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

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Message-ID: <b761ae52-5e3a-4d5e-8600-a23903abbf25@arm.com>
Date: Thu, 6 Mar 2025 14:40:57 +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 06/03/2025 14:26, Hongyan Xia wrote:
> On 06/03/2025 13:48, Dietmar Eggemann wrote:
>> 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.
>>
> 
> I doubt this would be a concern as there are other conditions after 
> checking p->se.sched_delayed. You would only skip the uclamp inc if you 
> are both sched_delayed and meet the second part of the if.
> 
> Another reason is that, I think whatever we do should be consistent with 
> what we did for util_est. If util_est also affects cpufreq then I doubt 
> uclamp should be enqueue/dequeued differently, as it would be difficult 
> to argue why sometimes util_est affects cpufreq while uclamp doesn't and 
> why sometimes uclamp does and util_est doesn't.
> 

Sorry what I just said might be a bit misleading. What I wanted to say 
was that util_est and uclamp should be in sync, but exactly when to 
enqueue or dequeue these two can be changed.

Do we want to first agree on one thing, which is, should we keep the 
util_est and uclamp on the rq while a task is being delayed?