Re: [PATCH] tick/nohz: Make the idle_exittime update correctly

[Frederic Weisbecker]

On Sat, Dec 05, 2020 at 05:41:52PM +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@xxxxxxxxxx>
> ---
>  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;
> 
>  	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);
>  }

Hi,

I've edited and queued on top of latest changes, see below. I'll post it after
the merge window.

---
From: Yunfeng Ye <yeyunfeng@xxxxxxxxxx>
Date: Wed, 10 Feb 2021 00:08:54 +0100
Subject: [PATCH] tick/nohz: Update idle_exittime on actual idle exit

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;

It's thus overwritten by mistake on nohz_full tick restart. Move the
update to the appropriate idle exit path instead.

Signed-off-by: Yunfeng Ye <yeyunfeng@xxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
 kernel/time/tick-sched.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 79796286a4ba..2907c762a8fe 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -918,8 +918,6 @@ 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);
 }
 
@@ -1231,6 +1229,7 @@ static void tick_nohz_idle_update_tick(struct tick_sched *ts, ktime_t now)
 	else
 		tick_nohz_restart_sched_tick(ts, now);
 
+	ts->idle_exittime = now;
 	tick_nohz_account_idle_ticks(ts);
 }
 
-- 
2.25.1