Re: [PATCH] sched/idle: Prevent stopping the tick when there is no cpuidle driver

[Vincent Guittot]

On Mon, 15 Jan 2024 at 13:40, Pierre Gondois <pierre.gondois@xxxxxxx> wrote:
>
> Hello Thomas,
>
> On 1/12/24 15:52, Thomas Gleixner wrote:
> > On Fri, Jan 12 2024 at 14:39, Pierre Gondois wrote:
> >> On 1/12/24 11:56, Anna-Maria Behnsen wrote:
> >>> Pierre Gondois <pierre.gondois@xxxxxxx> writes:
> >>>> I agree that the absence of cpuidle driver prevents from reaching deep
> >>>> idle states. FWIU, there is however still benefits in stopping the tick
> >>>> on such platform.
> >>>
> >>> What's the benefit?
> >>
> >> I did the following test:
> >> - on an arm64 Juno-r2 platform (2 big A-72 and 4 little A-53 CPUs)
> >> - booting with 'cpuidle.off=1'
> >> - using the energy counters of the platforms
> >>     (the counters measure energy for the whole cluster of big/little CPUs)
> >> - letting the platform idling during 10s
> >>
> >> So the energy consumption would be up:
> >> - ~6% for the big CPUs
> >> - ~10% for the litte CPUs
> >
> > Fair enough, but what's the actual usecase?
> >
> > NOHZ w/o cpuidle driver seems a rather academic exercise to me.

Don't know if it's really a valid use case but can't we have VMs in
such a configuration ?
NOHZ enabled and no cpuidle driver as VM doesn't manage HW anyway ?

>
> I thought Anna-Maria had a use-case for this.
> I just wanted to point out that this patch could potentially
> increase the energy consumption for her use-case, nothing more,
>
> Regards,
> Pierre
>
> >
> > Thanks,
> >
> >          tglx