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

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Date: Fri, 7 Jun 2024 16:20:49 +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

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,
the cpu_actual_cap = 512. Then the eenv->cpu_cap = 512, the eenv->pd_cap = 2048;
effective_cpu_util(4) = 1024;
effective_cpu_util(5) = 1024;
effective_cpu_util(6) = 256;
effective_cpu_util(7) = 0;

Without this patch, the eenv->pd_busy_time = 2048, because the clamp
of eenv->pd_cap.
However, indeed, the cpu4 and cpu5's util would not exceed the actual_cap(512),
so the cpu_util4/5 = 512, instead of 1024.
And with this patch, the eenv->pd_busy_time = 512+512+256 = 1280.
And the 1280 should be more reasonable.

BR
--
xuewen

> Looks like this was the case with 3e8c6c9aac42 already.
>
> > So clamp the cpu_busy_time with the eenv->cpu_cap, which is
> > the actual_cpu_capacity.
> >
> > Fixes: 3e8c6c9aac42 ("sched/fair: Remove task_util from effective utilization in feec()")
> > Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> > ---
> >  kernel/sched/fair.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 8a5b1ae0aa55..8939d725023a 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7870,7 +7870,9 @@ static inline void eenv_pd_busy_time(struct energy_env *eenv,
> >       for_each_cpu(cpu, pd_cpus) {
> >               unsigned long util = cpu_util(cpu, p, -1, 0);
> >
> > -             busy_time += effective_cpu_util(cpu, util, NULL, NULL);
> > +             util = effective_cpu_util(cpu, util, NULL, NULL);
> > +             util = min(eenv->cpu_cap, util);
> > +             busy_time += util;
> >       }
> >
> >       eenv->pd_busy_time = min(eenv->pd_cap, busy_time);
>