linux-kernel - Re: [PATCH] sched/fair: Prevent cpu_busy_time from exceeding actual_cpu

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Date: Mon, 17 Jun 2024 11:53:48 +0100
From: Qais Yousef <qyousef@...alina.io>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Xuewen Yan <xuewen.yan94@...il.com>, Xuewen Yan <xuewen.yan@...soc.com>,
	mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
	vincent.donnefort@....com, ke.wang@...soc.com,
	linux-kernel@...r.kernel.org, christian.loehle@....com
Subject: Re: [PATCH] sched/fair: Prevent cpu_busy_time from exceeding
 actual_cpu_capacity

On 06/17/24 11:07, Vincent Guittot wrote:

> > And should effective_cpu_util() return a value higher than
> > get_actual_cpu_capacity()?
> 
> I don't think we should because we want to return the effective
> utilization not the actual compute capacity.
> Having an utilization of the cpu or group of cpus above the actual
> capacity or the original capacity mainly means that we will have to
> run longer
> 
> By capping the utilization we filter this information.
> 
> capacity orig = 800
> util_avg = 700
> 
> if we cap the capacity to 400 the cpu is expected to run twice longer
> for the same amount of work to be done

Okay makes sense. Wouldn't the util be 'wrong' (to what degree will depend on
min/max freq ratio) though?

We cap with arch_scale_capacity() still, I guess we know at this stage it is
100% wrong if we allow returning higher values?