linux-kernel - Re: [PATCH] sched/fair: consider sched-idle CPU when selecting idle core

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Message-ID: <CAKfTPtCnewaivay7bftUY27+-qB=ct3eSNEoZW=-2Z6k0S4B4Q@mail.gmail.com>
Date:   Fri, 24 Jul 2020 09:23:48 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Jiang Biao <humjb_1983@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jiang Biao <benbjiang@...cent.com>
Subject: Re: [PATCH] sched/fair: consider sched-idle CPU when selecting idle core

On Fri, 24 Jul 2020 at 01:39, Jiang Biao <humjb_1983@....com> wrote:
>
> From: Jiang Biao <benbjiang@...cent.com>
>
> Sched-idle CPU has been considered in select_idle_cpu and
> select_idle_smt, it also needs to be considered in select_idle_core to
> be consistent and keep the same *idle* policy.

In the case of select_idle_core, we are looking for a core that is
fully idle but if one CPU of the core is running a sched_idle task,
the core will not be idle and we might end up having  the wakeup task
on a CPU and a sched_idle task on another CPU of the core which is not
what we want

>
> Signed-off-by: Jiang Biao <benbjiang@...cent.com>
> ---
>  kernel/sched/fair.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 04fa8dbcfa4d..f430a9820d08 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6014,7 +6014,7 @@ void __update_idle_core(struct rq *rq)
>                 if (cpu == core)
>                         continue;
>
> -               if (!available_idle_cpu(cpu))
> +               if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu))
>                         goto unlock;
>         }
>
> @@ -6045,7 +6045,7 @@ 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)) {
> -                       if (!available_idle_cpu(cpu)) {
> +                       if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu)) {
>                                 idle = false;
>                                 break;
>                         }
> --
> 2.21.0
>
>