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Diffstat (limited to 'drivers/cpuidle/governors/menu.c')
| -rw-r--r-- | drivers/cpuidle/governors/menu.c | 579 |
1 files changed, 579 insertions, 0 deletions
diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c new file mode 100644 index 000000000..c4922684f --- /dev/null +++ b/drivers/cpuidle/governors/menu.c @@ -0,0 +1,579 @@ +// 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.h> +#include <linux/sched/loadavg.h> +#include <linux/sched/stat.h> +#include <linux/math64.h> + +#define BUCKETS 12 +#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 3 decision factors for picking a C + * state: + * 1) Energy break even point + * 2) Performance impact + * 3) Latency tolerance (from pmqos infrastructure) + * These three 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 start with 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, however 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. A second independent factor that has big impact on + * the actual factor is if there is (disk) IO outstanding or not. + * (as a special twist, we consider every sleep longer than 50 milliseconds + * as perfect; there are no power gains for sleeping longer than this) + * + * For these two reasons we keep an array of 12 independent factors, that gets + * indexed based on the magnitude of the expected duration as well as the + * "is IO outstanding" property. + * + * 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 such as + * mice. + * For this, we use a different predictor: We track the duration of the last 8 + * intervals and if the stand deviation of these 8 intervals is below a + * threshold value, we use the average of these intervals as prediction. + * + * Limiting Performance Impact + * --------------------------- + * C states, especially those with large exit latencies, can have a real + * noticeable impact on workloads, which is not acceptable for most sysadmins, + * and in addition, less performance has a power price of its own. + * + * As a general rule of thumb, menu assumes that the following heuristic + * holds: + * The busier the system, the less impact of C states is acceptable + * + * This rule-of-thumb is implemented using a performance-multiplier: + * If the exit latency times the performance multiplier is longer than + * the predicted duration, the C state is not considered a candidate + * for selection due to a too high performance impact. So the higher + * this multiplier is, the longer we need to be idle to pick a deep C + * state, and thus the less likely a busy CPU will hit such a deep + * C state. + * + * Two factors are used in determing this multiplier: + * a value of 10 is added for each point of "per cpu load average" we have. + * a value of 5 points is added for each process that is waiting for + * IO on this CPU. + * (these values are experimentally determined) + * + * The load average factor gives a longer term (few seconds) input to the + * decision, while the iowait value gives a cpu local instantanious input. + * The iowait factor may look low, but realize that this is also already + * represented in the system load average. + * + */ + +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, unsigned int nr_iowaiters) +{ + int bucket = 0; + + /* + * We keep two groups of stats; one with no + * IO pending, one without. + * This allows us to calculate + * E(duration)|iowait + */ + if (nr_iowaiters) + bucket = BUCKETS/2; + + 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; +} + +/* + * Return a multiplier for the exit latency that is intended + * to take performance requirements into account. + * The more performance critical we estimate the system + * to be, the higher this multiplier, and thus the higher + * the barrier to go to an expensive C state. + */ +static inline int performance_multiplier(unsigned int nr_iowaiters) +{ + /* for IO wait tasks (per cpu!) we add 10x each */ + return 1 + 10 * nr_iowaiters; +} + +static DEFINE_PER_CPU(struct menu_device, menu_devices); + +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, + unsigned int predicted_us) +{ + int i, divisor; + unsigned int min, max, thresh, avg; + uint64_t sum, variance; + + thresh = INT_MAX; /* Discard outliers above this value */ + +again: + + /* First calculate the average of past intervals */ + min = UINT_MAX; + max = 0; + sum = 0; + divisor = 0; + for (i = 0; i < INTERVALS; i++) { + unsigned int value = data->intervals[i]; + if (value <= thresh) { + sum += value; + divisor++; + if (value > max) + max = value; + + if (value < min) + min = value; + } + } + + /* + * If the result of the computation is going to be discarded anyway, + * avoid the computation altogether. + */ + if (min >= predicted_us) + return UINT_MAX; + + if (divisor == INTERVALS) + avg = sum >> INTERVAL_SHIFT; + else + avg = div_u64(sum, divisor); + + /* Then try to determine variance */ + variance = 0; + for (i = 0; i < INTERVALS; i++) { + unsigned int value = data->intervals[i]; + if (value <= thresh) { + int64_t diff = (int64_t)value - avg; + variance += diff * diff; + } + } + if (divisor == INTERVALS) + variance >>= INTERVAL_SHIFT; + else + do_div(variance, divisor); + + /* + * 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 ((((u64)avg*avg > variance*36) && (divisor * 4 >= INTERVALS * 3)) + || variance <= 400) { + return avg; + } + } + + /* + * If we have outliers to the upside in our distribution, discard + * those by setting the threshold to exclude these outliers, then + * calculate the average and standard deviation again. Once we get + * down to the bottom 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. + */ + if ((divisor * 4) <= INTERVALS * 3) + return UINT_MAX; + + thresh = max - 1; + 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); + unsigned int predicted_us; + u64 predicted_ns; + u64 interactivity_req; + unsigned int nr_iowaiters; + ktime_t delta, delta_tick; + int i, idx; + + if (data->needs_update) { + menu_update(drv, dev); + data->needs_update = 0; + } + + /* determine the expected residency time, round up */ + delta = tick_nohz_get_sleep_length(&delta_tick); + if (unlikely(delta < 0)) { + delta = 0; + delta_tick = 0; + } + data->next_timer_ns = delta; + + nr_iowaiters = nr_iowait_cpu(dev->cpu); + data->bucket = which_bucket(data->next_timer_ns, nr_iowaiters); + + if (unlikely(drv->state_count <= 1 || 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; + } + + /* Round up the result for half microseconds. */ + predicted_us = div_u64(data->next_timer_ns * + data->correction_factor[data->bucket] + + (RESOLUTION * DECAY * NSEC_PER_USEC) / 2, + RESOLUTION * DECAY * NSEC_PER_USEC); + /* Use the lowest expected idle interval to pick the idle state. */ + predicted_ns = (u64)min(predicted_us, + get_typical_interval(data, predicted_us)) * + NSEC_PER_USEC; + + if (tick_nohz_tick_stopped()) { + /* + * If the tick is already stopped, the cost of possible short + * idle duration misprediction is much higher, because the CPU + * may be stuck in a shallow idle state for a long time as a + * result of it. In that case say we might mispredict and use + * the known time till the closest timer event for the idle + * state selection. + */ + if (predicted_ns < TICK_NSEC) + predicted_ns = data->next_timer_ns; + } else { + /* + * Use the performance multiplier and the user-configurable + * latency_req to determine the maximum exit latency. + */ + interactivity_req = div64_u64(predicted_ns, + performance_multiplier(nr_iowaiters)); + if (latency_req > interactivity_req) + latency_req = interactivity_req; + } + + /* + * 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->target_residency_ns > predicted_ns) { + /* + * Use a physical idle state, not busy polling, unless + * a timer is going to trigger soon enough. + */ + if ((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) && + s->exit_latency_ns <= latency_req && + s->target_residency_ns <= data->next_timer_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 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 (s->exit_latency_ns > latency_req) + break; + + idx = i; + } + + 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; + + /* update the repeating-pattern data */ + data->intervals[data->interval_ptr++] = ktime_to_us(measured_ns); + if (data->interval_ptr >= INTERVALS) + data->interval_ptr = 0; +} + +/** + * 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); |