh = min; else max_thresh = max; goto again; } /** * menu_select - selects the next idle state to enter * @drv: cpuidle driver containing state data * @dev: the CPU * @stop_tick: indication on whether or not to stop the tick */ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev, bool *stop_tick) { struct menu_device *data = this_cpu_ptr(&menu_devices); s64 latency_req = cpuidle_governor_latency_req(dev->cpu); u64 predicted_ns; ktime_t delta, delta_tick; int i, idx; if (data->needs_update) { menu_update(drv, dev); data->needs_update = 0; } else if (!dev->last_residency_ns) { /* * This happens when the driver rejects the previously selected * idle state and returns an error, so update the recent * intervals table to prevent invalid information from being * used going forward. */ menu_update_intervals(data, UINT_MAX); } /* Find the shortest expected idle interval. */ predicted_ns = get_typical_interval(data) * NSEC_PER_USEC; if (predicted_ns > RESIDENCY_THRESHOLD_NS || tick_nohz_tick_stopped()) { unsigned int timer_us; /* Determine the time till the closest timer. */ delta = tick_nohz_get_sleep_length(&delta_tick); if (unlikely(delta < 0)) { delta = 0; delta_tick = 0; } data->next_timer_ns = delta; data->bucket = which_bucket(data->next_timer_ns); /* Round up the result for half microseconds. */ timer_us = div_u64((RESOLUTION * DECAY * NSEC_PER_USEC) / 2 + data->next_timer_ns * data->correction_factor[data->bucket], RESOLUTION * DECAY * NSEC_PER_USEC); /* Use the lowest expected idle interval to pick the idle state. */ predicted_ns = min((u64)timer_us * NSEC_PER_USEC, predicted_ns); /* * If the tick is already stopped, the cost of possible short * idle duration misprediction is higher because the CPU may get * stuck in a shallow idle state then. To avoid that, if * predicted_ns is small enough, say it might be mispredicted * and use the known time till the closest timer for idle state * selection unless that timer is going to trigger within * SAFE_TIMER_RANGE_NS in which case it can be regarded as a * sufficient safety net. */ if (tick_nohz_tick_stopped() && predicted_ns < TICK_NSEC && data->next_timer_ns > SAFE_TIMER_RANGE_NS) predicted_ns = data->next_timer_ns; } else { /* * Because the next timer event is not going to be determined * in this case, assume that without the tick the closest timer * will be in distant future and that the closest tick will occur * after 1/2 of the tick period. */ data->next_timer_ns = KTIME_MAX; delta_tick = TICK_NSEC / 2; data->bucket = BUCKETS - 1; } if (latency_req == 0 || ((data->next_timer_ns < drv->states[1].target_residency_ns || latency_req < drv->states[1].exit_latency_ns) && !dev->states_usage[0].disable)) { /* * In this case state[0] will be used no matter what, so return * it right away and keep the tick running if state[0] is a * polling one. */ *stop_tick = !(drv->states[0].flags & CPUIDLE_FLAG_POLLING); return 0; } /* * Find the idle state with the lowest power while satisfying * our constraints. */ idx = -1; for (i = 0; i < drv->state_count; i++) { struct cpuidle_state *s = &drv->states[i]; if (dev->states_usage[i].disable) continue; if (idx == -1) idx = i; /* first enabled state */ if (s->exit_latency_ns > latency_req) break; if (s->target_residency_ns <= predicted_ns) { idx = i; continue; } /* * Use a physical idle state instead of busy polling so long as * its target residency is below the residency threshold, its * exit latency is not greater than the predicted idle duration, * and the next timer doesn't expire soon. */ if ((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) && s->target_residency_ns < RESIDENCY_THRESHOLD_NS && s->target_residency_ns <= data->next_timer_ns && s->exit_latency_ns <= predicted_ns) { predicted_ns = s->target_residency_ns; idx = i; break; } if (predicted_ns < TICK_NSEC) break; if (!tick_nohz_tick_stopped()) { /* * If the state selected so far is shallow, waking up * early won't hurt, so retain the tick in that case and * let the governor run again in the next iteration of * the idle loop. */ predicted_ns = drv->states[idx].target_residency_ns; break; } /* * If the state selected so far is shallow and this state's * target residency matches the time till the closest timer * event, select this one to avoid getting stuck in the shallow * one for too long. */ if (drv->states[idx].target_residency_ns < TICK_NSEC && s->target_residency_ns <= delta_tick) idx = i; return idx; } if (idx == -1) idx = 0; /* No states enabled. Must use 0. */ /* * 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) || predicted_ns < TICK_NSEC) && !tick_nohz_tick_stopped()) { *stop_tick = false; if (idx > 0 && drv->states[idx].target_residency_ns > delta_tick) { /* * The tick is not going to be stopped and the target * residency of the state to be returned is not within * the time until the next timer event including the * tick, so try to correct that. */ for (i = idx - 1; i >= 0; i--) { if (dev->states_usage[i].disable) continue; idx = i; if (drv->states[i].target_residency_ns <= delta_tick) break; } } } return idx; } /** * menu_reflect - records that data structures need update * @dev: the CPU * @index: the index of actual entered state * * NOTE: it's important to be fast here because this operation will add to * the overall exit latency. */ static void menu_reflect(struct cpuidle_device *dev, int index) { struct menu_device *data = this_cpu_ptr(&menu_devices); dev->last_state_idx = index; data->needs_update = 1; data->tick_wakeup = tick_nohz_idle_got_tick(); } /** * menu_update - attempts to guess what happened after entry * @drv: cpuidle driver containing state data * @dev: the CPU */ static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev) { struct menu_device *data = this_cpu_ptr(&menu_devices); int last_idx = dev->last_state_idx; struct cpuidle_state *target = &drv->states[last_idx]; u64 measured_ns; unsigned int new_factor; /* * Try to figure out how much time passed between entry to low * power state and occurrence of the wakeup event. * * If the entered idle state didn't support residency measurements, * we use them anyway if they are short, and if long, * truncate to the whole expected time. * * Any measured amount of time will include the exit latency. * Since we are interested in when the wakeup begun, not when it * was completed, we must subtract the exit latency. However, if * the measured amount of time is less than the exit latency, * assume the state was never reached and the exit latency is 0. */ if (data->tick_wakeup && data->next_timer_ns > TICK_NSEC) { /* * The nohz code said that there wouldn't be any events within * the tick boundary (if the tick was stopped), but the idle * duration predictor had a differing opinion. Since the CPU * was woken up by a tick (that wasn't stopped after all), the * predictor was not quite right, so assume that the CPU could * have been idle long (but not forever) to help the idle * duration predictor do a better job next time. */ measured_ns = 9 * MAX_INTERESTING / 10; } else if ((drv->states[last_idx].flags & CPUIDLE_FLAG_POLLING) && dev->poll_time_limit) { /* * The CPU exited the "polling" state due to a time limit, so * the idle duration prediction leading to the selection of that * state was inaccurate. If a better prediction had been made, * the CPU might have been woken up from idle by the next timer. * Assume that to be the case. */ measured_ns = data->next_timer_ns; } else { /* measured value */ measured_ns = dev->last_residency_ns; /* Deduct exit latency */ if (measured_ns > 2 * target->exit_latency_ns) measured_ns -= target->exit_latency_ns; else measured_ns /= 2; } /* Make sure our coefficients do not exceed unity */ if (measured_ns > data->next_timer_ns) measured_ns = data->next_timer_ns; /* Update our correction ratio */ new_factor = data->correction_factor[data->bucket]; new_factor -= new_factor / DECAY; if (data->next_timer_ns > 0 && measured_ns < MAX_INTERESTING) new_factor += div64_u64(RESOLUTION * measured_ns, data->next_timer_ns); else /* * we were idle so long that we count it as a perfect * prediction */ new_factor += RESOLUTION; /* * We don't want 0 as factor; we always want at least * a tiny bit of estimated time. Fortunately, due to rounding, * new_factor will stay nonzero regardless of measured_us values * and the compiler can eliminate this test as long as DECAY > 1. */ if (DECAY == 1 && unlikely(new_factor == 0)) new_factor = 1; data->correction_factor[data->bucket] = new_factor; menu_update_intervals(data, ktime_to_us(measured_ns)); } /** * menu_enable_device - scans a CPU's states and does setup * @drv: cpuidle driver * @dev: the CPU */ static int menu_enable_device(struct cpuidle_driver *drv, struct cpuidle_device *dev) { struct menu_device *data = &per_cpu(menu_devices, dev->cpu); int i; memset(data, 0, sizeof(struct menu_device)); /* * if the correction factor is 0 (eg first time init or cpu hotplug * etc), we actually want to start out with a unity factor. */ for(i = 0; i < BUCKETS; i++) data->correction_factor[i] = RESOLUTION * DECAY; return 0; } static struct cpuidle_governor menu_governor = { .name = "menu", .rating = 20, .enable = menu_enable_device, .select = menu_select, .reflect = menu_reflect, }; /** * init_menu - initializes the governor */ static int __init init_menu(void) { return cpuidle_register_governor(&menu_governor); } postcore_initcall(init_menu);
