linux-kernel - Re: [PATCH 7/7] sched/uclamp: Cater for uclamp in find_energy_efficient

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Date:   Fri, 22 Jul 2022 09:09:59 +0800
From:   Xuewen Yan <xuewen.yan94@...il.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org, Wei Wang <wvw@...gle.com>,
        Jonathan JMChen <Jonathan.JMChen@...iatek.com>,
        Hank <han.lin@...iatek.com>
Subject: Re: [PATCH 7/7] sched/uclamp: Cater for uclamp in find_energy_efficient_cpu()'s
 early exit condition

On Thu, Jul 21, 2022 at 10:24 PM Qais Yousef <qais.yousef@....com> wrote:
>
> On 07/20/22 15:39, Xuewen Yan wrote:
> > Hi Qais
> >
> > On Thu, Jun 30, 2022 at 3:48 AM Qais Yousef <qais.yousef@....com> wrote:
> > >
> > > If the utilization of the woken up task is 0, we skip the energy
> > > calculation because it has no impact.
> > >
> > > But if the task is boosted (uclamp_min != 0) will have an impact on task
> > > placement and frequency selection. Only skip if the util is truly
> > > 0 after applying uclamp values.
> > >
> > > Change uclamp_task_cpu() signature to avoid unnecessary additional calls
> > > to uclamp_eff_get(). feec() is the only user now.
> > >
> > > Fixes: 732cd75b8c920 ("sched/fair: Select an energy-efficient CPU on task wake-up")
> > > Signed-off-by: Qais Yousef <qais.yousef@....com>
> > > ---
> > >  kernel/sched/fair.c | 14 ++++++++------
> > >  1 file changed, 8 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 499ef7a7288c..a112ca45864c 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -4057,14 +4057,16 @@ static inline unsigned long task_util_est(struct task_struct *p)
> > >  }
> > >
> > >  #ifdef CONFIG_UCLAMP_TASK
> > > -static inline unsigned long uclamp_task_util(struct task_struct *p)
> > > +static inline unsigned long uclamp_task_util(struct task_struct *p,
> > > +                                            unsigned long uclamp_min,
> > > +                                            unsigned long uclamp_max)
> > >  {
> > > -       return clamp(task_util_est(p),
> > > -                    uclamp_eff_value(p, UCLAMP_MIN),
> > > -                    uclamp_eff_value(p, UCLAMP_MAX));
> > > +       return clamp(task_util_est(p), uclamp_min, uclamp_max);
> > >  }
> > >  #else
> > > -static inline unsigned long uclamp_task_util(struct task_struct *p)
> > > +static inline unsigned long uclamp_task_util(struct task_struct *p,
> > > +                                            unsigned long uclamp_min,
> > > +                                            unsigned long uclamp_max)
> > >  {
> > >         return task_util_est(p);
> > >  }
> > > @@ -6913,7 +6915,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> > >         target = prev_cpu;
> > >
> > >         sync_entity_load_avg(&p->se);
> > > -       if (!task_util_est(p))
> > > +       if (!uclamp_task_util(p, p_util_min, p_util_max))
> >
> > Is it not enough to just replace the task_util_est with the
> > uclamp_task_util? If change the definition of uclamp_task_util,
> > that means it have to get task's uclamp first if user want to call the
> > function, may increase the code complex farther more?
>
> Calling uclamp_eff_value() all the time is not cheap actually.
>
> We can easily add two versions when we need to:
>
>         __uclamp_task_util(p, uclamp_min, uclamp_max);
>
>         uclamp_task_util(p) {
>                 uclamp_min = uclamp_eff_value();
>                 uclamp_max = uclamp_eff_value();
>                 return __uclamp_eff_value(p, uclamp_min, uclamp_max);
>         }
>
> When we need to. Since we have a single user now, there's no need to do this
> now and if we ever get more users it'd be easy to refactor then?

Sounds good!

Thanks!

>
>
> Thanks!
>
> --
> Qais Yousef
>
> >
> > >                 goto unlock;
> > >
> > >         for (; pd; pd = pd->next) {
> > > --
> > > 2.25.1
> > >
> >
> > BR
> > ---
> > xuewen.yan