linux-kernel - Re: [PATCH 10/10] sched/timers: Explain why idle task schedules out on remote timer enqueue

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Message-ID: <CAJZ5v0jiUVu3C0AkUAK9B8+0Lu8nrQ=xzHC6r1cxBLHdmBY-sA@mail.gmail.com>
Date:   Fri, 11 Aug 2023 19:43:10 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: Re: [PATCH 10/10] sched/timers: Explain why idle task schedules out
 on remote timer enqueue

On Fri, Aug 11, 2023 at 7:01 PM Frederic Weisbecker <frederic@...nel.org> wrote:
>
> Trying to avoid that didn't bring much value after testing, add comment
> about this.
>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>

Acked-by: Rafael J. Wysocki <rafael@...nel.org>

> ---
>  kernel/sched/core.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c52c2eba7c73..e53b892167ad 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1135,6 +1135,28 @@ static void wake_up_idle_cpu(int cpu)
>         if (cpu == smp_processor_id())
>                 return;
>
> +       /*
> +        * Set TIF_NEED_RESCHED and send an IPI if in the non-polling
> +        * part of the idle loop. This forces an exit from the idle loop
> +        * and a round trip to schedule(). Now this could be optimized
> +        * because a simple new idle loop iteration is enough to
> +        * re-evaluate the next tick. Provided some re-ordering of tick
> +        * nohz functions that would need to follow TIF_NR_POLLING
> +        * clearing:
> +        *
> +        * - On most archs, a simple fetch_or on ti::flags with a
> +        *   "0" value would be enough to know if an IPI needs to be sent.
> +        *
> +        * - x86 needs to perform a last need_resched() check between
> +        *   monitor and mwait which doesn't take timers into account.
> +        *   There a dedicated TIF_TIMER flag would be required to
> +        *   fetch_or here and be checked along with TIF_NEED_RESCHED
> +        *   before mwait().
> +        *
> +        * However, remote timer enqueue is not such a frequent event
> +        * and testing of the above solutions didn't appear to report
> +        * much benefits.
> +        */
>         if (set_nr_and_not_polling(rq->idle))
>                 smp_send_reschedule(cpu);
>         else
> --
> 2.34.1
>