linux-kernel - Re: [PATCH 5/8] sched/fair: Make check_misfit_status() only compare dynamic capacities

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Message-ID: <1c4c5ff0-f29e-3f08-7ba0-6173bda888cd@arm.com>
Date:   Thu, 4 Feb 2021 15:57:00 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Valentin Schneider <valentin.schneider@....com>,
        Qais Yousef <qais.yousef@....com>
Cc:     linux-kernel@...r.kernel.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 5/8] sched/fair: Make check_misfit_status() only compare
 dynamic capacities

On 04/02/2021 12:34, Valentin Schneider wrote:
> On 04/02/21 11:49, Dietmar Eggemann wrote:
>> On 03/02/2021 16:15, Qais Yousef wrote:
>>> On 01/28/21 18:31, Valentin Schneider wrote:

[...]

> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 21bd71f58c06..ea7f0155e268 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1482,6 +1482,33 @@ DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
>  DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_cpucapacity);
>  extern struct static_key_false sched_asym_cpucapacity;
>  
> +/*
> + * Note that the static key is system-wide, but the visibility of
> + * SD_ASYM_CPUCAPACITY isn't. Thus the static key being enabled does not
> + * imply all CPUs can see asymmetry.
> + *
> + * Consider an asymmetric CPU capacity system such as:
> + *
> + * MC [           ]
> + *     0 1 2 3 4 5
> + *     L L L L B B
> + *
> + * w/ arch_scale_cpu_capacity(L) < arch_scale_cpu_capacity(B)
> + *
> + * By default, booting this system will enable the sched_asym_cpucapacity
> + * static key, and all CPUs will see SD_ASYM_CPUCAPACITY set at their MC
> + * sched_domain.
> + *
> + * Further consider exclusive cpusets creating a "symmetric island":
> + *
> + * MC [   ][      ]
> + *     0 1  2 3 4 5
> + *     L L  L L B B
> + *
> + * Again, booting this will enable the static key, but CPUs 0-1 will *not* have
> + * SD_ASYM_CPUCAPACITY set in any of their sched_domain. This is the intending

s/intending/intended

> + * behaviour, as CPUs 0-1 should be treated as a regular, isolated SMP system.
> + */

LGTM.