linux-kernel - Re: [PATCH 3/4 v2] sched/fair: minimize concurrent LBs between domain level

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Message-ID: <jhjblhwmu3d.mognet@arm.com>
Date:   Wed, 23 Sep 2020 15:43:50 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, linux-kernel@...r.kernel.org, pauld@...hat.com,
        hdanton@...a.com
Subject: Re: [PATCH 3/4 v2] sched/fair: minimize concurrent LBs between domain level


On 21/09/20 08:24, Vincent Guittot wrote:
> sched domains tend to trigger simultaneously the load balance loop but
> the larger domains often need more time to collect statistics. This
> slowness makes the larger domain trying to detach tasks from a rq whereas
> tasks already migrated somewhere else at a sub-domain level. This is not
> a real problem for idle LB because the period of smaller domains will
> increase with its CPUs being busy and this will let time for higher ones
> to pulled tasks. But this becomes a problem when all CPUs are already busy
> because all domains stay synced when they trigger their LB.
>
> A simple way to minimize simultaneous LB of all domains is to decrement the
> the busy interval by 1 jiffies. Because of the busy_factor, the interval of
> larger domain will not be a multiple of smaller ones anymore.
>
> Reviewed-by: Phil Auld <pauld@...hat.com>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>

Reviewed-by: Valentin Schneider <valentin.schneider@....com>