linux-kernel - Re: [PATCH] sched/util_est: fix util_est

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Message-ID: <20180621190832.GD27616@hirez.programming.kicks-ass.net>
Date:   Thu, 21 Jun 2018 21:08:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/util_est: fix util_est_dequeue() for throttled cfs
 rq

On Thu, Jun 14, 2018 at 12:32:32PM +0100, Patrick Bellasi wrote:
> On 14-Jun 12:33, Vincent Guittot wrote:
> > When a cfs_rq is throttled, parent cfs_rq->nr_running is decreased and
> > everything happens at cfs_rq level. Currently util_est stays unchanged
> > in such case and it keeps accounting the utilization of throttled tasks.
> > This can somewhat make sense as we don't dequeue tasks but only throttled
> > cfs_rq.

> > If a task of another group is enqueued/dequeued and root cfs_rq becomes
> > idle during the dequeue, util_est will be cleared whereas it was
> > accounting util_est of throttled tasks before.

> > So the behavior of util_est
> > is not always the same regarding throttled tasks and depends of side
> > activity. Furthermore, util_est will not be updated when the cfs_rq is
> > unthrottled

> > as everything happens at cfs rq level. Main results is that
> > util_est will stay null whereas we now have running tasks. We have to wait
> > for the next dequeue/enqueue of the previously throttled tasks to get an
> > up to date util_est.
> > 
> > Remove the assumption that cfs_rq's estimated utilization of a CPU is 0
> > if there is no running task so the util_est of a task remains until the
> > latter is dequeued even if its cfs_rq has been throttled.

> > Fixes: 7f65ea42eb00 ("sched/fair: Add util_est on top of PELT")
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>

> LGTM:
> 
> Reviewed-by: Patrick Bellasi <patrick.bellasi@....com>

Thanks guys!