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

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Message-ID: <fcc07133-c43d-4423-bf92-b1d720c7e864@arm.com>
Date: Mon, 15 Jan 2024 16:41:25 +0100
From: Pierre Gondois <pierre.gondois@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Thomas Gleixner <tglx@...utronix.de>,
 Anna-Maria Behnsen <anna-maria@...utronix.de>, linux-kernel@...r.kernel.org,
 Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>,
 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 1/15/24 14:29, Vincent Guittot wrote:
> On Mon, 15 Jan 2024 at 13:40, Pierre Gondois <pierre.gondois@....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@....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 ?

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