Re: [PATCH] sched/uclamp: Fix getting unreasonable ucalmp_max when rq is idle

[Xuewen Yan]

On Tue, Jun 29, 2021 at 9:50 PM Valentin Schneider
<valentin.schneider@xxxxxxx> wrote:
>
>
> +Cc Patrick's current address
>
> On 18/06/21 15:23, Xuewen Yan wrote:
> > From: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
> >
> > Now in uclamp_rq_util_with(), when the task != NULL, the uclamp_max as following:
> > uc_rq_max = rq->uclamp[UCLAMP_MAX].value;
> > uc_eff_max = uclamp_eff_value(p, UCLAMP_MAX);
> > uclamp_max = max{uc_rq_max, uc_eff_max};
> >
> > Consider the following scenario:
> > (1)the rq is idle, the uc_rq_max is last task's UCLAMP_MAX;
> > (2)the p's uc_eff_max < uc_rq_max.
> >
> > The result is the uclamp_max = uc_rq_max instead of uc_eff_max, it is unreasonable.
> >
> > The scenario often happens in find_energy_efficient_cpu(), when the task has smaller UCLAMP_MAX.
> >
> > Inserts whether the rq is idle in the uclamp_rq_util_with().
> >
> > Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
> > ---
> >  kernel/sched/sched.h | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index a189bec13729..0feef6af89f2 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -2550,7 +2550,10 @@ unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util,
> >
> >       if (p) {
> >               min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN));
> > -             max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX));
> > +             if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
> > +                     max_util = uclamp_eff_value(p, UCLAMP_MAX);
> > +             else
> > +                     max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX));
>
> That makes sense to me - enqueuing the task will lift UCLAMP_FLAG_IDLE and
> set the rq clamp as the task's via uclamp_idle_reset().
>
> Does this want a
>
>   Fixes: 9d20ad7dfc9a ("sched/uclamp: Add uclamp_util_with()")
>
> ?

Yes，add it.

>
> Also, when we have UCLAMP_FLAG_IDLE, we don't even need to read the rq max
> - and I'm pretty sure the same applies to the rq min. What about something like:

Good idea, I'll try it in V2.

>
> ---
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 6510f0a46789..a2c6f6ae6392 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2833,23 +2833,27 @@ static __always_inline
>  unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util,
>                                   struct task_struct *p)
>  {
> -       unsigned long min_util;
> -       unsigned long max_util;
> +       unsigned long min_util = 0;
> +       unsigned long max_util = 0;
>
>         if (!static_branch_likely(&sched_uclamp_used))
>                 return util;
>
> -       min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
> -       max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
> -
>         if (p) {
> -               min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN));
> +               min_util = uclamp_eff_value(p, UCLAMP_MIN);
> +               max_util = uclamp_eff_value(p, UCLAMP_MAX);
> +
> +               /*
> +                * Ignore last runnable task's max clamp, as this task will
> +                * reset it. Similarly, no need to read the rq's min clamp.
> +                */
>                 if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
> -                       max_util = uclamp_eff_value(p, UCLAMP_MAX);
> -               else
> -                       max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX));
> +                       goto out;
>         }
>
> +       min_util = max_t(unsigned long, min_util, READ_ONCE(rq->uclamp[UCLAMP_MIN].value));
> +       max_util = max_t(unsigned long, max_util, READ_ONCE(rq->uclamp[UCLAMP_MAX].value));

Is it necessary to use max_t here? although it is not the main problem...

> +out:
>         /*
>          * Since CPU's {min,max}_util clamps are MAX aggregated considering
>          * RUNNABLE tasks with _different_ clamps, we can end up with an

Thanks!
xuewen