linux-kernel - Re: [PATCH] sched/idle: Prevent stopping the tick when there is no cpuidle driver

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Date: Thu, 21 Dec 2023 16:22:48 +0100
From: Pierre Gondois <pierre.gondois@....com>
To: Anna-Maria Behnsen <anna-maria@...utronix.de>,
 linux-kernel@...r.kernel.org
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira
 <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>,
 Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/idle: Prevent stopping the tick when there is no
 cpuidle driver

Hello Anna-Maria,

On 12/15/23 14:05, Anna-Maria Behnsen wrote:
> When there is no cpuidle driver, the system tries to stop the tick even if
> the system is fully loaded. But stopping the tick is not for free and it
> decreases performance on a fully loaded system. As there is no (cpuidle)
> framework which brings CPU in a power saving state when nothing needs to be
> done, there is also no power saving benefit when stopping the tick.

Just in case is wasn't taken into consideration:
-
Stopping the tick isn't free on a busy system, but it should also cost
something to regularly handle ticks on each CPU of an idle system.

FWIU, disabling the ticks also allows to add a CPU to the 'nohz.idle_cpus_mask'
mask, which helps the idle load balancer picking an idle CPU to do load
balancing for all the idle CPUs (cf. kick_ilb()).

It seems better to do one periodic balancing for all the idle CPUs rather
than periodically waking-up all CPUs to try to balance.

-
I would have assumed that if the system was fully loaded, ticks would
not be stopped, or maybe I misunderstood the case.
I assume the wake-up latency would be improved if the tick doesn't
have to be re-setup again.

Regards,
Pierre

> 
> Therefore do not stop the tick when there is no cpuidle driver.
> 
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> ---
>   kernel/sched/idle.c | 2 --
>   1 file changed, 2 deletions(-)
> 
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 565f8374ddbb..fd111686aaf3 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -165,8 +165,6 @@ static void cpuidle_idle_call(void)
>   	 */
>   
>   	if (cpuidle_not_available(drv, dev)) {
> -		tick_nohz_idle_stop_tick();
> -
>   		default_idle_call();
>   		goto exit_idle;
>   	}