Re: [PATCH 7/7] sched/uclamp: Cater for uclamp in find_energy_efficient_cpu()'s early exit condition

[Qais Yousef <qais.yousef@arm.com>]

On 07/20/22 15:39, Xuewen Yan wrote:
> Hi Qais
> 
> On Thu, Jun 30, 2022 at 3:48 AM Qais Yousef <qais.yousef@arm.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@arm.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?


Thanks!

--
Qais Yousef

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