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

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Aug 17, 2018 at 11:32:07AM +0200, Rafael J. Wysocki wrote:
> On Thursday, August 16, 2018 3:27:24 PM CEST Frederic Weisbecker wrote:
> > On Thu, Aug 09, 2018 at 07:08:34PM +0200, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@intel.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.
> > > 
> > > Fixes: 554c8aa8ecad (sched: idle: Select idle state before stopping the tick)
> > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.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();
> > 
> > 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?
> 
> Yes, but the governor is expected to avoid using shallow states when the
> tick is stopped already.

So what kind of sleep do we enter to when an idle tick fires and we go
back to idle? Is it always deep?

I believe that ts->tick_stopped == 1 shouldn't be too relevant for the governor.
We can definetly have scenarios where the idle tick is stopped for a long while,
then it fires and schedules the next timer at NOW() + TICK_NSEC (as if the tick
had been restarted). This can even repeat that way for some time, because
ts->tick_stopped == 1 only implies that the tick has been stopped once since
we entered the idle loop. After that we may well have a periodic tick behaviour.
In that case we probably don't want deep idle state. Especially if we have:

              idle_loop() {
                  tick_stop (scheduled several seconds forward)
                  deep_idle_sleep()
                  //several seconds later
                  tick()
                  tick_stop (scheduled TICK_NSEC forward)
                  deep_idle_sleep()
                  tick() {
                      set_need_resched()
                  }
                  exit idle loop
              }

Here the last deep idle state isn't necessary.

> 
> > 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
> > 
> 
> We could do that, but my concern with that approach is that we may end up
> stopping and starting the tick back and forth without exiting the loop
> in do_idle() just because somebody uses a periodic timer behind our
> back and the governor gets confused.
> 
> Besides, that would be a change in behavior, while the $subject patch
> simply fixes a mistake in the original design.

Ok, let's take the safe approach for now as this is a fix and it should even be
routed to stable.

But then in the longer term, perhaps cpuidle_select() should think that
through.

Thanks.