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

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Message-ID: <0dcf8ac1-29ec-44e8-80ef-4c0f4a6d34e7@arm.com>
Date: Thu, 6 Mar 2025 14:48:37 +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 06/03/2025 11:53, Hongyan Xia wrote:
> 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?

Let's say 'p->se.sched_delayed = 1' and we are in

enqueue_task()

  enqueue_task_fair()

    if (!(p->se.sched_delayed && ...)

      uclamp_rq_inc(rq, p);

So p wouldn't be included here.

But then p would be requeued in

      requeue_delayed_entity(se)

since you removed the uclamp_rq_inc() from enqueue_task() (after the
p->sched_class->enqueue_task) p will not be considered for uclamp.

[...]