linux-kernel - Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed

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Date:   Mon, 27 Feb 2023 15:37:37 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Roman Kagan <rkagan@...zon.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Valentin Schneider <vschneid@...hat.com>,
        Zhang Qiao <zhangqiao22@...wei.com>,
        Ben Segall <bsegall@...gle.com>,
        Waiman Long <longman@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed

On Mon, 27 Feb 2023 at 09:43, Roman Kagan <rkagan@...zon.de> wrote:
>
> On Tue, Feb 21, 2023 at 06:26:11PM +0100, Vincent Guittot wrote:
> > On Tue, 21 Feb 2023 at 17:57, Roman Kagan <rkagan@...zon.de> wrote:
> > > What scares me, though, is that I've got a message from the test robot
> > > that this commit drammatically affected hackbench results, see the quote
> > > below.  I expected the commit not to affect any benchmarks.
> > >
> > > Any idea what could have caused this change?
> >
> > Hmm, It's most probably because se->exec_start is reset after a
> > migration and the condition becomes true for newly migrated task
> > whereas its vruntime should be after min_vruntime.
> >
> > We have missed this condition
>
> Makes sense to me.
>
> But what would then be the reliable way to detect a sched_entity which
> has slept for long and risks overflowing in .vruntime comparison?

For now I don't have a better idea than adding the same check in
migrate_task_rq_fair()

>
> Thanks,
> Roman.
>
>
>
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