linux-kernel - Re: [PATCH] sched/pelt: Fix task util

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Date:   Fri, 19 Feb 2021 11:48:28 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Vincent Donnefort <vincent.donnefort@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH] sched/pelt: Fix task util_est update filtering

On Tue, 16 Feb 2021 at 17:39, <vincent.donnefort@....com> wrote:
>
> From: Vincent Donnefort <vincent.donnefort@....com>
>
> Being called for each dequeue, util_est reduces the number of its updates
> by filtering out when the EWMA signal is different from the task util_avg
> by less than 1%. It is a problem for a sudden util_avg ramp-up. Due to the
> decay from a previous high util_avg, EWMA might now be close enough to
> the new util_avg. No update would then happen while it would leave
> ue.enqueued with an out-of-date value.
>
> Taking into consideration the two util_est members, EWMA and enqueued for
> the filtering, ensures, for both, an up-to-date value.
>
> This is for now an issue only for the trace probe that might return the
> stale value. Functional-wise, it isn't (yet) a problem, as the value is

What do you mean by "it isn't (yet) a problem" ? How could this become
a problem ?

> always accessed through max(enqueued, ewma).
>

This adds more tests and or update of  struct avg.util_est. It would
be good to have an idea of the perf impact. Especially because this
only fixes a tracing problem


> This problem has been observed using LISA's UtilConvergence:test_means on
> the sd845c board.
>
> Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 794c2cb945f8..9008e0c42def 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3941,24 +3941,27 @@ static inline void util_est_dequeue(struct cfs_rq *cfs_rq,
>         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,
> + * Check if a (signed) value is within the (unsigned) util_est margin,
>   * based on the observation that:
>   *
>   *     abs(x) < y := (unsigned)(x + y - 1) < (2 * y - 1)
>   *
> - * NOTE: this only works when value + maring < INT_MAX.
> + * NOTE: this only works when value + UTIL_EST_MARGIN < INT_MAX.
>   */
> -static inline bool within_margin(int value, int margin)
> +static inline bool util_est_within_margin(int value)
>  {
> -       return ((unsigned int)(value + margin - 1) < (2 * margin - 1));
> +       return ((unsigned int)(value + UTIL_EST_MARGIN - 1) <
> +               (2 * UTIL_EST_MARGIN - 1));
>  }
>
>  static inline void util_est_update(struct cfs_rq *cfs_rq,
>                                    struct task_struct *p,
>                                    bool task_sleep)
>  {
> -       long last_ewma_diff;
> +       long last_ewma_diff, last_enqueued_diff;
>         struct util_est ue;
>
>         if (!sched_feat(UTIL_EST))
> @@ -3979,6 +3982,8 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
>         if (ue.enqueued & UTIL_AVG_UNCHANGED)
>                 return;
>
> +       last_enqueued_diff = ue.enqueued;
> +
>         /*
>          * Reset EWMA on utilization increases, the moving average is used only
>          * to smooth utilization decreases.
> @@ -3992,12 +3997,19 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
>         }
>
>         /*
> -        * Skip update of task's estimated utilization when its EWMA is
> +        * 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;
> -       if (within_margin(last_ewma_diff, (SCHED_CAPACITY_SCALE / 100)))
> +       last_enqueued_diff -= ue.enqueued;
> +       if (util_est_within_margin(last_ewma_diff)) {
> +               if (!util_est_within_margin(last_enqueued_diff)) {
> +                       ue.ewma = ue.enqueued;
> +                       goto done;
> +               }
> +
>                 return;
> +       }
>
>         /*
>          * To avoid overestimation of actual task utilization, skip updates if
> --
> 2.25.1
>