linux-kernel - Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask of CPUs searched

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Date:   Thu, 3 Dec 2020 17:38:03 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Ziljstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux-ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask
 of CPUs searched

On Thu, 3 Dec 2020 at 15:11, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> The target CPU is definitely not idle in both select_idle_core and
> select_idle_cpu. For select_idle_core(), the SMT is potentially
> checked unnecessarily as the core is definitely not idle if the
> target is busy. For select_idle_cpu(), the first CPU checked is
> simply a waste.

>
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> ---
>  kernel/sched/fair.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 68dd9cd62fbd..1d8f5c4b4936 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6077,6 +6077,7 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
>                 return -1;
>
>         cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +       __cpumask_clear_cpu(target, cpus);

should clear cpu_smt_mask(target) as we are sure that the core will not be idle

>
>         for_each_cpu_wrap(core, cpus, target) {
>                 bool idle = true;
> @@ -6181,6 +6182,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>         time = cpu_clock(this);
>
>         cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +       __cpumask_clear_cpu(target, cpus);
>
>         for_each_cpu_wrap(cpu, cpus, target) {
>                 schedstat_inc(this_rq()->sis_scanned);
> --
> 2.26.2
>