Re: [PATCH v2 2/2] sched/fair: Simplify util_est

From: Alex Shi
Date: Wed Dec 06 2023 - 22:47:24 EST

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In reply to: Vincent Guittot: "Re: [PATCH v2 2/2] sched/fair: Simplify util_est"
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Looks good to me.

Reviewed-by: Alex Shi <alexs@xxxxxxxxxx>

On Sat, Dec 2, 2023 at 12:17 AM Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> With UTIL_EST_FASTUP now being permanent, we can take advantage of the
> fact that the ewma jumps directly to a higher utilization at dequeue to
> simplify util_est and remove the enqueued field.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Reviewed-and-tested-by: Lukasz Luba <lukasz.luba@xxxxxxx>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Reviewed-by: Hongyan Xia <hongyan.xia2@xxxxxxx>
> ---
> include/linux/sched.h | 49 +++++++-------------------
> kernel/sched/debug.c | 7 ++--
> kernel/sched/fair.c | 82 ++++++++++++++++---------------------------
> kernel/sched/pelt.h | 4 +--
> 4 files changed, 48 insertions(+), 94 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 8d258162deb0..03bfe9ab2951 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -415,42 +415,6 @@ struct load_weight {
> u32 inv_weight;
> };
>
> -/**
> - * struct util_est - Estimation utilization of FAIR tasks
> - * @enqueued: instantaneous estimated utilization of a task/cpu
> - * @ewma: the Exponential Weighted Moving Average (EWMA)
> - * utilization of a task
> - *
> - * Support data structure to track an Exponential Weighted Moving Average
> - * (EWMA) of a FAIR task's utilization. New samples are added to the moving
> - * average each time a task completes an activation. Sample's weight is chosen
> - * so that the EWMA will be relatively insensitive to transient changes to the
> - * task's workload.
> - *
> - * The enqueued attribute has a slightly different meaning for tasks and cpus:
> - * - task: the task's util_avg at last task dequeue time
> - * - cfs_rq: the sum of util_est.enqueued for each RUNNABLE task on that CPU
> - * Thus, the util_est.enqueued of a task represents the contribution on the
> - * estimated utilization of the CPU where that task is currently enqueued.
> - *
> - * Only for tasks we track a moving average of the past instantaneous
> - * estimated utilization. This allows to absorb sporadic drops in utilization
> - * of an otherwise almost periodic task.
> - *
> - * The UTIL_AVG_UNCHANGED flag is used to synchronize util_est with util_avg
> - * updates. When a task is dequeued, its util_est should not be updated if its
> - * util_avg has not been updated in the meantime.
> - * This information is mapped into the MSB bit of util_est.enqueued at dequeue
> - * time. Since max value of util_est.enqueued for a task is 1024 (PELT util_avg
> - * for a task) it is safe to use MSB.
> - */
> -struct util_est {
> - unsigned int enqueued;
> - unsigned int ewma;
> -#define UTIL_EST_WEIGHT_SHIFT 2
> -#define UTIL_AVG_UNCHANGED 0x80000000
> -} __attribute__((__aligned__(sizeof(u64))));
> -
> /*
> * The load/runnable/util_avg accumulates an infinite geometric series
> * (see __update_load_avg_cfs_rq() in kernel/sched/pelt.c).
> @@ -505,9 +469,20 @@ struct sched_avg {
> unsigned long load_avg;
> unsigned long runnable_avg;
> unsigned long util_avg;
> - struct util_est util_est;
> + unsigned int util_est;
> } ____cacheline_aligned;
>
> +/*
> + * The UTIL_AVG_UNCHANGED flag is used to synchronize util_est with util_avg
> + * updates. When a task is dequeued, its util_est should not be updated if its
> + * util_avg has not been updated in the meantime.
> + * This information is mapped into the MSB bit of util_est at dequeue time.
> + * Since max value of util_est for a task is 1024 (PELT util_avg for a task)
> + * it is safe to use MSB.
> + */
> +#define UTIL_EST_WEIGHT_SHIFT 2
> +#define UTIL_AVG_UNCHANGED 0x80000000
> +
> struct sched_statistics {
> #ifdef CONFIG_SCHEDSTATS
> u64 wait_start;
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 168eecc209b4..8d5d98a5834d 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -684,8 +684,8 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
> cfs_rq->avg.runnable_avg);
> SEQ_printf(m, " .%-30s: %lu\n", "util_avg",
> cfs_rq->avg.util_avg);
> - SEQ_printf(m, " .%-30s: %u\n", "util_est_enqueued",
> - cfs_rq->avg.util_est.enqueued);
> + SEQ_printf(m, " .%-30s: %u\n", "util_est",
> + cfs_rq->avg.util_est);
> SEQ_printf(m, " .%-30s: %ld\n", "removed.load_avg",
> cfs_rq->removed.load_avg);
> SEQ_printf(m, " .%-30s: %ld\n", "removed.util_avg",
> @@ -1075,8 +1075,7 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
> P(se.avg.runnable_avg);
> P(se.avg.util_avg);
> P(se.avg.last_update_time);
> - P(se.avg.util_est.ewma);
> - PM(se.avg.util_est.enqueued, ~UTIL_AVG_UNCHANGED);
> + PM(se.avg.util_est, ~UTIL_AVG_UNCHANGED);
> #endif
> #ifdef CONFIG_UCLAMP_TASK
> __PS("uclamp.min", p->uclamp_req[UCLAMP_MIN].value);
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e94d65da8d66..823dd76d0546 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4781,9 +4781,7 @@ static inline unsigned long task_runnable(struct task_struct *p)
>
> static inline unsigned long _task_util_est(struct task_struct *p)
> {
> - struct util_est ue = READ_ONCE(p->se.avg.util_est);
> -
> - return max(ue.ewma, (ue.enqueued & ~UTIL_AVG_UNCHANGED));
> + return READ_ONCE(p->se.avg.util_est) & ~UTIL_AVG_UNCHANGED;
> }
>
> static inline unsigned long task_util_est(struct task_struct *p)
> @@ -4800,9 +4798,9 @@ static inline void util_est_enqueue(struct cfs_rq *cfs_rq,
> return;
>
> /* Update root cfs_rq's estimated utilization */
> - enqueued = cfs_rq->avg.util_est.enqueued;
> + enqueued = cfs_rq->avg.util_est;
> enqueued += _task_util_est(p);
> - WRITE_ONCE(cfs_rq->avg.util_est.enqueued, enqueued);
> + WRITE_ONCE(cfs_rq->avg.util_est, enqueued);
>
> trace_sched_util_est_cfs_tp(cfs_rq);
> }
> @@ -4816,34 +4814,20 @@ static inline void util_est_dequeue(struct cfs_rq *cfs_rq,
> return;
>
> /* Update root cfs_rq's estimated utilization */
> - enqueued = cfs_rq->avg.util_est.enqueued;
> + enqueued = cfs_rq->avg.util_est;
> enqueued -= min_t(unsigned int, enqueued, _task_util_est(p));
> - WRITE_ONCE(cfs_rq->avg.util_est.enqueued, enqueued);
> + WRITE_ONCE(cfs_rq->avg.util_est, enqueued);
>
> trace_sched_util_est_cfs_tp(cfs_rq);
> }
>
> #define UTIL_EST_MARGIN (SCHED_CAPACITY_SCALE / 100)
>
> -/*
> - * Check if a (signed) value is within a specified (unsigned) margin,
> - * based on the observation that:
> - *
> - * abs(x) < y := (unsigned)(x + y - 1) < (2 * y - 1)
> - *
> - * NOTE: this only works when value + margin < INT_MAX.
> - */
> -static inline bool within_margin(int value, int margin)
> -{
> - return ((unsigned int)(value + margin - 1) < (2 * margin - 1));
> -}
> -
> static inline void util_est_update(struct cfs_rq *cfs_rq,
> struct task_struct *p,
> bool task_sleep)
> {
> - long last_ewma_diff, last_enqueued_diff;
> - struct util_est ue;
> + unsigned int ewma, dequeued, last_ewma_diff;
>
> if (!sched_feat(UTIL_EST))
> return;
> @@ -4855,23 +4839,25 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
> if (!task_sleep)
> return;
>
> + /* Get current estimate of utilization */
> + ewma = READ_ONCE(p->se.avg.util_est);
> +
> /*
> * If the PELT values haven't changed since enqueue time,
> * skip the util_est update.
> */
> - ue = p->se.avg.util_est;
> - if (ue.enqueued & UTIL_AVG_UNCHANGED)
> + if (ewma & UTIL_AVG_UNCHANGED)
> return;
>
> - last_enqueued_diff = ue.enqueued;
> + /* Get utilization at dequeue */
> + dequeued = task_util(p);
>
> /*
> * Reset EWMA on utilization increases, the moving average is used only
> * to smooth utilization decreases.
> */
> - ue.enqueued = task_util(p);
> - if (ue.ewma < ue.enqueued) {
> - ue.ewma = ue.enqueued;
> + if (ewma <= dequeued) {
> + ewma = dequeued;
> goto done;
> }
>
> @@ -4879,27 +4865,22 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
> * Skip update of task's estimated utilization when its members are
> * already ~1% close to its last activation value.
> */
> - last_ewma_diff = ue.enqueued - ue.ewma;
> - last_enqueued_diff -= ue.enqueued;
> - if (within_margin(last_ewma_diff, UTIL_EST_MARGIN)) {
> - if (!within_margin(last_enqueued_diff, UTIL_EST_MARGIN))
> - goto done;
> -
> - return;
> - }
> + last_ewma_diff = ewma - dequeued;
> + if (last_ewma_diff < UTIL_EST_MARGIN)
> + goto done;
>
> /*
> * To avoid overestimation of actual task utilization, skip updates if
> * we cannot grant there is idle time in this CPU.
> */
> - if (task_util(p) > arch_scale_cpu_capacity(cpu_of(rq_of(cfs_rq))))
> + if (dequeued > arch_scale_cpu_capacity(cpu_of(rq_of(cfs_rq))))
> return;
>
> /*
> * To avoid underestimate of task utilization, skip updates of EWMA if
> * we cannot grant that thread got all CPU time it wanted.
> */
> - if ((ue.enqueued + UTIL_EST_MARGIN) < task_runnable(p))
> + if ((dequeued + UTIL_EST_MARGIN) < task_runnable(p))
> goto done;
>
>
> @@ -4907,25 +4888,24 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
> * Update Task's estimated utilization
> *
> * When *p completes an activation we can consolidate another sample
> - * of the task size. This is done by storing the current PELT value
> - * as ue.enqueued and by using this value to update the Exponential
> - * Weighted Moving Average (EWMA):
> + * of the task size. This is done by using this value to update the
> + * Exponential Weighted Moving Average (EWMA):
> *
> * ewma(t) = w * task_util(p) + (1-w) * ewma(t-1)
> * = w * task_util(p) + ewma(t-1) - w * ewma(t-1)
> * = w * (task_util(p) - ewma(t-1)) + ewma(t-1)
> - * = w * ( last_ewma_diff ) + ewma(t-1)
> - * = w * (last_ewma_diff + ewma(t-1) / w)
> + * = w * ( -last_ewma_diff ) + ewma(t-1)
> + * = w * (-last_ewma_diff + ewma(t-1) / w)
> *
> * Where 'w' is the weight of new samples, which is configured to be
> * 0.25, thus making w=1/4 ( >>= UTIL_EST_WEIGHT_SHIFT)
> */
> - ue.ewma <<= UTIL_EST_WEIGHT_SHIFT;
> - ue.ewma += last_ewma_diff;
> - ue.ewma >>= UTIL_EST_WEIGHT_SHIFT;
> + ewma <<= UTIL_EST_WEIGHT_SHIFT;
> + ewma -= last_ewma_diff;
> + ewma >>= UTIL_EST_WEIGHT_SHIFT;
> done:
> - ue.enqueued |= UTIL_AVG_UNCHANGED;
> - WRITE_ONCE(p->se.avg.util_est, ue);
> + ewma |= UTIL_AVG_UNCHANGED;
> + WRITE_ONCE(p->se.avg.util_est, ewma);
>
> trace_sched_util_est_se_tp(&p->se);
> }
> @@ -7653,16 +7633,16 @@ cpu_util(int cpu, struct task_struct *p, int dst_cpu, int boost)
> if (sched_feat(UTIL_EST)) {
> unsigned long util_est;
>
> - util_est = READ_ONCE(cfs_rq->avg.util_est.enqueued);
> + util_est = READ_ONCE(cfs_rq->avg.util_est);
>
> /*
> * During wake-up @p isn't enqueued yet and doesn't contribute
> - * to any cpu_rq(cpu)->cfs.avg.util_est.enqueued.
> + * to any cpu_rq(cpu)->cfs.avg.util_est.
> * If @dst_cpu == @cpu add it to "simulate" cpu_util after @p
> * has been enqueued.
> *
> * During exec (@dst_cpu = -1) @p is enqueued and does
> - * contribute to cpu_rq(cpu)->cfs.util_est.enqueued.
> + * contribute to cpu_rq(cpu)->cfs.util_est.
> * Remove it to "simulate" cpu_util without @p's contribution.
> *
> * Despite the task_on_rq_queued(@p) check there is still a
> diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h
> index 3a0e0dc28721..9e1083465fbc 100644
> --- a/kernel/sched/pelt.h
> +++ b/kernel/sched/pelt.h
> @@ -52,13 +52,13 @@ static inline void cfs_se_util_change(struct sched_avg *avg)
> return;
>
> /* Avoid store if the flag has been already reset */
> - enqueued = avg->util_est.enqueued;
> + enqueued = avg->util_est;
> if (!(enqueued & UTIL_AVG_UNCHANGED))
> return;
>
> /* Reset flag to report util_avg has been updated */
> enqueued &= ~UTIL_AVG_UNCHANGED;
> - WRITE_ONCE(avg->util_est.enqueued, enqueued);
> + WRITE_ONCE(avg->util_est, enqueued);
> }
>
> static inline u64 rq_clock_pelt(struct rq *rq)
> --
> 2.34.1
>

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