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

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Message-ID: <87y1chy8e4.fsf@somnus>
Date: Mon, 22 Jan 2024 11:22:59 +0100
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: Pierre Gondois <pierre.gondois@....com>, Vincent Guittot
 <vincent.guittot@...aro.org>
Cc: Thomas Gleixner <tglx@...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

Hi,

Pierre Gondois <pierre.gondois@....com> writes:
> 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
>

So it's not on me to decide whether it is valid to skip stopping the
tick in this setting or not. I observed this unconditional call (which
is not for free) on a fully loaded system which decreases performance.

If there is a reasonable condition that could be added for stopping the
tick, this might also be a good solution or even a better solution. But
only checking whether cpuidle driver is available or not and then
unconditionally stopping the tick, doesn't make sense IMHO.

Thanks,

	Anna-Maria