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

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Date: Mon, 17 Jun 2024 12:03:48 +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 06/17/24 15:27, Xuewen Yan wrote:
> Hi Qais,
> 
> On Mon, Jun 17, 2024 at 6:20 AM Qais Yousef <qyousef@...alina.io> wrote:
> >
> > On 06/12/24 16:11, Xuewen Yan wrote:
> > > Hi Qais
> > >
> > > On Mon, Jun 10, 2024 at 6:55 AM Qais Yousef <qyousef@...alina.io> wrote:
> > > >
> > > > On 06/06/24 15:06, Xuewen Yan wrote:
> > > > > Because the effective_cpu_util() would return a util which
> > > > > maybe bigger than the actual_cpu_capacity, this could cause
> > > > > the pd_busy_time calculation errors.
> > > > > So clamp the cpu_busy_time with the eenv->cpu_cap, which is
> > > > > the actual_cpu_capacity.
> > > >
> > > > I actually think capping by pd_cap is something we should remove. Saturated
> > > > systems aren't calculated properly especially when uclamp_max is used.
> > > >
> > > > But this might a bigger change and out of scope of what you're proposing..
> > >
> > > I agree, there are other things to consider before doing this.
> > >
> > > >
> > > > Did this 'wrong' calculation cause an actual problem for task placement?
> > > > I assume the pd looked 'busier' because some CPUs were too busy.
> > >
> > > This will not only affect calculations in scenarios with high temperatures.
> > > Sometimes, users will set scalimg_max_freq to actively limit the CPU frequency,
> > > so that even if the CPU load is large, the CPU frequency will not be very high.
> > > At this time, even if tasks are placed on other CPUs in the same PD,
> > > the energy increment may not be too large, thus affecting core selection.
> > >
> > > >
> > > > Was the system in overutilzied state? Usually if one CPU is that busy
> > > > overutilized should be set and we'd skip EAS - unless uclamp_max was used.
> > >
> > > As Christian said, This case occurs not only in the overutil scenario,
> > > this scenario holds true as long as the actual-cpu-capacity caused by
> > > the reduction in max cpu frequency is smaller than the cpu util.
> >
> > Hmm. Sorry I might be thick here, but shouldn't fits_capacity() use
> > capacity_of() which should return capacity based on get_actual_cpu_capacity()
> > to compare if we are overutilized? If we are higher than this value that you
> > need to cap, then the CPU must be overutilized and we shouldn't be in feec() in
> > the first place, no? Unless the rq is capped with uclamp_max that is.
> 
> 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?

> 
> >
> > I generally think our current definition of overutilized is wrong and the use
> > case you're talking about should hold true if it's just a single CPU that is
> > overutilized. But I can't see how you end up in feec() if the util is higher
> > than the CPU in our current code base.
> >
> > What did I miss?
> >
> > And should effective_cpu_util() return a value higher than
> > get_actual_cpu_capacity()?
> 
> I also thought about changing this at first, but because this function
> is called in many places,
> I am not 100% sure that changing it will not have any unexpected consequences,
> so I only changed feec():)

Yes I had similar doubts. But the question had to be asked :-)