linux-kernel - Re: [PATCH 7/7 v3] sched/fair: reduce the window for duplicated update

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Date:   Fri, 12 Feb 2021 19:19:58 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org,
        joel@...lfernandes.org
Cc:     fweisbec@...il.com, tglx@...utronix.de, qais.yousef@....com,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 7/7 v3] sched/fair: reduce the window for duplicated update

On 12/02/21 15:17, Vincent Guittot wrote:
> Start to update last_blocked_load_update_tick to reduce the possibility
> of another cpu starting the update one more time
>

IIUC this can happen if e.g. a CPU is busy updating load in
update_blocked_averages() while another enters update_nohz_stats() for the
same rq. They'll be serialized by the RQ lock, but both can still enter
update_blocked_averages() at roughly the same time.

Shouldn't then the rq->last_blocked_load_update_tick check be either
deferred to or re-done within update_blocked_averages(), with the rq lock
held?