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

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Message-ID: <e123524c-8c27-4ee7-8799-10931375712f@arm.com>
Date: Thu, 6 Mar 2025 10:53:46 +0000
From: Hongyan Xia <hongyan.xia2@....com>
To: Dietmar Eggemann <dietmar.eggemann@....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 05/03/2025 18:22, Dietmar Eggemann wrote:
> 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)?

Sorry I'm not sure I'm following. The only condition of the 
uclamp_rq_inc() here should be

     if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags & 
ENQUEUE_RESTORE))))

Could you elaborate why it doesn't get enqueued?