linux-kernel - Re: [PATCH v4.19] sched/fair: handle case of task_h

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Date:   Mon, 20 Jul 2020 14:25:41 +0200
From:   Greg KH <greg@...ah.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,
        stable@...r.kernel.org, valentin.schneider@....com,
        sashal@...nel.org
Subject: Re: [PATCH v4.19] sched/fair: handle case of task_h_load() returning
 0

On Mon, Jul 20, 2020 at 10:34:01AM +0200, Vincent Guittot wrote:
> [ Upstream commit 01cfcde9c26d8555f0e6e9aea9d6049f87683998 ]
> 
> task_h_load() can return 0 in some situations like running stress-ng
> mmapfork, which forks thousands of threads, in a sched group on a 224 cores
> system. The load balance doesn't handle this correctly because
> env->imbalance never decreases and it will stop pulling tasks only after
> reaching loop_max, which can be equal to the number of running tasks of
> the cfs. Make sure that imbalance will be decreased by at least 1.
> 
> We can't simply ensure that task_h_load() returns at least one because it
> would imply to handle underflow in other places.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> [removed misfit part which was not implemented yet]
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> Reviewed-by: Valentin Schneider <valentin.schneider@....com>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
> Tested-by: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: <stable@...r.kernel.org> # v4.19 v4.14 v4.9 v4.4
> cc: Sasha Levin <sashal@...nel.org>
> Link: https://lkml.kernel.org/r/20200710152426.16981-1-vincent.guittot@linaro.org
> ---
> 
> This patch also applies on v4.14.188 v4.9.230 and v4.4.230

Thanks for all of the backports, now queued up.

greg k-h