linux-kernel - Re: [PATCH v3 4/7] sched/fair: Introduce a CPU capacity comparison helper

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Message-ID: <CAKfTPtCdT4Depo4BX4bT4tsYLxDRcfwx+znKpt7MU5GSTwEtDg@mail.gmail.com>
Date:   Mon, 15 Mar 2021 15:24:36 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Qais Yousef <qais.yousef@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Rik van Riel <riel@...riel.com>,
        Lingutla Chandrasekhar <clingutla@...eaurora.org>
Subject: Re: [PATCH v3 4/7] sched/fair: Introduce a CPU capacity comparison helper

On Thu, 11 Mar 2021 at 13:05, Valentin Schneider
<valentin.schneider@....com> 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.
>
> Reviewed-by: Qais Yousef <qais.yousef@....com>
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> ---
>  kernel/sched/fair.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index db892f6e222f..ddb2ab3edf6d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -113,6 +113,13 @@ int __weak arch_asym_cpu_priority(int cpu)
>   */
>  #define fits_capacity(cap, max)        ((cap) * 1280 < (max) * 1024)
>
> +/*
> + * The margin used when comparing CPU capacities.
> + * is 'cap1' noticeably greater than 'cap2'
> + *
> + * (default: ~5%)
> + */
> +#define capacity_greater(cap1, cap2) ((cap1) * 1024 > (cap2) * 1078)

defined but not used.

Should be merged with next patch which start to use it

>  #endif
>
>  #ifdef CONFIG_CFS_BANDWIDTH
> --
> 2.25.1
>