Re: [PATCH] cpuidle: Always stop scheduler tick on adaptive-tick CPUs

[Frederic Weisbecker]

On Tue, Jul 16, 2019 at 05:25:10PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> 
> Running the scheduler tick on idle adaptive-tick CPUs is not useful

Judging by the below change, you mean full dynticks, right?

> and it may also be not expected by users (as reported by Thomas), so
> add a check to cpuidle_idle_call() to always stop the tick on them
> regardless of the idle duration predicted by the governor.
> 
> Fixes: 554c8aa8ecad ("sched: idle: Select idle state before stopping the tick")
> Reported-by: Thomas Lindroth <thomas.lindroth@xxxxxxxxx>
> Tested-by: Thomas Lindroth <thomas.lindroth@xxxxxxxxx>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
>  kernel/sched/idle.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> Index: linux-pm/kernel/sched/idle.c
> ===================================================================
> --- linux-pm.orig/kernel/sched/idle.c
> +++ linux-pm/kernel/sched/idle.c
> @@ -191,7 +191,8 @@ static void cpuidle_idle_call(void)
>  		 */
>  		next_state = cpuidle_select(drv, dev, &stop_tick);
>  
> -		if (stop_tick || tick_nohz_tick_stopped())
> +		if (stop_tick || tick_nohz_tick_stopped() ||
> +		    !housekeeping_cpu(dev->cpu, HK_FLAG_TICK))

But tick_nohz_tick_stopped() also works on full dynticks CPUs. If the
tick isn't stopped on a full dynticks CPU by the time we reach this path,
it means that the conditions for the tick to be stopped are not met anyway
(eg: more than one task and sched tick is needed, perf event requires the tick,
posix CPU timer, etc...)

Or am I missing something else?

Thanks.

>  			tick_nohz_idle_stop_tick();
>  		else
>  			tick_nohz_idle_retain_tick();
> 
> 
>