linux-kernel - Re: [PATCH] sched/uclamp: Let each sched

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Message-ID: <4394f2a7-b4e8-419a-a299-f1afa560c944@arm.com>
Date: Wed, 5 Mar 2025 19:22:00 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Hongyan Xia <hongyan.xia2@....com>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
 Tejun Heo <tj@...nel.org>, David Vernet <void@...ifault.com>,
 Andrea Righi <arighi@...dia.com>, Changwoo Min <changwoo@...lia.com>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/uclamp: Let each sched_class handle uclamp

On 27/02/2025 14:54, Hongyan Xia wrote:

[...]

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 857808da23d8..7e5a653811ad 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6941,8 +6941,10 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	 * Let's add the task's estimated utilization to the cfs_rq's
>  	 * estimated utilization, before we update schedutil.
>  	 */
> -	if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags & ENQUEUE_RESTORE))))
> +	if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags & ENQUEUE_RESTORE)))) {
> +		uclamp_rq_inc(rq, p);
>  		util_est_enqueue(&rq->cfs, p);
> +	}

So you want to have p uclamp-enqueued so that its uclamp_min value
counts for the cpufreq_update_util()/cfs_rq_util_change() calls later in
enqueue_task_fair?

  if (p->in_iowait)
    cpufreq_update_util(rq, SCHED_CPUFREQ_IOWAIT);

  enqueue_entity() -> update_load_avg() -> cfs_rq_util_change() ->
  cpufreq_update_util()

But if you do this before requeue_delayed_entity() (1) you will not
uclamp-enqueue p which got his ->sched_delayed just cleared in (1)?

[...]