Messages in this thread | | | Date | Sat, 11 Feb 2023 18:01:19 +0000 | From | Qais Yousef <> | Subject | Re: [PATCH v2 2/3] sched/uclamp: Ignore (util == 0) optimization in feec() when p_util_max = 0 |
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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 will not be 0.
Would such an explanation clarify things better?
> > >> 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.
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.
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.
I don't mind keeping it - but I think worth thinking if it is really adding much.
Cheers
-- Qais Yousef
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