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

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Message-ID: <20240618145851.je4a7cu4ltrt3qxa@airbuntu>
Date: Tue, 18 Jun 2024 15:58:51 +0100
From: Qais Yousef <qyousef@...alina.io>
To: Xuewen Yan <xuewen.yan94@...il.com>
Cc: Xuewen Yan <xuewen.yan@...soc.com>, vincent.guittot@...aro.org,
	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 12:03, Qais Yousef wrote:

> > Sorry, I miss the "fits_capacity() use capacity_of()", and without
> > uclamp_max, the rd is over-utilized,
> > and would not use feec().
> > But I notice the uclamp_max, if the rq's uclamp_max is smaller than
> > SCHED_CAPACITY_SCALE,
> > and is bigger than actual_cpu_capacity, the util_fits_cpu() would
> > return true, and the rd is not over-utilized.
> > Is this setting intentional?
> 
> Hmm. To a great extent yes. We didn't want to take all types of rq pressure
> into account for uclamp_max. But this corner case could be debatable.
> 
> Is this the source of your problem? If you change util_fits_cpu() to return
> false here, would this fix the problem you're seeing?

FWIW, if this happens due to uclamp_max, then this patch to do the capping is
still needed.

I think it's good to understand first how we end up in feec() when a CPU is
supposed to be overutlized. uclamp_max is the only way to override this
decision AFAICT..