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

From: Rafael J. Wysocki
Date: Fri Aug 10 2018 - 03:15:58 EST

On Fri, Aug 10, 2018 at 8:19 AM, <leo.yan@xxxxxxxxxx> wrote:
> 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.
> IMHO, I don't think this patch is on the right way;

So we disagree here, quite obviously.

> from the idle loop side, it needs to provide sane fundamental supports,
> for example, it can stop or restart the tick per idle governor's request.

No, if the tick is stopped, restarting it is pointless until we exit
the loop in do_idle().

> On the other hand, the idle governors can decide their own policy for how to
> use the tick in idle loop. This patch seems mixes two things and
> finally it's possible to couple the implementation between idle loop
> and 'menu' governor for sched tick usage.

I'm not following this, sorry.

> I still think my patch to restart the tick is valid :)

It changes the behavior significantly, though, and it is not clear if
the new behavior is desirable.

The patch here simply fixes a problem while leaving the overall behavior as is.

>> 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();