linux-kernel - Re: [PATCH] sched/pelt: avoid underestimate of task utilization

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Date:   Fri, 24 Nov 2023 10:43:59 +0000
From:   Hongyan Xia <hongyan.xia2@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     lukasz.luba@....com, mingo@...hat.com, mgorman@...e.de,
        rostedt@...dmis.org, bristot@...hat.com, peterz@...radead.org,
        rafael@...nel.org, vschneid@...hat.com, dietmar.eggemann@....com,
        juri.lelli@...hat.com, bsegall@...gle.com, qyousef@...alina.io,
        viresh.kumar@...aro.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH] sched/pelt: avoid underestimate of task utilization

On 22/11/2023 14:01, Vincent Guittot wrote:
> [...]
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 07f555857698..eeb505d28905 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4774,6 +4774,11 @@ static inline unsigned long task_util(struct task_struct *p)
>   	return READ_ONCE(p->se.avg.util_avg);
>   }
>   
> +static inline unsigned long task_runnable(struct task_struct *p)
> +{
> +	return READ_ONCE(p->se.avg.runnable_avg);
> +}
> +
>   static inline unsigned long _task_util_est(struct task_struct *p)
>   {
>   	struct util_est ue = READ_ONCE(p->se.avg.util_est);
> @@ -4892,6 +4897,14 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
>   	if (task_util(p) > 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))
> +		goto done;
> +
> +

Actually, does this also skip util_est increases as well, assuming no 
FASTUP? When a task is ramping up, another task could join and then 
blocks this task from ramping up its util_est.

Or do we think this is intended behavior for !FASTUP?

>   	/*
>   	 * Update Task's estimated utilization
>   	 *