Messages in this thread Patch in this message | | | Date | Tue, 14 Feb 2023 13:47:35 +0100 | Subject | Re: [PATCH v2 2/3] sched/uclamp: Ignore (util == 0) optimization in feec() when p_util_max = 0 | From | Dietmar Eggemann <> |
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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.
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