linux-kernel - Re: [PATCH] sched/pelt: avoid underestimate of task utilization

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Date:   Fri, 24 Nov 2023 13:04:51 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Hongyan Xia <hongyan.xia2@....com>
Cc:     lukasz.luba@....com, mingo@...hat.com, mgorman@...e.de,
        rostedt@...dmis.org, bristot@...hat.com, peterz@...radead.org,
        rafael@...nel.org, vschneid@...hat.com, dietmar.eggemann@....com,
        juri.lelli@...hat.com, bsegall@...gle.com, qyousef@...alina.io,
        viresh.kumar@...aro.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH] sched/pelt: avoid underestimate of task utilization

On Fri, 24 Nov 2023 at 11:44, Hongyan Xia <hongyan.xia2@....com> wrote:
>
> On 22/11/2023 14:01, Vincent Guittot wrote:
> > [...]
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 07f555857698..eeb505d28905 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4774,6 +4774,11 @@ static inline unsigned long task_util(struct task_struct *p)
> >       return READ_ONCE(p->se.avg.util_avg);
> >   }
> >
> > +static inline unsigned long task_runnable(struct task_struct *p)
> > +{
> > +     return READ_ONCE(p->se.avg.runnable_avg);
> > +}
> > +
> >   static inline unsigned long _task_util_est(struct task_struct *p)
> >   {
> >       struct util_est ue = READ_ONCE(p->se.avg.util_est);
> > @@ -4892,6 +4897,14 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
> >       if (task_util(p) > arch_scale_cpu_capacity(cpu_of(rq_of(cfs_rq))))
> >               return;
> >
> > +     /*
> > +      * To avoid underestimate of task utilization, skip updates of ewma if
> > +      * we cannot grant that thread got all CPU time it wanted.
> > +      */
> > +     if ((ue.enqueued + UTIL_EST_MARGIN) < task_runnable(p))
> > +             goto done;
> > +
> > +
>
> Actually, does this also skip util_est increases as well, assuming no
> FASTUP? When a task is ramping up, another task could join and then
> blocks this task from ramping up its util_est.
>
> Or do we think this is intended behavior for !FASTUP?

sched_feat(UTIL_EST_FASTUP) has been there to disable faster ramp up
in case of regression but I'm not aware of anybody having to disable
it during the last 3 year so we should just delete the sched_feat()
and make faster ramp up  permanent.

>
> >       /*
> >        * Update Task's estimated utilization
> >        *