linux-kernel - Re: [PATCH v3] sched/fair: Avoid stale CPU util

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Date:   Fri, 18 Dec 2020 15:35:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Xuewen Yan <xuewen.yan94@...il.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        王科 (Ke Wang) <Ke.Wang@...soc.com>,
        Ryan Y <xuewyan@...mail.com>,
        Xuewen Yan <Xuewen.Yan@...soc.com>
Subject: Re: [PATCH v3] sched/fair: Avoid stale CPU util_est value for
 schedutil in task dequeue

On Fri, Dec 18, 2020 at 02:12:50PM +0100, Vincent Guittot wrote:
> On Fri, 18 Dec 2020 at 10:28, Xuewen Yan <xuewen.yan94@...il.com> wrote:
> >
> > From: Xuewen Yan <xuewen.yan@...soc.com>
> >
> > CPU (root cfs_rq) estimated utilization (util_est) is currently used in
> > dequeue_task_fair() to drive frequency selection before it is updated.
> >
> > with:
> >
> > CPU_util        : rq->cfs.avg.util_avg
> > CPU_util_est    : rq->cfs.avg.util_est
> > CPU_utilization : max(CPU_util, CPU_util_est)
> > task_util       : p->se.avg.util_avg
> > task_util_est   : p->se.avg.util_est
> >
> > dequeue_task_fair():
> >
> >     /* (1) CPU_util and task_util update + inform schedutil about
> >            CPU_utilization changes */
> >     for_each_sched_entity() /* 2 loops */
> >         (dequeue_entity() ->) update_load_avg() -> cfs_rq_util_change()
> >          -> cpufreq_update_util() ->...-> sugov_update_[shared\|single]
> >          -> sugov_get_util() -> cpu_util_cfs()
> >
> >     /* (2) CPU_util_est and task_util_est update */
> >     util_est_dequeue()
> >
> > cpu_util_cfs() uses CPU_utilization which could lead to a false (too
> > high) utilization value for schedutil in task ramp-down or ramp-up
> > scenarios during task dequeue.
> >
> > To mitigate the issue split the util_est update (2) into:
> >
> >  (A) CPU_util_est update in util_est_dequeue()
> >  (B) task_util_est update in util_est_update()
> >
> > Place (A) before (1) and keep (B) where (2) is. The latter is necessary
> > since (B) relies on task_util update in (1).
> >
> 
> maybe add a
> Fixes: 7f65ea42eb00 ("sched/fair: Add util_est on top of PELT")
> 
> > Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> > Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
> 
> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

Thanks!