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

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Message-ID: <bf5a70bf-3d13-4b3b-a3ef-804998b21fe9@arm.com>
Date: Fri, 7 Mar 2025 19:32:07 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Xuewen Yan <xuewen.yan94@...il.com>
Cc: 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>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/uclamp: Let each sched_class handle uclamp

On 06/03/2025 13:01, Xuewen Yan wrote:
> On Thu, Mar 6, 2025 at 2:24 AM Dietmar Eggemann
> <dietmar.eggemann@....com> 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)?
>>
> 
> Could we change to the following:
> 
> when enqueue:
> 
> -     if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags
> & ENQUEUE_RESTORE))))
> +     if (!(p->se.sched_delayed && !(flags & ENQUEUE_DELAYED)))

Why you want to check ENQUEUE_DELAYED as well here? Isn't
!p->se.sched_delayed implying !ENQUEUE_DELAYED).

> +             uclamp_rq_inc(rq, p);
>                util_est_enqueue(&rq->cfs, p);
>  +     }
> 
> 
> when dequeue:
> 
> -       if (!(p->se.sched_delayed && (task_on_rq_migrating(p) ||
> (flags & DEQUEUE_SAVE))))
> +       if (!p->se.sched_delayed) {
> +               uclamp_rq_dec(rq, p);
>                 util_est_dequeue(&rq->cfs, p);
> +       }

So you want to exclude all delayed tasks from util_est and uclamp?