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

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Date:   Wed, 2 Aug 2023 12:34:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     anna-maria@...utronix.de, tglx@...utronix.de, frederic@...nel.org,
        gautham.shenoy@....com, linux-kernel@...r.kernel.org,
        daniel.lezcano@...aro.org, linux-pm@...r.kernel.org,
        mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com
Subject: Re: [RFC][PATCH 1/3] cpuidle: Inject tick boundary state

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.

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.