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

[Vincent Guittot]

Hi Patrick,

On Thu, 20 Jun 2019 at 17:06, Patrick Bellasi <patrick.bellasi@xxxxxxx> wrote:
>
> The estimated utilization for a task is currently defined based on:
>  - enqueued: the utilization value at the end of the last activation
>  - ewma:     an exponential moving average which samples are the enqueued values
>
> According to this definition, when a task suddenly change it's bandwidth
> requirements from small to big, the EWMA will need to collect multiple
> samples before converging up to track the new big utilization.
>
> Moreover, after the PELT scale invariance update [1], in the above scenario we
> can see that the utilization of the task has a significant drop from the first
> big activation to the following one. That's implied by the new "time-scaling"

Could you give us more details about this? I'm not sure to understand
what changes between the 1st big activation and the following one ?
The utilization implied by new "time-scaling" should be the same as
always running at max frequency with previous  method

> mechanisms instead of the previous "delta-scaling" approach.
>
> Unfortunately, these drops cannot be fully absorbed by the current util_est
> implementation. Indeed, the low-frequency filtering introduced by the "ewma" is
> entirely useless while converging up and it does not help in stabilizing sooner
> the PELT signal.
>
> To make util_est do better service in the above scenario, do change its
> definition to slow down only utilization decreases. Do that by resetting the
> "ewma" every time the last collected sample increases.
>
> This change makes also the default util_est implementation more aligned with
> the major scheduler behavior, which is to optimize for performance.
> In the future, this implementation can be further refined to consider
> task specific hints.
>
> [1] sched/fair: Update scale invariance of PELT
>     Message-ID: <tip-23127296889fe84b0762b191b5d041e8ba6f2599@xxxxxxxxxxxxxx>
>
> Signed-off-by: Patrick Bellasi <patrick.bellasi@xxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> ---
>  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 3c11dcdedcbc..27b33caaaaf4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3685,11 +3685,22 @@ 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;
> @@ -3722,6 +3733,7 @@ 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)
> --
> 2.21.0
>