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

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Message-ID: <20230309142825.GB273121@hirez.programming.kicks-ass.net>
Date:   Thu, 9 Mar 2023 15:28:25 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.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, 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.

> side note, why do we reset exec_start ? Is it only to make task_hot
> returns false ?

Yeah, god aweful hack.