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

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Date: Wed, 19 Jun 2024 19:10:21 +0100
From: Qais Yousef <qyousef@...alina.io>
To: Xuewen Yan <xuewen.yan94@...il.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>,
	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 06/19/24 11:05, Xuewen Yan wrote:
> On Tue, Jun 18, 2024 at 11:39 PM Qais Yousef <qyousef@...alina.io> wrote:
> >
> > On 06/18/24 17:23, Vincent Guittot wrote:
> > > On Mon, 17 Jun 2024 at 12:53, Qais Yousef <qyousef@...alina.io> wrote:
> > > >
> > > > 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?
> > >
> > > I think that capping utilization to max capacity generates some energy
> > > estimation error because it filters the fact that we run longer in
> > > some cases.
> >
> > Yes, I think so too and that was my first statement. But I think this is
> > a bigger change to do separately.
> 
> I saw the the sched_cpu_util() was used by teo.c and cpufreq_cooling.c
> If we change the arch_scale_capacity() to actual_cpu_capacity(), it may cause
> some errors?

The plan to revert this now.

> 
> For-example:
> In teo:
> 212 static bool teo_cpu_is_utilized(int cpu, struct teo_cpu *cpu_data)
> 213 {
> 214         return sched_cpu_util(cpu) > cpu_data->util_threshold;
> 215 }
> It may cause the teo_cpu_is_utilized() to return false forever if the
> actual_cpu_capacity is smaller than util_threshold.
> However, the util_threshold is frome actual_cpu_capacity.
> 
> In cpufreq_cooling.c:
> May we should change:
> 
> diff --git a/drivers/thermal/cpufreq_cooling.c
> b/drivers/thermal/cpufreq_cooling.c
> index 280071be30b1..a8546d69cc10 100644
> --- a/drivers/thermal/cpufreq_cooling.c
> +++ b/drivers/thermal/cpufreq_cooling.c
> @@ -164,7 +164,7 @@ static u32 get_load(struct cpufreq_cooling_device
> *cpufreq_cdev, int cpu,
>  {
>         unsigned long util = sched_cpu_util(cpu);
> 
> -       return (util * 100) / arch_scale_cpu_capacity(cpu);
> +       return (util * 100) / get_actual_cpu_capacity(cpu);
>  }
>  #else /* !CONFIG_SMP */
>  static u32 get_load(struct cpufreq_cooling_device *cpufreq_cdev, int cpu,
> 
> 
> Because if still use arch_scale_cpu_capacity(), the load pct may be decreased,
> It may affect the thermal-IPA-governor's power consideration.

I am not sure about this one. But looks plausible. Vincent?