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From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Mon, 15 Jan 2024 14:29:26 +0100
Message-ID: <CAKfTPtC3kN7gsgh+W4YZutQDMfs6TeT_F+Y5ba9txcM2jbbX6g@mail.gmail.com>
Subject: Re: [PATCH] sched/idle: Prevent stopping the tick when there is no
 cpuidle driver
To: Pierre Gondois <pierre.gondois@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Anna-Maria Behnsen <anna-maria@linutronix.de>, 
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>, 
	Peter Zijlstra <peterz@infradead.org>, Juri Lelli <juri.lelli@redhat.com>, 
	Dietmar Eggemann <dietmar.eggemann@arm.com>, Steven Rostedt <rostedt@goodmis.org>, 
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>, 
	Daniel Bristot de Oliveira <bristot@redhat.com>, Valentin Schneider <vschneid@redhat.com>
Content-Type: text/plain; charset="UTF-8"

On Mon, 15 Jan 2024 at 13:40, Pierre Gondois <pierre.gondois@arm.com> 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@arm.com> 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