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

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Message-ID: <370e4908-eb8a-74d5-b771-8e657c3b6461@arm.com>
Date:   Tue, 14 Mar 2023 14:37:53 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     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 14/03/2023 14:29, Dietmar Eggemann wrote:
> On 14/03/2023 13:07, Peter Zijlstra wrote:
>> On Tue, Mar 14, 2023 at 08:41:30AM +0100, Vincent Guittot wrote:

[...]

>> So when a task is not running for a long time (our case at hand), then
>> there's two cases:
>>
>>  - it wakes up locally and place_entity() gets to reset vruntime;
>>  - it wakes up remotely and migrate_task_rq_fair() can reset vruntime.
>>
>> So if we can rely on ENQUEUE_MIGRATED to differentiate between these
>> cases, when wouldn't something like this work?
> 
> I guess so. We would avoid rq_clock_task skews or to be forced to pass
> state that migrating se's vruntime is too old.

... just saw Vincent's reply ... I forgot the limitation that we can't
all rq_clock_task() in migrate_task_rq_fair() again.

[...]