linux-kernel - Re: [PATCH] sched/fair: Allow decaying util_est when util

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Message-ID: <CAKfTPtCfaH6SvBesbKBHRNfjZHJXXC1h4NF8GoFUczE5NiRunQ@mail.gmail.com>
Date: Wed, 26 Mar 2025 18:25:30 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Pierre Gondois <pierre.gondois@....com>
Cc: linux-kernel@...r.kernel.org, Chritian Loehle <christian.loehle@....com>, 
	Hongyan Xia <hongyan.xia2@....com>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH] sched/fair: Allow decaying util_est when util_avg > CPU capa

On Tue, 25 Mar 2025 at 16:06, Pierre Gondois <pierre.gondois@....com> wrote:
>
> commit 10a35e6812aa ("sched/pelt: Skip updating util_est when
> utilization is higher than CPU's capacity")
> prevents util_est from being updated if util_avg is higher than the
> underlying CPU capacity to avoid overestimating the task when the CPU
> is capped (due to thermal issue for instance). In this scenario, the
> task will miss its deadlines and start overlapping its wake-up events
> for instance. The task will appear as always running when the CPU is
> just not powerful enough to allow having a good estimation of the
> task.
>
> commit b8c96361402a ("sched/fair/util_est: Implement faster ramp-up
> EWMA on utilization increases")
> sets ewma to util_avg when ewma > util_avg, allowing ewma to quickly
> grow instead of slowly converge to the new util_avg value when a task
> profile changes from small to big.
>
> However, the 2 conditions:
> - Check util_avg against max CPU capacity
> - Check whether util_est > util_avg
> are placed in an order such as it is possible to set util_est to a
> value higher than the CPU capacity if util_est > util_avg, but
> util_est is prevented to decay as long as:
> CPU capacity < util_avg < util_est.
>
> Just remove the check as either:
> 1.
> There is idle time on the CPU. In that case the util_avg value of the
> task is actually correct. It is possible that the task missed a
> deadline and appears bigger, but this is also the case when the
> util_avg of the task is lower than the maximum CPU capacity.
> 2.
> There is no idle time. In that case, the util_avg value might aswell
> be an under estimation of the size of the task.
> It is possible that undesired frequency spikes will appear when the
> task is later enqueued with an inflated util_est value, but the
> frequency spike might aswell be deserved. The absence of idle time
> prevents from drawing any conclusion.
>
> Signed-off-by: Pierre Gondois <pierre.gondois@....com>

This change looks reasonable to me. Did you face problems related to
this in a particular use case ?

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.rog>


> ---
>  kernel/sched/fair.c | 7 -------
>  1 file changed, 7 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c798d2795243..de7687e579c2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4918,13 +4918,6 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
>         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 (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.
> --
> 2.25.1
>