Re: [PATCH v2 2/3] sched/uclamp: Ignore (util == 0) optimization in feec() when p_util_max = 0

[Vincent Guittot]

On Tue, 14 Feb 2023 at 13:47, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
>
> On 11/02/2023 19:01, Qais Yousef wrote:
> > On 02/08/23 12:52, Dietmar Eggemann wrote:
> >> On 07/02/2023 11:04, Vincent Guittot wrote:
> >>> On Sun, 5 Feb 2023 at 23:43, Qais Yousef <qyousef@xxxxxxxxxxx> wrote:
> >>>>
> >>>> find_energy_efficient_cpu() bails out early if effective util of the
> >>>> task is 0. When uclamp is being used, this could lead to wrong decisions
> >>>> when uclamp_max is set to 0. Cater for that.
> >>
> >> IMHO this needs a little bit more explanation. Someone could argue that
> >> 'util > 0, uclamp_min=0, uclamp_max=0' is a valid setup for a task which
> >> should let it appear as a task with 0 util (capped to 0).
> >
> > You want me to explain the purpose of the optimization then?
> >
> > The optimization skips energy calculation when util is 0 because the delta will
> > be 0. But when uclamp_max = 0 util is not really 0 - consequently  the delta
>
> I would say:
>
> s/really/necessarily
> s/delta/energy delta
>
> > will not be 0.
> >
> > Would such an explanation clarify things better?
>
> Yes. It key to understand that there is a 2-step process. First,
> admittance to a possible target (util and uclamp) and second, energy
> diff based target-selection (util).
>
> >>>> Fixes: d81304bc6193 ("sched/uclamp: Cater for uclamp in find_energy_efficient_cpu()'s early exit condition")
> >>>> Signed-off-by: Qais Yousef <qyousef@xxxxxxxxxxx>
> >>>> ---
> >>>>  kernel/sched/fair.c | 2 +-
> >>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >>>> index 7a21ee74139f..a8c3d92ff3f6 100644
> >>>> --- a/kernel/sched/fair.c
> >>>> +++ b/kernel/sched/fair.c
> >>>> @@ -7374,7 +7374,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >>>>         target = prev_cpu;
> >>>>
> >>>>         sync_entity_load_avg(&p->se);
> >>>> -       if (!uclamp_task_util(p, p_util_min, p_util_max))
> >>>> +       if (!uclamp_task_util(p, p_util_min, p_util_max) && p_util_max != 0)
> >>>
> >>> The below should do the same without testing twice p_util_max:
> >>> uclamp_task_util(p, p_util_min, ULONG_MAX)
> >>
> >> Since uclamp_task_util() is only used here and we don't want to test for
> >> capping to 0 anymore, why not just get rid of this function and use:
> >>
> >>   !(task_util_est(p) || p_util_min)
> >
> > That would be better, yes!
> >
> > Question for you and Vincent. Do we really want this optimization? I started
> > with removing it - then erred on the conservative side and kept it.
>
> Hard to say ... at least we know that util=0 will have absolutely no
> effect on task placement. So we can spare the heavy EAS algorithm in
> this case for sure.
>
> > I don't know how often we hit this case and I didn't see any benchmark run to
> > be able to verify anything when I looked at the history.
>
> There are very few EAS wakeups with `task_util_est(p) = 0`. Probably not
> relevant.
>
> > It seems helpful in theory - but why we save something if we ignore 0 but not
> > 1 which I suspect will not produce a significant delta either.
>
> True, it's hard to find the real line of significance here.
>
> > I don't mind keeping it - but I think worth thinking if it is really adding
> > much.
>
> I would keep it and just remove uclamp_task_util(). We still have a lot
> of uclamp/util related functions, we should try to keep the number as
> low as possible. Just checked it, this check has been there from the
> beginning of EAS.

Yes make sense to keep the test as proposed by Dietmar and save the cycles

>