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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Mon, 27 Feb 2023 15:37:37 +0100
Message-ID: <CAKfTPtAq3yBYBxpR=RO8zxrQduOymqkdAEhigjfCuGfsY1uHsg@mail.gmail.com>
Subject: Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed
To:     Roman Kagan <rkagan@amazon.de>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org,
        Valentin Schneider <vschneid@redhat.com>,
        Zhang Qiao <zhangqiao22@huawei.com>,
        Ben Segall <bsegall@google.com>,
        Waiman Long <longman@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>
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On Mon, 27 Feb 2023 at 09:43, Roman Kagan <rkagan@amazon.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@amazon.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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