linux-kernel - Re: Stopping the tick on a fully loaded system

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Message-ID: <20230727075922.GA3886590@hirez.programming.kicks-ass.net>
Date:   Thu, 27 Jul 2023 09:59:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        g@...ez.programming.kicks-ass.net
Cc:     Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        "Gautham R. Shenoy" <gautham.shenoy@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>
Subject: Re: Stopping the tick on a fully loaded system

On Wed, Jul 26, 2023 at 06:14:32PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 26, 2023 at 05:53:46PM +0200, Rafael J. Wysocki wrote:
> 
> > > > That means we don't track nearly enough data to reliably tell anything
> > > > about disabling the tick or not. We should have at least one bucket
> > > > beyond TICK_NSEC for this.
> > >
> > > Quite likely.
> > 
> > So the reasoning here was that those additional bins would not be
> > necessary for idle state selection, but the problem of whether or not
> > to stop the tick is kind of separate from the idle state selection
> > problem if the target residency values for all of the idle states are
> > relatively short.  And so it should be addressed separately which
> > currently it is not.  Admittedly, this is a mistake.
> 
> Right, the C state buckets are enough to pick a state, but not to handle
> the tick thing.
> 
> The below hack boots on my ivb-ep with extra (disabled) states. Now let
> me go hack up teo to make use of that.
> 
> name		residency
> 
> POLL		0
> C1              1
> C1E             80
> C3              156
> C6              300
> TICK            1000
> POST-TICK       2000
> 

Ah, so last night (or rather, somewhat realy today) I realized I has the
buckets wrong.

We don't have buckets to the left, but buckets to the right, so the
above would give:

0: [0,1)
1: [1,80)
2: [80,156)
3: [156,300)
4: [300,1000)
5: [1000,2000)
6: [2000,...)

Which also means I can ditch the whole POST-TICK bucket. Let me get
breakfast and try all this again.