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

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Message-ID: <CAKfTPtADUas2QHZCQyu0ad-JTKRQ=PcsB=o7+PuJNVxHwAzkCQ@mail.gmail.com>
Date:   Thu, 9 Mar 2023 16:14:38 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Zhang Qiao <zhangqiao22@...wei.com>, linux-kernel@...r.kernel.org,
        mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....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 Thu, 9 Mar 2023 at 15:37, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Mar 09, 2023 at 03:28:25PM +0100, Peter Zijlstra wrote:
> > On Thu, Mar 09, 2023 at 02:34:05PM +0100, Vincent Guittot wrote:
> >
> > > Then, even if we don't clear exec_start before migrating and keep
> > > current value to be used in place_entity on the new cpu, we can't
> > > compare the rq_clock_task(rq_of(cfs_rq)) of 2 different rqs AFAICT
> >
> > Blergh -- indeed, irq and steal time can skew them between CPUs :/
> > I suppose we can fudge that... wait_start (which is basically what we're
> > making it do) also does that IIRC.
> >
> > I really dislike having this placement muck spreadout like proposed.
>
> Also, I think we might be over-engineering this, we don't care about
> accuracy at all, all we really care about is 'long-time'.

you mean taking the patch 1/2 that you mentioned here to add a
migrated field:
https://lore.kernel.org/all/68832dfbb60fda030540b5f4e39c5801942689b1.1648228023.git.tim.c.chen@linux.intel.com/T/#ma5637eb8010f3f4a4abff778af8db705429d003b

And assume that the divergence between the rq_clock_task() can be ignored ?

That could probably work but we need to replace the (60LL *
NSEC_PER_SEC) by ((1ULL << 63) / NICE_0_LOAD) because 60sec divergence
would not be unrealistic.
and a comment to explain why it's acceptable


>
>