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

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Date:   Mon, 27 Feb 2023 18:00:09 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Roman Kagan <rkagan@...zon.de>,
        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>,
        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 27/02/2023 15:37, Vincent Guittot wrote:
> 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()

Don't we have the issue that we could have a non-up-to-date rq clock in
migrate? No rq lock held in `!task_on_rq_migrating(p)`.

Also deferring `se->exec_start = 0` from `migrate` into `enqueue ->
place entity` doesn't seem to work since the rq clocks of different CPUs
are not in sync.