linux-kernel - Re: [PATCH v4 3/3] sched/fair: Introduce a CPU capacity comparison helper

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Message-ID: <b03ae299-0b0c-3090-4d16-edbf2bdd5edb@arm.com>
Date:   Tue, 6 Apr 2021 17:37:18 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Cc:     Qais Yousef <qais.yousef@....com>,
        Lingutla Chandrasekhar <clingutla@...eaurora.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v4 3/3] sched/fair: Introduce a CPU capacity comparison
 helper

On 01/04/2021 21:30, Valentin Schneider wrote:
> During load-balance, groups classified as group_misfit_task are filtered
> out if they do not pass
> 
>   group_smaller_max_cpu_capacity(<candidate group>, <local group>);
> 
> which itself employs fits_capacity() to compare the sgc->max_capacity of
> both groups.
> 
> Due to the underlying margin, fits_capacity(X, 1024) will return false for
> any X > 819. Tough luck, the capacity_orig's on e.g. the Pixel 4 are
> {261, 871, 1024}. If a CPU-bound task ends up on one of those "medium"
> CPUs, misfit migration will never intentionally upmigrate it to a CPU of
> higher capacity due to the aforementioned margin.
> 
> One may argue the 20% margin of fits_capacity() is excessive in the advent
> of counter-enhanced load tracking (APERF/MPERF, AMUs), but one point here
> is that fits_capacity() is meant to compare a utilization value to a
> capacity value, whereas here it is being used to compare two capacity
> values. As CPU capacity and task utilization have different dynamics, a
> sensible approach here would be to add a new helper dedicated to comparing
> CPU capacities.
> 
> While at it, replace group_smaller_{min, max}_cpu_capacity() with
> comparisons of the source group's min/max capacity and the destination
> CPU's capacity.

IMHO, you haven't mentioned why you replace local sched group with dst
CPU. I can see that only the capacity of the dst CPU makes really sense
here. Might be worth mentioning in the patch header why. There is some
of it in v3 6/7 but that's a different change.

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>

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