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

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Date: Wed, 19 Jun 2024 19:03:48 +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 06/19/24 10:46, Xuewen Yan wrote:
> On Tue, Jun 18, 2024 at 10:58 PM Qais Yousef <qyousef@...alina.io> wrote:
> >
> > 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..
> 
> Sorry for the late reply...
> In our own tree, we removed the check for rd overutil in feec(), so
> the above case often occurs.

Yeah, the current definition of overutilized is not good enough. So I can see
the need to do that..

> And now it seems that on the mainline, uclamp_max is the only way to
> override this.

Thanks for checking. Your patch is correct and if we combine this with another
patch to use get_actual_cpu_capacity() in util_fits_cpu(), they are good
improvements.

Are you still happy to send an updated patches?


Thanks!

--
Qais Yousef