Re: [PATCH v2] sched/fair: util_est: fast ramp-up EWMA on utilization increases

[Vincent Guittot]

On Wed, 23 Oct 2019 at 22:56, Patrick Bellasi
<patrick.bellasi@xxxxxxxxxx> wrote:
>
> The estimated utilization for a task:
>
>    util_est = max(util_avg, est.enqueue, est.ewma)
>
> is defined based on:
>  - util_avg: the PELT defined utilization
>  - est.enqueued: the util_avg at the end of the last activation
>  - est.ewma:     a exponential moving average on the est.enqueued
>                  samples
>
> According to this definition, when a task suddenly change its bandwidth
> requirements from small to big, the EWMA will need to collect multiple
> samples before converging up to track the new big utilization.
>
> This slow convergence towards bigger utilization values is not
> aligned to the default scheduler behavior, which is to optimize for
> performance. Moreover, the est.ewma component fails to compensate for
> temporarely utilization drops which spans just few est.enqueued samples.
>
> To let util_est do a better job in the scenario depicted above, change
> its definition by making util_est directly follow upward motion and
> only decay the est.ewma on downward.
>
> Signed-off-by: Patrick Bellasi <patrick.bellasi@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Acked-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

> ---
>  kernel/sched/fair.c     | 14 +++++++++++++-
>  kernel/sched/features.h |  1 +
>  2 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a81c36472822..a14487462b6c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3768,11 +3768,22 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
>         if (ue.enqueued & UTIL_AVG_UNCHANGED)
>                 return;
>
> +       /*
> +        * Reset EWMA on utilization increases, the moving average is used only
> +        * to smooth utilization decreases.
> +        */
> +       ue.enqueued = (task_util(p) | UTIL_AVG_UNCHANGED);
> +       if (sched_feat(UTIL_EST_FASTUP)) {
> +               if (ue.ewma < ue.enqueued) {
> +                       ue.ewma = ue.enqueued;
> +                       goto done;
> +               }
> +       }
> +
>         /*
>          * Skip update of task's estimated utilization when its EWMA is
>          * already ~1% close to its last activation value.
>          */
> -       ue.enqueued = (task_util(p) | UTIL_AVG_UNCHANGED);
>         last_ewma_diff = ue.enqueued - ue.ewma;
>         if (within_margin(last_ewma_diff, (SCHED_CAPACITY_SCALE / 100)))
>                 return;
> @@ -3805,6 +3816,7 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
>         ue.ewma <<= UTIL_EST_WEIGHT_SHIFT;
>         ue.ewma  += last_ewma_diff;
>         ue.ewma >>= UTIL_EST_WEIGHT_SHIFT;
> +done:
>         WRITE_ONCE(p->se.avg.util_est, ue);
>  }
>
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index 2410db5e9a35..7481cd96f391 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -89,3 +89,4 @@ SCHED_FEAT(WA_BIAS, true)
>   * UtilEstimation. Use estimated CPU utilization.
>   */
>  SCHED_FEAT(UTIL_EST, true)
> +SCHED_FEAT(UTIL_EST_FASTUP, true)
> --
> 2.17.1
>