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

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Message-ID: <CAB8ipk-SUFDATb=euJQFebxQ513SRwTEpSbBSD6K=batQKELHg@mail.gmail.com>
Date: Mon, 10 Mar 2025 10:41:39 +0800
From: Xuewen Yan <xuewen.yan94@...il.com>
To: Dietmar Eggemann <dietmar.eggemann@....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, 
	Xuewen Yan <xuewen.yan@...soc.com>
Subject: Re: [PATCH] sched/uclamp: Let each sched_class handle uclamp

On Sat, Mar 8, 2025 at 2:32 AM Dietmar Eggemann
<dietmar.eggemann@....com> wrote:
>
> 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).

Indeed, the (!(p->se.sched_delayed && !(flags & ENQUEUE_DELAYED))) is equal to
the  (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags &
ENQUEUE_RESTORE)))).
I just think it might be easier to read using the ENQUEUE_DELAYED flag.
Because we only allow enq the uclamp and util_est when wake up the delayed-task.


>
> > +             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?

Yes, Because we had dequeued the task's uclamp and util_est when first sleeping。
This is achieved by putting the update of uclamp and util_est before
dequeue_entity.
For other dequeue, we should always ignore it. So there is no need to
judge the flags and migration.

BR