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

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Date: Sun, 16 Jun 2024 22:48:12 +0100
From: Qais Yousef <qyousef@...alina.io>
To: 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>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Todd Kjos <tkjos@...gle.com>, "wvw@...gle.com" <wvw@...gle.com>
Subject: Re: [PATCH v6 2/2] cpuidle: teo: Introduce util-awareness

On 06/12/24 08:53, Lukasz Luba wrote:

> > 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.
> 
> This is not true. When you up-migrate a task to big CPU, then CPU idle
> gov can instantly benefit from utilization information and won't make
> mistake based on old local history and won't use deep idle state.
> So migrating the utilization from one CPU to another CPU says a lot
> about the idleness to that destination CPU.

You can migrate a 1024 tasks to a bigger core, but it could run for 3ms and
sleep for 40ms. So unfortunately I have to disagree with you here.
Generally a high util value doesn't mean we are not going to be idle long
enough to satisfy min_residency of the CPU.

> When Christian removed the util he got -4.5% lower score in GB5, so
> this util has impact [1].

We need the idle governor to help with power saving. We can disable DVFS and
all idle states and always get a better performance.

The residency is ~50% worse on some clusters with this change. Overall power
impact is 2-4% in many use cases.

GB5 is not really a representative use case to measure the usefulness of the
idle governor. Task placement to avoid the cost of idle exit latency is
a completely different problem.