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

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Message-ID: <9cd7ee64-e5f4-4dd0-9e92-e84d96487f50@arm.com>
Date: Thu, 6 Mar 2025 14:26:22 +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 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.