cpuidle: menu: Avoid computations when result will be discarded

[linux.git] / drivers / cpuidle / governors / menu.c

/*
 * 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>
 *
 * This code is licenced under the GPL version 2 as described
 * in the COPYING file that acompanies the Linux Kernel.
 */

#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>

/*
 * Please note when changing the tuning values:
 * If (MAX_INTERESTING-1) * RESOLUTION > UINT_MAX, the result of
 * a scaling operation multiplication may overflow on 32 bit platforms.
 * In that case, #define RESOLUTION as ULL to get 64 bit result:
 * #define RESOLUTION 1024ULL
 *
 * The default values do not overflow.
 */
#define BUCKETS 12
#define INTERVAL_SHIFT 3
#define INTERVALS (1UL << INTERVAL_SHIFT)
#define RESOLUTION 1024
#define DECAY 8
#define MAX_INTERESTING 50000


/*
 * 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 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             last_state_idx;
        int             needs_update;
        int             tick_wakeup;

        unsigned int    next_timer_us;
        unsigned int    bucket;
        unsigned int    correction_factor[BUCKETS];
        unsigned int    intervals[INTERVALS];
        int             interval_ptr;
};


#define LOAD_INT(x) ((x) >> FSHIFT)
#define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100)

static inline int get_loadavg(unsigned long load)
{
        return LOAD_INT(load) * 10 + LOAD_FRAC(load) / 10;
}

static inline int which_bucket(unsigned int duration, unsigned long 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 < 10)
                return bucket;
        if (duration < 100)
                return bucket + 1;
        if (duration < 1000)
                return bucket + 2;
        if (duration < 10000)
                return bucket + 3;
        if (duration < 100000)
                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 long nr_iowaiters, unsigned long load)
{
        int mult = 1;

        /* for higher loadavg, we are more reluctant */

        mult += 2 * get_loadavg(load);

        /* for IO wait tasks (per cpu!) we add 5x each */
        mult += 10 * nr_iowaiters;

        return mult;
}

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 = UINT_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);
        int latency_req = cpuidle_governor_latency_req(dev->cpu);
        int i;
        int idx;
        unsigned int interactivity_req;
        unsigned int predicted_us;
        unsigned long nr_iowaiters, cpu_load;
        ktime_t delta_next;

        if (data->needs_update) {
                menu_update(drv, dev);
                data->needs_update = 0;
        }

        /* determine the expected residency time, round up */
        data->next_timer_us = ktime_to_us(tick_nohz_get_sleep_length(&delta_next));

        get_iowait_load(&nr_iowaiters, &cpu_load);
        data->bucket = which_bucket(data->next_timer_us, nr_iowaiters);

        if (unlikely(drv->state_count <= 1 || latency_req == 0) ||
            ((data->next_timer_us < drv->states[1].target_residency ||
              latency_req < drv->states[1].exit_latency) &&
             !drv->states[0].disabled && !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.
                 */
                *stop_tick = false;
                return 0;
        }

        /*
         * Force the result of multiplication to be 64 bits even if both
         * operands are 32 bits.
         * Make sure to round up for half microseconds.
         */
        predicted_us = DIV_ROUND_CLOSEST_ULL((uint64_t)data->next_timer_us *
                                         data->correction_factor[data->bucket],
                                         RESOLUTION * DECAY);
        /*
         * Use the lowest expected idle interval to pick the idle state.
         */
        predicted_us = min(predicted_us, get_typical_interval(data, predicted_us));

        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_us < TICK_USEC)
                        predicted_us = ktime_to_us(delta_next);
        } else {
                /*
                 * Use the performance multiplier and the user-configurable
                 * latency_req to determine the maximum exit latency.
                 */
                interactivity_req = predicted_us / performance_multiplier(nr_iowaiters, cpu_load);
                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];
                struct cpuidle_state_usage *su = &dev->states_usage[i];

                if (s->disabled || su->disable)
                        continue;

                if (idx == -1)
                        idx = i; /* first enabled state */

                if (s->target_residency > predicted_us) {
                        /*
                         * 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 <= latency_req &&
                            s->target_residency <= data->next_timer_us) {
                                predicted_us = s->target_residency;
                                idx = i;
                                break;
                        }
                        if (predicted_us < TICK_USEC)
                                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_us = drv->states[idx].target_residency;
                                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 < TICK_USEC &&
                            s->target_residency <= ktime_to_us(delta_next))
                                idx = i;

                        return idx;
                }
                if (s->exit_latency > latency_req) {
                        /*
                         * If we break out of the loop for latency reasons, use
                         * the target residency of the selected state as the
                         * expected idle duration so that the tick is retained
                         * as long as that target residency is low enough.
                         */
                        predicted_us = drv->states[idx].target_residency;
                        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_us < TICK_USEC) && !tick_nohz_tick_stopped()) {
                unsigned int delta_next_us = ktime_to_us(delta_next);

                *stop_tick = false;

                if (idx > 0 && drv->states[idx].target_residency > delta_next_us) {
                        /*
                         * 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 (drv->states[i].disabled ||
                                    dev->states_usage[i].disable)
                                        continue;

                                idx = i;
                                if (drv->states[i].target_residency <= delta_next_us)
                                        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);

        data->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 = data->last_state_idx;
        struct cpuidle_state *target = &drv->states[last_idx];
        unsigned int measured_us;
        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_us > TICK_USEC) {
                /*
                 * 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_us = 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_us = data->next_timer_us;
        } else {
                /* measured value */
                measured_us = dev->last_residency;

                /* Deduct exit latency */
                if (measured_us > 2 * target->exit_latency)
                        measured_us -= target->exit_latency;
                else
                        measured_us /= 2;
        }

        /* Make sure our coefficients do not exceed unity */
        if (measured_us > data->next_timer_us)
                measured_us = data->next_timer_us;

        /* Update our correction ratio */
        new_factor = data->correction_factor[data->bucket];
        new_factor -= new_factor / DECAY;

        if (data->next_timer_us > 0 && measured_us < MAX_INTERESTING)
                new_factor += RESOLUTION * measured_us / data->next_timer_us;
        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++] = measured_us;
        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);