Re: [PATCH] sched: idle: Avoid retaining the tick when it has been stopped

[Frederic Weisbecker]

On Thu, Aug 09, 2018 at 07:08:34PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> 
> If the tick has been stopped already, but the governor has not asked to
> stop it (which it can do sometimes), the idle loop should invoke
> tick_nohz_idle_stop_tick(), to let tick_nohz_stop_tick() take care
> of this case properly.
> 
> Fixes: 554c8aa8ecad (sched: idle: Select idle state before stopping the tick)
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
>  kernel/sched/idle.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux-pm/kernel/sched/idle.c
> ===================================================================
> --- linux-pm.orig/kernel/sched/idle.c
> +++ linux-pm/kernel/sched/idle.c
> @@ -190,7 +190,7 @@ static void cpuidle_idle_call(void)
>  		 */
>  		next_state = cpuidle_select(drv, dev, &stop_tick);
>  
> -		if (stop_tick)
> +		if (stop_tick || tick_nohz_tick_stopped())
>  			tick_nohz_idle_stop_tick();
>  		else
>  			tick_nohz_idle_retain_tick();

So what if tick_nohz_idle_stop_tick() sees no timer to schedule and
cancels it, we may remain idle in a shallow state for a long while?

Otherwise we can have something like this:

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index da9455a..408c985 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -806,6 +806,9 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
 static void tick_nohz_retain_tick(struct tick_sched *ts)
 {
 	ts->timer_expires_base = 0;
+
+	if (ts->tick_stopped)
+		tick_nohz_restart(ts, ktime_get());
 }
 
 #ifdef CONFIG_NO_HZ_FULL