Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

Peter Zijlstra
2017-07-11T16:34:35Z

On Tue, Jul 11, 2017 at 06:09:27PM +0200, Frederic Weisbecker wrote:

> > > - tick_nohz_idle_enter costs 7058ns - 10726ns
> > > - tick_nohz_idle_exit costs 8372ns - 20850ns
> >
> > Right, those are horrible expensive, but skipping them isn't 'hard', the
> > only tricky bit is finding a condition that makes sense.
>
> Note you can statically disable it with nohz=0 boot parameter.

Yeah, but that's bad for power usage, nobody wants that.

> > See Mike's patch: https://patchwork.kernel.org/patch/2839221/
> >
> > Combined with the above, and possibly a better condition, that should
> > get rid of most of this.
>
> Such a patch could work well if the decision from the scheduler to not stop the tick
> happens on idle entry.
>
> Now if sched_needs_cpu() first allows to stop the tick then refuses it later
> in the end of an idle IRQ, this won't have the desired effect. As long as ts->tick_stopped=1,
> it stays so until we really restart the tick. So the whole costly nohz machinery stays on.
>
> I guess it doesn't matter though, as we are talking about making fast idle entry so the
> decision not to stop the tick is likely to be done once on idle entry, when ts->tick_stopped=0.
>
> One exception though: if the tick is already stopped when we enter idle (full nohz case). And
> BTW stopping the tick outside idle shouldn't be concerned here.
>
> So I'd rather put that on can_stop_idle_tick().

Mike's patch much predates the existence of that function I think ;-) But
sure..

> >
> > > - totally from arch_cpu_idle_enter entry to arch_cpu_idle_exit return costs
> > > 9122ns - 15318ns.
> > > --In this period, rcu_idle_enter costs 1985ns - 2262ns, rcu_idle_exit costs
> > > 1813ns - 3507ns
> >
> > Is that the POPF being painful? or something else?
>
> Probably that and the atomic_add_return().

I got properly lost in the RCU machinery. It wasn't at all clear to me
if rcu_eqs_enter_common() was a slow-path function or not.

Also, RCU_FAST_NO_HZ will make a fairly large difference here.. Paul
what's the state of that thing, do we actually want that or not?

But I think we can at the very least do this; it only gets called from
kernel/sched/idle.c and both callsites have IRQs explicitly disabled by
that point.

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 51d4c3acf32d..dccf2dc8155a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -843,13 +843,8 @@ static void rcu_eqs_enter(bool user)
*/
void rcu_idle_enter(void)
{
- unsigned long flags;
-
- local_irq_save(flags);
rcu_eqs_enter(false);
- local_irq_restore(flags);
}
-EXPORT_SYMBOL_GPL(rcu_idle_enter);

#ifdef CONFIG_NO_HZ_FULL
/**