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Message-ID: <fe5038e1-e06b-41dd-9584-cbd992bef1fe@arm.com>
Date: Mon, 15 Jan 2024 13:40:54 +0100
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Subject: Re: [PATCH] sched/idle: Prevent stopping the tick when there is no
 cpuidle driver
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To: Thomas Gleixner <tglx@linutronix.de>,
 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>
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> <87mstaioy6.ffs@tglx>
From: Pierre Gondois <pierre.gondois@arm.com>
In-Reply-To: <87mstaioy6.ffs@tglx>
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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.

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