linux-kernel - Re: [PATCH v6 2/2] cpuidle: teo: Introduce util-awareness

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Date: Wed, 29 May 2024 14:09:44 +0100
From: Christian Loehle <christian.loehle@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>,
 Lukasz Luba <lukasz.luba@....com>
Cc: Kajetan Puchalski <kajetan.puchalski@....com>, rafael@...nel.org,
 daniel.lezcano@...aro.org, Dietmar.Eggemann@....com, dsmythies@...us.net,
 yu.chen.surf@...il.com, linux-pm@...r.kernel.org,
 linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
 Ulf Hansson <ulf.hansson@...aro.org>, Qais Yousef <qyousef@...alina.io>
Subject: Re: [PATCH v6 2/2] cpuidle: teo: Introduce util-awareness

On 5/28/24 15:07, Vincent Guittot wrote:
> On Tue, 28 May 2024 at 11:59, Lukasz Luba <lukasz.luba@....com> wrote:
>>
>> Hi Vincent,
>>
>> On 5/28/24 10:29, Vincent Guittot wrote:
>>> Hi All,
>>>
>>> I'm quite late on this thread but this patchset creates a major
>>> regression for psci cpuidle driver when using the OSI mode (OS
>>> initiated mode).  In such a case, cpuidle driver takes care only of
>>> CPUs power state and the deeper C-states ,which includes cluster and
>>> other power domains, are handled with power domain framework. In such
>>> configuration ,cpuidle has only 2 c-states : WFI and cpu off states
>>> and others states that include the clusters, are managed by genpd and
>>> its governor.
>>>
>>> This patch selects cpuidle c-state N-1 as soon as the utilization is
>>> above CPU capacity / 64 which means at most a level of 16 on the big
>>> core but can be as low as 4 on little cores. These levels are very low
>>> and the main result is that as soon as there is very little activity
>>> on a CPU, cpuidle always selects WFI states whatever the estimated
>>> sleep duration and which prevents any deeper states. Another effect is
>>> that it also keeps the tick firing every 1ms in my case.
>>
>> Thanks for reporting this.
>> Could you add what regression it's causing, please?
>> Performance or higher power?
> 
> It's not a perf but rather a power regression. I don't have a power
> counter so it's difficult to give figures but I found it while running
> a unitary test below on my rb5:
> run 500us every 19457ms on medium core (uclamp_min: 600).

Is that supposed to say 19.457ms?
(Because below you say idle time is >18ms and total test time 5sec)
Is the utilisation more like 1/20000 or 1/20?
In any case what you describe is probably an issue, I'll try to reproduce.
Note also my findings here:
https://lore.kernel.org/lkml/0ce2d536-1125-4df8-9a5b-0d5e389cd8af@arm.com/

Kind Regards,
Christian