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

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Date: Fri, 7 Jun 2024 18:37:54 +0800
From: Xuewen Yan <xuewen.yan94@...il.com>
To: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Xuewen Yan <xuewen.yan@...soc.com>, vincent.guittot@...aro.org, mingo@...hat.com, 
	peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com, 
	vincent.donnefort@....com, qyousef@...alina.io, ke.wang@...soc.com, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Prevent cpu_busy_time from exceeding actual_cpu_capacity

On Fri, Jun 7, 2024 at 6:30 PM Dietmar Eggemann
<dietmar.eggemann@....com> wrote:
>
> On 07/06/2024 10:20, Xuewen Yan wrote:
> > Hi Dietmar
> >
> > On Fri, Jun 7, 2024 at 3:19 PM Dietmar Eggemann
> > <dietmar.eggemann@....com> wrote:
> >>
> >> On 06/06/2024 09:06, 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.
> >>
> >> Doesn't return effective_cpu_util() either scale or min(scale, util)
> >> with scale = arch_scale_cpu_capacity(cpu)? So the util sum over the PD
> >> cannot exceed eenv->cpu_cap?
> >
> > In effective_cpu_util, the scale = arch_scale_cpu_capacity(cpu);
> >  Although there is the clamp of eenv->pd_cap, but let us consider the
> > following simple scenario:
> > The pd cpus are 4-7, and the arch_scale_capacity is 1024, and because
> > of cpufreq-limit,
>
> Ah, this is due to:
>
> find_energy_efficient_cpu()
>
>    ...
>    for (; pd; pd = pd->next)
>        ...
>        cpu_actual_cap = get_actual_cpu_capacity(cpu)
>
>        for_each_cpu(cpu, cpus)
>            ...
>            eenv.pd_cap += cpu_actual_cap
>
> and:
>
> get_actual_cpu_capacity()
>
>    ...
>    capacity = arch_scale_cpu_capacity(cpu)
>
>    capacity -= max(hw_load_avg(cpu_rq(cpu)), cpufreq_get_pressure(cpu))
>
> which got introduced by f1f8d0a22422 ("sched/cpufreq: Take cpufreq
> feedback into account").

I don't think it was introduced by f1f8d0a22422, because f1f8d0a22422
just replaced the cpu_thermal_cap with get_actual_cpu_capacity(cpu).
The eenv.cpu_cap was  introduced by 3e8c6c9aac42 ("sched/fair: Remove
task_util from effective utilization in feec()").

BR
---
xuewen

>
> [...]