linux-kernel - Re: [PATCH v2] sched/fair: fix imbalance due to CPU affinity

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Message-ID: <7111f9d1-62f2-504c-a7ba-958b1c659cc8@arm.com>
Date:   Tue, 2 Jul 2019 10:34:49 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org
Subject: Re: [PATCH v2] sched/fair: fix imbalance due to CPU affinity

On 01/07/2019 16:47, Vincent Guittot wrote:
> The load_balance() has a dedicated mecanism to detect when an imbalance
> is due to CPU affinity and must be handled at parent level. In this case,
> the imbalance field of the parent's sched_group is set.
> 
> The description of sg_imbalanced() gives a typical example of two groups
> of 4 CPUs each and 4 tasks each with a cpumask covering 1 CPU of the first
> group and 3 CPUs of the second group. Something like:
> 
> 	{ 0 1 2 3 } { 4 5 6 7 }
> 	        *     * * *
> 
> But the load_balance fails to fix this UC on my octo cores system
> made of 2 clusters of quad cores.
> 
> Whereas the load_balance is able to detect that the imbalanced is due to
> CPU affinity, it fails to fix it because the imbalance field is cleared
> before letting parent level a chance to run. In fact, when the imbalance is
> detected, the load_balance reruns without the CPU with pinned tasks. But
> there is no other running tasks in the situation described above and
> everything looks balanced this time so the imbalance field is immediately
> cleared.
> 
> The imbalance field should not be cleared if there is no other task to move
> when the imbalance is detected.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>

Does that want a

Cc: stable@...r.kernel.org
Fixes: afdeee0510db ("sched: Fix imbalance flag reset")

?