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

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

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@layalina.io> 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@layalina.io>
>>>> ---
>>>>  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.