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From: Thomas Gleixner <tglx@linutronix.de>
To: Pierre Gondois <pierre.gondois@arm.com>, Anna-Maria Behnsen
 <anna-maria@linutronix.de>, linux-kernel@vger.kernel.org
Cc: Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
 Juri Lelli <juri.lelli@redhat.com>, Vincent Guittot
 <vincent.guittot@linaro.org>, 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>
Subject: Re: [PATCH] sched/idle: Prevent stopping the tick when there is no
 cpuidle driver
In-Reply-To: <d0f1617e-0088-4bd9-bea6-e89c63b0e2ae@arm.com>
References: <20231215130501.24542-1-anna-maria@linutronix.de>
 <c09fb78b-5bf9-4c0b-b93f-10fd19a4ab36@arm.com> <87ttnmiif9.fsf@somnus>
 <06a2561f-557b-4eaa-8f11-75883bbbaef9@arm.com> <87a5pag6q7.fsf@somnus>
 <d0f1617e-0088-4bd9-bea6-e89c63b0e2ae@arm.com>
Date: Fri, 12 Jan 2024 15:52:33 +0100
Message-ID: <87mstaioy6.ffs@tglx>
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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.

Thanks,

        tglx