linux-kernel - Re: [PATCH v3] sched/fair: Fix per-CPU kthread and wakee stacking for asym CPU capacity

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Message-ID: <20211204094040.GN16608@worktop.programming.kicks-ass.net>
Date:   Sat, 4 Dec 2021 10:40:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Donnefort <vincent.donnefort@....com>
Cc:     mingo@...hat.com, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        valentin.schneider@....com
Subject: Re: [PATCH v3] sched/fair: Fix per-CPU kthread and wakee stacking
 for asym CPU capacity

On Mon, Nov 29, 2021 at 05:31:15PM +0000, Vincent Donnefort wrote:
> select_idle_sibling() has a special case for tasks woken up by a per-CPU
> kthread where the selected CPU is the previous one. For asymmetric CPU
> capacity systems, the assumption was that the wakee couldn't have a
> bigger utilization during task placement than it used to have during the
> last activation. That was not considering uclamp.min which can completely
> change between two task activations and as a consequence mandates the
> fitness criterion asym_fits_capacity(), even for the exit path described
> above.
> 
> Fixes: b4c9c9f15649 ("sched/fair: Prefer prev cpu in asymmetric wakeup path")
> Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
> Reviewed-by: Valentin Schneider <valentin.schneider@....com>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>

Thanks!