linux-kernel - Re: [PATCH] sched/fair: Optimize select_idle

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Date:   Thu, 5 Dec 2019 18:27:51 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH] sched/fair: Optimize select_idle_core

Hi Srikar,

On Thu, 5 Dec 2019 at 18:23, Srikar Dronamraju
<srikar@...ux.vnet.ibm.com> wrote:
>
> Currently we loop through all threads of a core to evaluate if the core
> is idle or not. This is unnecessary. If a thread of a core is not
> idle, skip evaluating other threads of a core.

I think that the goal is also to clear all CPUs of the core from the
cpumask  of the loop above so it will not try the same core next time

>
> Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> ---
>  kernel/sched/fair.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 69a81a5709ff..b9d628128cfc 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5872,10 +5872,12 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
>                 bool idle = true;
>
>                 for_each_cpu(cpu, cpu_smt_mask(core)) {
> -                       __cpumask_clear_cpu(cpu, cpus);
> -                       if (!available_idle_cpu(cpu))
> +                       if (!available_idle_cpu(cpu)) {
>                                 idle = false;
> +                               break;
> +                       }
>                 }
> +               cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
>
>                 if (idle)
>                         return core;
> --
> 2.18.1
>