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

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Date: Fri, 12 Jan 2024 15:52:33 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Pierre Gondois <pierre.gondois@....com>, Anna-Maria Behnsen
 <anna-maria@...utronix.de>, linux-kernel@...r.kernel.org
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot
 <vincent.guittot@...aro.org>, 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: [PATCH] sched/idle: Prevent stopping the tick when there is no
 cpuidle driver

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@....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.

Thanks,

        tglx