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

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Date:   Fri, 10 Aug 2018 09:15:54 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] sched: idle: Avoid retaining the tick when it has been stopped

On Fri, Aug 10, 2018 at 8:19 AM,  <leo.yan@...aro.org> wrote:
> On Thu, Aug 09, 2018 at 07:08:34PM +0200, Rafael J . Wysocki wrote:
>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>
>> 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@...el.com>
>> ---
>>  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();
>>