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

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Message-ID: <Y/xtDWYTKLutOqrM@u40bc5e070a0153.ant.amazon.com>
Date:   Mon, 27 Feb 2023 09:42:54 +0100
From:   Roman Kagan <rkagan@...zon.de>
To:     Vincent Guittot <vincent.guittot@...aro.org>
CC:     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 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?

Thanks,
Roman.



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