Re: Stopping the tick on a fully loaded system

Peter Zijlstra
2023-07-27T08:03:59Z

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.