kernel/sched/pelt.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Per Entity Load Tracking
 *
 *  Copyright (C) 2007 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
 *
 *  Interactivity improvements by Mike Galbraith
 *  (C) 2007 Mike Galbraith <efault@gmx.de>
 *
 *  Various enhancements by Dmitry Adamushko.
 *  (C) 2007 Dmitry Adamushko <dmitry.adamushko@gmail.com>
 *
 *  Group scheduling enhancements by Srivatsa Vaddagiri
 *  Copyright IBM Corporation, 2007
 *  Author: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
 *
 *  Scaled math optimizations by Thomas Gleixner
 *  Copyright (C) 2007, Thomas Gleixner <tglx@linutronix.de>
 *
 *  Adaptive scheduling granularity, math enhancements by Peter Zijlstra
 *  Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra
 *
 *  Move PELT related code from fair.c into this pelt.c file
 *  Author: Vincent Guittot <vincent.guittot@linaro.org>
 */

#include <linux/sched.h>
#include "sched.h"
#include "pelt.h"

int pelt_load_avg_period = PELT32_LOAD_AVG_PERIOD;
int pelt_load_avg_max = PELT32_LOAD_AVG_MAX;
const u32 *pelt_runnable_avg_yN_inv = pelt32_runnable_avg_yN_inv;

static int __init set_pelt(char *str)
{
    int rc, num;

    rc = kstrtoint(str, 0, &num);
    if (rc) {
        pr_err("%s: kstrtoint failed. rc=%d\n", __func__, rc);
        return 0;
    }

    switch (num) {
        case PELT8_LOAD_AVG_PERIOD:
            pelt_load_avg_period = PELT8_LOAD_AVG_PERIOD;
            pelt_load_avg_max = PELT8_LOAD_AVG_MAX;
            pelt_runnable_avg_yN_inv = pelt8_runnable_avg_yN_inv;
            pr_info("PELT half life is set to %dms\n", num);
            break;
        case PELT32_LOAD_AVG_PERIOD:
            pelt_load_avg_period = PELT32_LOAD_AVG_PERIOD;
            pelt_load_avg_max = PELT32_LOAD_AVG_MAX;
            pelt_runnable_avg_yN_inv = pelt32_runnable_avg_yN_inv;
            pr_info("PELT half life is set to %dms\n", num);
            break;
        default:
            pr_err("Default PELT half life is 32ms\n");
    }

    return 0;
}

early_param("pelt", set_pelt);

/*
 * Approximate:
 *   val * y^n,    where y^32 ~= 0.5 (~1 scheduling period)
 */
static u64 decay_load(u64 val, u64 n)
{
    unsigned int local_n;

    if (unlikely(n > LOAD_AVG_PERIOD * 0x3f)) {
        return 0;
    }

    /* after bounds checking we can collapse to 32-bit */
    local_n = n;

    /*
     * As y^PERIOD = 1/2, we can combine
     *    y^n = 1/2^(n/PERIOD) * y^(n%PERIOD)
     * With a look-up table which covers y^n (n<PERIOD)
     *
     * To achieve constant time decay_load.
     */
    if (unlikely(local_n >= LOAD_AVG_PERIOD)) {
        val >>= local_n / LOAD_AVG_PERIOD;
        local_n %= LOAD_AVG_PERIOD;
    }

    val = mul_u64_u32_shr(val, pelt_runnable_avg_yN_inv[local_n], 0x20);
    return val;
}

static u32 __accumulate_pelt_segments(u64 periods, u32 d1, u32 d3)
{
    u32 c1, c2, c3 = d3; /* y^0 == 1 */

    /*
     * c1 = d1 y^p
     */
    c1 = decay_load((u64)d1, periods);

    /*
     *            p-1
     * c2 = 1024 \Sum y^n
     *            n=1
     *
     *              inf        inf
     *    = 1024 ( \Sum y^n - \Sum y^n - y^0 )
     *              n=0        n=p
     */
    c2 = LOAD_AVG_MAX - decay_load(LOAD_AVG_MAX, periods) - 0x400;

    return c1 + c2 + c3;
}

/*
 * Accumulate the three separate parts of the sum; d1 the remainder
 * of the last (incomplete) period, d2 the span of full periods and d3
 * the remainder of the (incomplete) current period.
 *
 *           d1          d2           d3
 *           ^           ^            ^
 *           |           |            |
 *         |<->|<----------------->|<--->|
 * ... |---x---|------| ... |------|-----x (now)
 *
 *                           p-1
 * u' = (u + d1) y^p + 1024 \Sum y^n + d3 y^0
 *                           n=1
 *
 *    = u y^p +                    (Step 1)
 *
 *                     p-1
 *      d1 y^p + 1024 \Sum y^n + d3 y^0        (Step 2)
 *                     n=1
 */
static __always_inline u32 accumulate_sum(u64 delta, struct sched_avg *sa, unsigned long load, unsigned long runnable,
                                          int running)
{
    u32 contrib = (u32)delta; /* p == 0 -> delta < 1024 */
    u64 periods;

    delta += sa->period_contrib;
    periods = delta / 0x400; /* A period is 1024us (~1ms) */

    /*
     * Step 1: decay old *_sum if we crossed period boundaries.
     */
    if (periods) {
        sa->load_sum = decay_load(sa->load_sum, periods);
        sa->runnable_sum = decay_load(sa->runnable_sum, periods);
        sa->util_sum = decay_load((u64)(sa->util_sum), periods);

        /*
         * Step 2
         */
        delta %= 0x400;
        if (load) {
            /*
             * This relies on the:
             *
             * if (!load)
             *    runnable = running = 0;
             *
             * clause from ___update_load_sum(); this results in
             * the below usage of @contrib to dissapear entirely,
             * so no point in calculating it.
             */
            contrib = __accumulate_pelt_segments(periods, 0x400 - sa->period_contrib, delta);
        }
    }
    sa->period_contrib = delta;

    if (load) {
        sa->load_sum += load * contrib;
    }
    if (runnable) {
        sa->runnable_sum += runnable * contrib << SCHED_CAPACITY_SHIFT;
    }
    if (running) {
        sa->util_sum += contrib << SCHED_CAPACITY_SHIFT;
    }

    return periods;
}

/*
 * We can represent the historical contribution to runnable average as the
 * coefficients of a geometric series.  To do this we sub-divide our runnable
 * history into segments of approximately 1ms (1024us); label the segment that
 * occurred N-ms ago p_N, with p_0 corresponding to the current period, e.g.
 *
 * [<- 1024us ->|<- 1024us ->|<- 1024us ->| ...
 *      p0            p1           p2
 *     (now)       (~1ms ago)  (~2ms ago)
 *
 * Let u_i denote the fraction of p_i that the entity was runnable.
 *
 * We then designate the fractions u_i as our co-efficients, yielding the
 * following representation of historical load:
 *   u_0 + u_1*y + u_2*y^2 + u_3*y^3 + ...
 *
 * We choose y based on the with of a reasonably scheduling period, fixing:
 *   y^32 = 0.5
 *
 * This means that the contribution to load ~32ms ago (u_32) will be weighted
 * approximately half as much as the contribution to load within the last ms
 * (u_0).
 *
 * When a period "rolls over" and we have new u_0`, multiplying the previous
 * sum again by y is sufficient to update:
 *   load_avg = u_0` + y*(u_0 + u_1*y + u_2*y^2 + ... )
 *            = u_0 + u_1*y + u_2*y^2 + ... [re-labeling u_i --> u_{i+1}]
 */
static __always_inline int ___update_load_sum(u64 now, struct sched_avg *sa, unsigned long load, unsigned long runnable,
                                              int running)
{
    u64 delta;

    delta = now - sa->last_update_time;
    /*
     * This should only happen when time goes backwards, which it
     * unfortunately does during sched clock init when we swap over to TSC.
     */
    if ((s64)delta < 0) {
        sa->last_update_time = now;
        return 0;
    }

    /*
     * Use 1024ns as the unit of measurement since it's a reasonable
     * approximation of 1us and fast to compute.
     */
    delta >>= 0xa;
    if (!delta) {
        return 0;
    }

    sa->last_update_time += delta << 0xa;

    /*
     * running is a subset of runnable (weight) so running can't be set if
     * runnable is clear. But there are some corner cases where the current
     * se has been already dequeued but cfs_rq->curr still points to it.
     * This means that weight will be 0 but not running for a sched_entity
     * but also for a cfs_rq if the latter becomes idle. As an example,
     * this happens during idle_balance() which calls
     * update_blocked_averages().
     *
     * Also see the comment in accumulate_sum().
     */
    if (!load) {
        runnable = running = 0;
    }

    /*
     * Now we know we crossed measurement unit boundaries. The *_avg
     * accrues by two steps:
     *
     * Step 1: accumulate *_sum since last_update_time. If we haven't
     * crossed period boundaries, finish.
     */
    if (!accumulate_sum(delta, sa, load, runnable, running)) {
        return 0;
    }

    return 1;
}

/*
 * When syncing *_avg with *_sum, we must take into account the current
 * position in the PELT segment otherwise the remaining part of the segment
 * will be considered as idle time whereas it's not yet elapsed and this will
 * generate unwanted oscillation in the range [1002..1024[.
 *
 * The max value of *_sum varies with the position in the time segment and is
 * equals to :
 *
 *   LOAD_AVG_MAX*y + sa->period_contrib
 *
 * which can be simplified into:
 *
 *   LOAD_AVG_MAX - 1024 + sa->period_contrib
 *
 * because LOAD_AVG_MAX*y == LOAD_AVG_MAX-1024
 *
 * The same care must be taken when a sched entity is added, updated or
 * removed from a cfs_rq and we need to update sched_avg. Scheduler entities
 * and the cfs rq, to which they are attached, have the same position in the
 * time segment because they use the same clock. This means that we can use
 * the period_contrib of cfs_rq when updating the sched_avg of a sched_entity
 * if it's more convenient.
 */
static __always_inline void ___update_load_avg(struct sched_avg *sa, unsigned long load)
{
    u32 divider = get_pelt_divider(sa);

    /*
     * Step 2: update *_avg.
     */
    sa->load_avg = div_u64(load * sa->load_sum, divider);
    sa->runnable_avg = div_u64(sa->runnable_sum, divider);
    WRITE_ONCE(sa->util_avg, sa->util_sum / divider);
}

/*
 * sched_entity
 *
 *   task:
 *     se_weight()   = se->load.weight
 *     se_runnable() = !!on_rq
 *
 *   group: [ see update_cfs_group() ]
 *     se_weight()   = tg->weight * grq->load_avg / tg->load_avg
 *     se_runnable() = grq->h_nr_running
 *
 *   runnable_sum = se_runnable() * runnable = grq->runnable_sum
 *   runnable_avg = runnable_sum
 *
 *   load_sum := runnable
 *   load_avg = se_weight(se) * load_sum
 *
 * cfq_rq
 *
 *   runnable_sum = \Sum se->avg.runnable_sum
 *   runnable_avg = \Sum se->avg.runnable_avg
 *
 *   load_sum = \Sum se_weight(se) * se->avg.load_sum
 *   load_avg = \Sum se->avg.load_avg
 */

int __update_load_avg_blocked_se(u64 now, struct sched_entity *se)
{
    if (___update_load_sum(now, &se->avg, 0, 0, 0)) {
        ___update_load_avg(&se->avg, se_weight(se));
        trace_pelt_se_tp(se);
        return 1;
    }

    return 0;
}

int __update_load_avg_se(u64 now, struct cfs_rq *cfs_rq, struct sched_entity *se)
{
    if (___update_load_sum(now, &se->avg, !!se->on_rq, se_runnable(se), cfs_rq->curr == se)) {
        ___update_load_avg(&se->avg, se_weight(se));
        cfs_se_util_change(&se->avg);
        trace_pelt_se_tp(se);
        return 1;
    }

    return 0;
}

int __update_load_avg_cfs_rq(u64 now, struct cfs_rq *cfs_rq)
{
    if (___update_load_sum(now, &cfs_rq->avg, scale_load_down(cfs_rq->load.weight), cfs_rq->h_nr_running,
                           cfs_rq->curr != NULL)) {
        ___update_load_avg(&cfs_rq->avg, 1);
        trace_pelt_cfs_tp(cfs_rq);
        return 1;
    }

    return 0;
}

/*
 * rt_rq
 *
 *   util_sum = \Sum se->avg.util_sum but se->avg.util_sum is not tracked
 *   util_sum = cpu_scale * load_sum
 *   runnable_sum = util_sum
 *
 *   load_avg and runnable_avg are not supported and meaningless.
 *
 */

int update_rt_rq_load_avg(u64 now, struct rq *rq, int running)
{
    if (___update_load_sum(now, &rq->avg_rt, running, running, running)) {
        ___update_load_avg(&rq->avg_rt, 1);
        trace_pelt_rt_tp(rq);
        return 1;
    }

    return 0;
}

/*
 * dl_rq
 *
 *   util_sum = \Sum se->avg.util_sum but se->avg.util_sum is not tracked
 *   util_sum = cpu_scale * load_sum
 *   runnable_sum = util_sum
 *
 *   load_avg and runnable_avg are not supported and meaningless.
 *
 */

int update_dl_rq_load_avg(u64 now, struct rq *rq, int running)
{
    if (___update_load_sum(now, &rq->avg_dl, running, running, running)) {
        ___update_load_avg(&rq->avg_dl, 1);
        trace_pelt_dl_tp(rq);
        return 1;
    }

    return 0;
}

#ifdef CONFIG_SCHED_THERMAL_PRESSURE
/*
 * thermal
 *
 *   load_sum = \Sum se->avg.load_sum but se->avg.load_sum is not tracked
 *
 *   util_avg and runnable_load_avg are not supported and meaningless.
 *
 * Unlike rt/dl utilization tracking that track time spent by a cpu
 * running a rt/dl task through util_avg, the average thermal pressure is
 * tracked through load_avg. This is because thermal pressure signal is
 * time weighted "delta" capacity unlike util_avg which is binary.
 * "delta capacity" =  actual capacity  -
 *            capped capacity a cpu due to a thermal event.
 */

int update_thermal_load_avg(u64 now, struct rq *rq, u64 capacity)
{
    if (___update_load_sum(now, &rq->avg_thermal, capacity, capacity, capacity)) {
        ___update_load_avg(&rq->avg_thermal, 1);
        trace_pelt_thermal_tp(rq);
        return 1;
    }

    return 0;
}
#endif

#ifdef CONFIG_HAVE_SCHED_AVG_IRQ
/*
 * irq
 *
 *   util_sum = \Sum se->avg.util_sum but se->avg.util_sum is not tracked
 *   util_sum = cpu_scale * load_sum
 *   runnable_sum = util_sum
 *
 *   load_avg and runnable_avg are not supported and meaningless.
 *
 */

int update_irq_load_avg(struct rq *rq, u64 running)
{
    int ret = 0;

    /*
     * We can't use clock_pelt because irq time is not accounted in
     * clock_task. Instead we directly scale the running time to
     * reflect the real amount of computation
     */
    running = cap_scale(running, arch_scale_freq_capacity(cpu_of(rq)));
    running = cap_scale(running, arch_scale_cpu_capacity(cpu_of(rq)));

    /*
     * We know the time that has been used by interrupt since last update
     * but we don't when. Let be pessimistic and assume that interrupt has
     * happened just before the update. This is not so far from reality
     * because interrupt will most probably wake up task and trig an update
     * of rq clock during which the metric is updated.
     * We start to decay with normal context time and then we add the
     * interrupt context time.
     * We can safely remove running from rq->clock because
     * rq->clock += delta with delta >= running
     */
    ret = ___update_load_sum(rq->clock - running, &rq->avg_irq, 0, 0, 0);
    ret += ___update_load_sum(rq->clock, &rq->avg_irq, 1, 1, 1);
    if (ret) {
        ___update_load_avg(&rq->avg_irq, 1);
        trace_pelt_irq_tp(rq);
    }

    return ret;
}
#endif