linux-kernel - Re: [PATCH] sched/fair: Clear SMT siblings after determining the core is not idle

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Message-ID: <CAKfTPtBcZnxR073=eRsWNAGOnA8K4emL0BYEUzbkzZx6qJWmSg@mail.gmail.com>
Date:   Mon, 30 Nov 2020 15:47:01 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: Clear SMT siblings after determining the core
 is not idle

On Mon, 30 Nov 2020 at 15:40, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> The clearing of SMT siblings from the SIS mask before checking for an idle
> core is a small but unnecessary cost. Defer the clearing of the siblings
> until the scan moves to the next potential target. The cost of this was
> not measured as it is borderline noise but it should be self-evident.

Good point

>
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

> ---
>  kernel/sched/fair.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0d54d69ba1a5..d9acd55d309b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6087,10 +6087,11 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
>                                 break;
>                         }
>                 }
> -               cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
>
>                 if (idle)
>                         return core;
> +
> +               cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
>         }
>
>         /*