linux-kernel - Re: [PATCH 1/5] sched/fair: Filter false overloaded

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Message-ID: <CAKfTPtCaQkzGZdfi_-yoYmYY-49ZD7+nWfrbETryJKg=K-JB3w@mail.gmail.com>
Date: Fri, 6 Sep 2024 08:51:13 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Hongyan Xia <hongyan.xia2@....com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, 
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, 
	mgorman@...e.de, vschneid@...hat.com, lukasz.luba@....com, 
	rafael.j.wysocki@...el.com, linux-kernel@...r.kernel.org, qyousef@...alina.io
Subject: Re: [PATCH 1/5] sched/fair: Filter false overloaded_group case for EAS

On Mon, 2 Sept 2024 at 11:01, Hongyan Xia <hongyan.xia2@....com> wrote:
>
> On 30/08/2024 14:03, Vincent Guittot wrote:
> > With EAS, a group should be set overloaded if at least 1 CPU in the group
> > is overutilized bit it can happen that a CPU is fully utilized by tasks
> > because of clamping the compute capacity of the CPU. In such case, the CPU
> > is not overutilized and as a result should not be set overloaded as well.
> >
> > group_overloaded being a higher priority than group_misfit, such group can
> > be selected as the busiest group instead of a group with a mistfit task
> > and prevents load_balance to select the CPU with the misfit task to pull
> > the latter on a fitting CPU.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >   kernel/sched/fair.c | 12 +++++++++++-
> >   1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index fea057b311f6..e67d6029b269 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9806,6 +9806,7 @@ struct sg_lb_stats {
> >       enum group_type group_type;
> >       unsigned int group_asym_packing;        /* Tasks should be moved to preferred CPU */
> >       unsigned int group_smt_balance;         /* Task on busy SMT be moved */
> > +     unsigned long group_overutilized;       /* No CPU is overutilized in the group */
>
> Does this have to be unsigned long? I think a shorter width like bool
> (or int to be consistent with other fields) expresses the intention.

yes an unsigned int is enough

>
> Also the comment to me is a bit confusing. All other fields are positive
> but this one's comment is in a negative tone.

Coming from the 1st way I implemented it but then I forgot to update
the comment. Should be:
/* At least one CPU is overutilized in the group */

>
> >       unsigned long group_misfit_task_load;   /* A CPU has a task too big for its capacity */
> >   #ifdef CONFIG_NUMA_BALANCING
> >       unsigned int nr_numa_running;
> > @@ -10039,6 +10040,13 @@ group_has_capacity(unsigned int imbalance_pct, struct sg_lb_stats *sgs)
> >   static inline bool
> >   group_is_overloaded(unsigned int imbalance_pct, struct sg_lb_stats *sgs)
> >   {
> > +     /*
> > +      * With EAS and uclamp, 1 CPU in the group must be overutilized to
> > +      * consider the group overloaded.
> > +      */
> > +     if (sched_energy_enabled() && !sgs->group_overutilized)
> > +             return false;
> > +
> >       if (sgs->sum_nr_running <= sgs->group_weight)
> >               return false;
> >
> > @@ -10252,8 +10260,10 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> >               if (nr_running > 1)
> >                       *sg_overloaded = 1;
> >
> > -             if (cpu_overutilized(i))
> > +             if (cpu_overutilized(i)) {
> >                       *sg_overutilized = 1;
> > +                     sgs->group_overutilized = 1;
> > +             }
> >
> >   #ifdef CONFIG_NUMA_BALANCING
> >               sgs->nr_numa_running += rq->nr_numa_running;