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

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Date:   Wed, 1 Feb 2023 17:06:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Roman Kagan <rkagan@...zon.de>
Cc:     linux-kernel@...r.kernel.org, Juri Lelli <juri.lelli@...hat.com>,
        Zhang Qiao <zhangqiao22@...wei.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Waiman Long <longman@...hat.com>,
        Ben Segall <bsegall@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <vschneid@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being placed

On Mon, Jan 30, 2023 at 01:22:16PM +0100, Roman Kagan wrote:
> From: Zhang Qiao <zhangqiao22@...wei.com>
> 
> When a scheduling entity is placed onto cfs_rq, its vruntime is pulled
> to the base level (around cfs_rq->min_vruntime), so that the entity
> doesn't gain extra boost when placed backwards.
> 
> However, if the entity being placed wasn't executed for a long time, its
> vruntime may get too far behind (e.g. while cfs_rq was executing a
> low-weight hog), which can inverse the vruntime comparison due to s64
> overflow.  This results in the entity being placed with its original
> vruntime way forwards, so that it will effectively never get to the cpu.
> 
> To prevent that, ignore the vruntime of the entity being placed if it
> didn't execute for much longer than the characteristic sheduler time
> scale.
> 
> Signed-off-by: Zhang Qiao <zhangqiao22@...wei.com>
> [rkagan: formatted, adjusted commit log, comments, cutoff value]
> Co-developed-by: Roman Kagan <rkagan@...zon.de>
> Signed-off-by: Roman Kagan <rkagan@...zon.de>

Thanks!