Date: Mon, 1 Jul 2024 20:13:12 +0800
From: Xuewen Yan <xuewen.yan94@...il.com>
To: Qais Yousef <qyousef@...alina.io>
Cc: Xuewen Yan <xuewen.yan@...soc.com>, vincent.guittot@...aro.org, 
	dietmar.eggemann@....com, 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, 
	christian.loehle@....com, vincent.donnefort@....com, ke.wang@...soc.com, 
	di.shen@...soc.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/2] sched/fair: Use actual_cpu_capacity everywhere in util_fits_cpu()

On Fri, Jun 28, 2024 at 9:28 AM Qais Yousef <qyousef@...alina.io> wrote:
>
> On 06/24/24 16:20, Xuewen Yan wrote:
> > Commit f1f8d0a22422 ("sched/cpufreq: Take cpufreq feedback into account")
> > introduced get_actual_cpu_capacity(), and it had aggregated the
> > different pressures applied on the capacity of CPUs.
> > And in util_fits_cpu(), it would return true when uclamp_max
> > is smaller than SCHED_CAPACITY_SCALE, althought the uclamp_max
> > is bigger than actual_cpu_capacity.
> >
> > So use actual_cpu_capacity everywhere in util_fits_cpu() to
> > cover all cases.
> >
> > Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> > ---
> >  kernel/sched/fair.c | 19 +++++++++----------
> >  1 file changed, 9 insertions(+), 10 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 5ca6396ef0b7..9c16ae192217 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4980,7 +4980,7 @@ static inline int util_fits_cpu(unsigned long util,
> >                               int cpu)
> >  {
> >       unsigned long capacity = capacity_of(cpu);
> > -     unsigned long capacity_orig;
> > +     unsigned long capacity_actual;
> >       bool fits, uclamp_max_fits;
> >
> >       /*
> > @@ -4992,15 +4992,15 @@ static inline int util_fits_cpu(unsigned long util,
> >               return fits;
> >
> >       /*
> > -      * We must use arch_scale_cpu_capacity() for comparing against uclamp_min and
> > +      * We must use actual_cpu_capacity() for comparing against uclamp_min and
> >        * uclamp_max. We only care about capacity pressure (by using
> >        * capacity_of()) for comparing against the real util.
> >        *
> >        * If a task is boosted to 1024 for example, we don't want a tiny
> >        * pressure to skew the check whether it fits a CPU or not.
> >        *
> > -      * Similarly if a task is capped to arch_scale_cpu_capacity(little_cpu), it
> > -      * should fit a little cpu even if there's some pressure.
> > +      * Similarly if a task is capped to actual_cpu_capacity, it should fit
> > +      * the cpu even if there's some pressure.
>
> This statement is not clear now. We need to be specific since
> actual_cpu_capacity() includes thermal pressure and cpufreq limits.
>
> /even if there's some pressure/even if there is non OPP based pressure ie: RT,
> DL or irq/?
>
> >        *
> >        * Only exception is for HW or cpufreq pressure since it has a direct impact
> >        * on available OPP of the system.
> > @@ -5011,7 +5011,7 @@ static inline int util_fits_cpu(unsigned long util,
> >        * For uclamp_max, we can tolerate a drop in performance level as the
> >        * goal is to cap the task. So it's okay if it's getting less.
> >        */
> > -     capacity_orig = arch_scale_cpu_capacity(cpu);
> > +     capacity_actual = get_actual_cpu_capacity(cpu);
> >
> >       /*
> >        * We want to force a task to fit a cpu as implied by uclamp_max.
> > @@ -5039,7 +5039,7 @@ static inline int util_fits_cpu(unsigned long util,
> >        *     uclamp_max request.
> >        *
> >        *   which is what we're enforcing here. A task always fits if
> > -      *   uclamp_max <= capacity_orig. But when uclamp_max > capacity_orig,
> > +      *   uclamp_max <= capacity_actual. But when uclamp_max > capacity_actual,
> >        *   the normal upmigration rules should withhold still.
> >        *
> >        *   Only exception is when we are on max capacity, then we need to be
> > @@ -5050,8 +5050,8 @@ static inline int util_fits_cpu(unsigned long util,
> >        *     2. The system is being saturated when we're operating near
> >        *        max capacity, it doesn't make sense to block overutilized.
> >        */
> > -     uclamp_max_fits = (capacity_orig == SCHED_CAPACITY_SCALE) && (uclamp_max == SCHED_CAPACITY_SCALE);
> > -     uclamp_max_fits = !uclamp_max_fits && (uclamp_max <= capacity_orig);
> > +     uclamp_max_fits = (capacity_actual == SCHED_CAPACITY_SCALE) && (uclamp_max == SCHED_CAPACITY_SCALE);
>
> We should use capacity_orig here. We are checking if the CPU is the max
> capacity CPU.

Maybe we could remove the uclamp_max_fits = (capacity_orig ==
SCHED_CAPACITY_SCALE) && (uclamp_max == SCHED_CAPACITY_SCALE);
and just judge the uclamp_max <= capacity_actual?

-     uclamp_max_fits = (capacity_orig == SCHED_CAPACITY_SCALE) &&
(uclamp_max == SCHED_CAPACITY_SCALE);
-     uclamp_max_fits = !uclamp_max_fits && (uclamp_max <= capacity_orig);
+     uclamp_max_fits =  (uclamp_max <= capacity_actual);


>
> > +     uclamp_max_fits = !uclamp_max_fits && (uclamp_max <= capacity_actual);
> >       fits = fits || uclamp_max_fits;
> >
> >       /*
> > @@ -5086,8 +5086,7 @@ static inline int util_fits_cpu(unsigned long util,
> >        * handle the case uclamp_min > uclamp_max.
> >        */
> >       uclamp_min = min(uclamp_min, uclamp_max);
> > -     if (fits && (util < uclamp_min) &&
> > -         (uclamp_min > get_actual_cpu_capacity(cpu)))
> > +     if (fits && (util < uclamp_min) && (uclamp_min > capacity_actual))
> >               return -1;
> >
> >       return fits;
> > --
> > 2.25.1
> >

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