linux-kernel - Re: [PATCH] tick/nohz: Make the idle

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Date:   Tue, 15 Dec 2020 15:47:57 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Yunfeng Ye <yeyunfeng@...wei.com>
Cc:     fweisbec@...il.com, tglx@...utronix.de, mingo@...nel.org,
        linux-kernel@...r.kernel.org, Shiyuan Hu <hushiyuan@...wei.com>,
        Hewenliang <hewenliang4@...wei.com>
Subject: Re: [PATCH] tick/nohz: Make the idle_exittime update correctly

On Tue, Dec 15, 2020 at 08:06:34PM +0800, Yunfeng Ye wrote:
> The idle_exittime field of tick_sched is used to record the time when
> the idle state was left. but currently the idle_exittime is updated in
> the function tick_nohz_restart_sched_tick(), which is not always in idle
> state when nohz_full is configured.
> 
>   tick_irq_exit
>     tick_nohz_irq_exit
>       tick_nohz_full_update_tick
>         tick_nohz_restart_sched_tick
>           ts->idle_exittime = now;
> 
> So move to tick_nohz_stop_idle() to make the idle_exittime update
> correctly.
> 
> Signed-off-by: Yunfeng Ye <yeyunfeng@...wei.com>
> ---
>  kernel/time/tick-sched.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 749ec2a583de..be2e5d772d50 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -591,6 +591,7 @@ static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
>  {
>  	update_ts_time_stats(smp_processor_id(), ts, now, NULL);
>  	ts->idle_active = 0;
> +	ts->idle_exittime = now;

This changes a bit the meaning of idle_exittime then since this is also called
from idle interrupt entry.

__tick_nohz_idle_restart_tick() would be a better place.

Thanks.

> 
>  	sched_clock_idle_wakeup_event();
>  }
> @@ -901,7 +902,6 @@ static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
>  	 * Cancel the scheduled timer and restore the tick
>  	 */
>  	ts->tick_stopped  = 0;
> -	ts->idle_exittime = now;
> 
>  	tick_nohz_restart(ts, now);
>  }
> -- 
> 2.18.4
>