Re: [RFC][PATCH 1/3] cpuidle: Inject tick boundary state

[Rafael J. Wysocki]

On Wed, Aug 2, 2023 at 12:34 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Jul 31, 2023 at 06:55:35PM +0200, Rafael J. Wysocki wrote:
>
> > > In that case you cannot tell the difference between I'm good to use this
> > > state and I'm good to disable the tick and still use this state.
> >
> > No, you don't, but is it really worth the fuss?
>
> My somewhat aged IVB-EP sits around 25 us for restarting the tick.
>
> Depending on the C state, that is a significant chunk of exit latency,
> and depending on how often you do the whole NOHZ dance, this can add up
> to significant lost runtime too.
>
> And these are all machines that have a usable TSC, these numbers all go
> up significantly when you somehow end up on the HPET or similar wreckage.
>
> Stopping the tick is slightly more expensive, but in the same order, I
> get around 30 us on the IVB, vs 25 for restarting it. Reprogramming the
> timer (LAPIC/TSC-DEADLINE) is the main chunk of it I suspect.
>
> So over-all that's 55 us extra latency for the full idle path, which can
> definitely hurt.
>
> So yeah, I would say this is all worth it.

I agree that, in general, it is good to avoid stopping the tick when
it is not necessary to stop it.

> My ADL is somewhat better, but also much higher clocked, and gets around
> 10 us for a big core and 16 us for a little core for restarting the
> tick.

But my overall point is different.

An additional bin would possibly help if the deepest state has been
selected and its target residency is below the tick, and the closest
timer (other than the tick) is beyond the tick.  So how much of a
difference would be made by making this particular case more accurate?