Messages in this thread | | | From | Vincent Guittot <> | Date | Mon, 20 Feb 2023 18:24:53 +0100 | Subject | Re: [PATCH v2 2/3] sched/uclamp: Ignore (util == 0) optimization in feec() when p_util_max = 0 |
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On Tue, 14 Feb 2023 at 13:47, Dietmar Eggemann <dietmar.eggemann@arm.com> 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@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.
Yes make sense to keep the test as proposed by Dietmar and save the cycles
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