linux-kernel - Re: [PATCH] sched/fair: Use rq->lock when checking cfs

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Message-ID: <20211013143900.GB48428@blackbody.suse.cz>
Date:   Wed, 13 Oct 2021 16:39:01 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Odin Ugedal <odin@...d.al>
Cc:     open list <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH] sched/fair: Use rq->lock when checking cfs_rq list
 presence

On Mon, Oct 11, 2021 at 08:12:08PM +0100, Odin Ugedal <odin@...d.al> wrote:
> To be 100% clear, this can only happen when a control group is
> throttled while it has load
> (cfs_rq_is_decayed(cfs_rq) is false); and then its unthrottling race
> with its deletion?
> Is that a correct understanding Michal?

Yes, that's my working hypothesis but Vincent found a loophole in the
proposed fix under this assumption.


> Do you agree that that will also solve the problem Michal,
> or am I missing something obvious here?

It's not easy for me to verify this with a reproducer and as suggested
by your discomfort, let's dismiss this idea for the time being.

Michal