linux-kernel - Re: [PATCH v4 4/7] sched/fair: remove useless check in select_idle

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Message-ID: <CABk29NuXvCTkbQ=MdX5F+YswhOAPZSAQTAXn7B_uuKRr4=XVnQ@mail.gmail.com>
Date:   Mon, 27 Jun 2022 16:42:49 -0700
From:   Josh Don <joshdon@...gle.com>
To:     Abel Wu <wuyun.abel@...edance.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Chen Yu <yu.c.chen@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 4/7] sched/fair: remove useless check in select_idle_core

On Sun, Jun 19, 2022 at 5:05 AM Abel Wu <wuyun.abel@...edance.com> wrote:
>
> The function only gets called when sds->has_idle_cores is true which can
> be possible only when sched_smt_present is enabled.
>
> Signed-off-by: Abel Wu <wuyun.abel@...edance.com>
> ---
>  kernel/sched/fair.c | 3 ---
>  1 file changed, 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index aba1dad19574..1cc86e76e38e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6256,9 +6256,6 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
>         bool idle = true;
>         int cpu;
>
> -       if (!static_branch_likely(&sched_smt_present))
> -               return __select_idle_cpu(core, p);
> -

The static branch is basically free; although you're right that we
currently don't take !smt_present branch direction here, it doesn't
seem harmful to leave this check in case assumptions change about when
we call select_idle_core().

>         for_each_cpu(cpu, cpu_smt_mask(core)) {
>                 if (!available_idle_cpu(cpu)) {
>                         idle = false;
> --
> 2.31.1
>