linux-kernel - Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

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Date:   Wed, 15 Mar 2023 16:32:27 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Zhang Qiao <zhangqiao22@...wei.com>,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        juri.lelli@...hat.com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        rkagan@...zon.de
Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

On Wed, 15 Mar 2023 at 14:36, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>
> On 15/03/2023 11:21, Vincent Guittot wrote:
> > On Wed, 15 Mar 2023 at 11:15, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >>
> >> On 15/03/2023 09:42, Vincent Guittot wrote:
> >>> On Wed, 15 Mar 2023 at 08:18, Vincent Guittot
> >>> <vincent.guittot@...aro.org> wrote:
> >>>>
> >>>> On Tue, 14 Mar 2023 at 18:16, Peter Zijlstra <peterz@...radead.org> wrote:
> >>>>>
> >>>>> On Tue, Mar 14, 2023 at 02:24:37PM +0100, Vincent Guittot wrote:
> >>>>>
>
> [...]
>
> >> Isn't there an issue with this approach on asymmetric CPU capacity systems?
> >>
> >> We do a sync_entity_load_avg() in select_task_rq_fair()
> >> (find_energy_efficient_cpu() for EAS and select_idle_sibling() for CAS)
> >> to sync cfs_rq and se.
> >
> > ah yes, I forgot this point.
> > That being said, is it a valid problem for EAS based system ? I mean
> > we are trying to fix a vruntime comparison overflow that can happen
> > with a very long sleeping task (around 200 days) while only a very low
> > weight entity is always running  during those 200 days.
>
> True. Definitively very unlikely. But it's not only EAS, any asymmetric
> CPU capacity wakeup wouldn't have this check in this case.
>
> This dependency between sync_entity_load_avg() and the overflow
> detection would be very hard to spot though (in case
> sync_entity_load_avg() would get introduced in more common wakeup paths
> later).

fair enough