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

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Date: Wed, 29 May 2024 11:19:50 +0100
From: Qais Yousef <qyousef@...alina.io>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Kajetan Puchalski <kajetan.puchalski@....com>, rafael@...nel.org,
	daniel.lezcano@...aro.org, lukasz.luba@....com,
	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>
Subject: Re: [PATCH v6 2/2] cpuidle: teo: Introduce util-awareness

On 05/28/24 11:29, 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.

Unfortunately I think we need to revert this. We've been seeing the power
regressions for a long while now and it doesn't seem we'll see an improvement
soon based on last discussion.

> 
> IMO, we should at least increase the utilization level

This won't help. We tried different values, unfortunately the logic is flawed.
Utilization value on its own says nothing about the idleness of the system.
I think best to revert and rethink the logic. Which is something we're pursuing
and we'll share outcome when we have something to share. As it stands, this
doesn't help. And we should really strive to avoid magic thresholds and values.
They don't scale.


Thanks!

--
Qais Yousef