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

Paul E. McKenney
2017-07-11T18:09:40Z

On Tue, Jul 11, 2017 at 06:34:22PM +0200, Peter Zijlstra wrote:
> 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.

It is called on pretty much every transition to idle.

> 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?

If you are battery powered and don't have tight real-time latency
constraints, you want it -- it has represent a 30-40% boost in battery
lifetime for some low-utilization battery-powered devices. Otherwise,
probably 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);

With this addition, I am all for it:

RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_idle_enter() invoked with irqs enabled!!!");

If you are OK with this addition, may I please have your Signed-off-by?

Thanx, Paul

> rcu_eqs_enter(false);
> - local_irq_restore(flags);
> }
> -EXPORT_SYMBOL_GPL(rcu_idle_enter);
>
> #ifdef CONFIG_NO_HZ_FULL
> /**
>