linux-kernel - Re: [PATCH v7 02/21] timer: Do not IPI for deferrable timers

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Date:   Mon, 5 Jun 2023 23:29:37 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Arjan van de Ven <arjan@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rik van Riel <riel@...riel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        Lukasz Luba <lukasz.luba@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>
Subject: Re: [PATCH v7 02/21] timer: Do not IPI for deferrable timers

Le Wed, May 24, 2023 at 09:06:10AM +0200, Anna-Maria Behnsen a écrit :
> Deferrable timers do not prevent CPU from going idle and are not taken into
> account on idle path. Sending an IPI to a remote CPU when a new first
> deferrable timer was enqueued will wake up the remote CPU and but nothing
> will be done regarding the deferrable timers.
> 
> Drop IPI completely when a new first deferrable timer was enqueued.
> 
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> ---
> v6: new patch
> ---
>  kernel/time/timer.c | 15 ++++++---------
>  1 file changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index 63a8ce7177dd..6e251e3cf659 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -571,18 +571,15 @@ static int calc_wheel_index(unsigned long expires, unsigned long clk,
>  static void
>  trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
>  {
> -	if (!is_timers_nohz_active())
> -		return;
> -
>  	/*
> -	 * TODO: This wants some optimizing similar to the code below, but we
> -	 * will do that when we switch from push to pull for deferrable timers.
> +	 * Deferrable timers do not prevent CPU from going idle and are not
> +	 * taken into account on idle path. An IPI when a new deferrable

Just to make sure everyone is aware that this concerns also nohz_full,
this could be:

	/*
	 * Deferrable timers do not prevent CPU from entering dynticks
	 * and are not taken into account on idle/nohz_full path. An IPI
	 * when a new deferrable timer is enqueued will wake up the remote
	 * CPU but nothing will be done with the deferrable timer base.
	 * Therefore skip remote IPI for deferrable timers completely.
	 */

But anyway:

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>