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

[Pierre Gondois]

On 1/15/24 14:29, Vincent Guittot wrote:
> 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 ?

Yes right,
I tried with a kvmtool generated VM and it seemed to be the case:

$ grep . /sys/devices/system/cpu/cpuidle/*
/sys/devices/system/cpu/cpuidle/available_governors:menu
/sys/devices/system/cpu/cpuidle/current_driver:none
/sys/devices/system/cpu/cpuidle/current_governor:menu
/sys/devices/system/cpu/cpuidle/current_governor_ro:menu


> > 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