Re: [PATCH] sched/fair: Dequeue sched_delayed tasks when waking to a busy CPU

Phil Auld
2024-11-12T15:41:51Z

On Tue, Nov 12, 2024 at 03:23:38PM +0100 Mike Galbraith wrote:
> On Tue, 2024-11-12 at 07:41 -0500, Phil Auld wrote:
> > On Tue, Nov 12, 2024 at 08:05:04AM +0100 Mike Galbraith wrote:
> > >
> > > Unfortunate change log place holder below aside, I think this patch may
> > > need to be yanked as trading one not readily repeatable regression for
> > > at least one that definitely is, and likely multiple others.
> > >
> > > (adds knob)
> > >
> >
> > Yes, I ws just coming here to reply. I have the results from the first
> > version of the patch (I don't think the later one fundemtally changed
> > enough that it will matter but those results are still pending).
> >
> > Not entirely surprisingly we've traded a ~10% rand write regression for
> > 5-10% rand read regression. This makes sense to me since the reads are
> > more likely to be synchronous and thus be more buddy-like and benefit
> > from flipping back and forth on the same cpu.
>
> Ok, that would seem to second "shoot it".
>

Yes, drop it please, I think. Thanks!

> > I'd probably have to take the reads over the writes in such a trade off :)
> >
> > > tbench 8
> > >
> > > NO_MIGRATE_DELAYED    3613.49 MB/sec
> > > MIGRATE_DELAYED       3145.59 MB/sec
> > > NO_DELAY_DEQUEUE      3355.42 MB/sec
> > >
> > > First line is DELAY_DEQUEUE restoring pre-EEVDF tbench throughput as
> > > I've mentioned it doing, but $subject promptly did away with that and
> > > then some.
> > >
> >
> > Yep, that's not pretty.
>
> Yeah, not to mention annoying.
>
> I get the "adds bounce cache pain" aspect, but not why pre-EEVDF
> wouldn't be just as heavily affected, it having nothing blocking high
> frequency migration (the eternal scheduler boogieman:).
>
> Bottom line would appear to be that these survivors should be left
> where they ended up, either due to LB or more likely bog standard
> prev_cpu locality, for they are part and parcel of a progression.
>
> > > I thought I might be able to do away with the reservation like side
> > > effect of DELAY_DEQUEUE by borrowing h_nr_delayed from...
> > >
> > >      sched/eevdf: More PELT vs DELAYED_DEQUEUE
> > >
> > > ...for cgroups free test config, but Q/D poke at idle_cpu() helped not
> > > at all.
>
> We don't however have to let sched_delayed block SIS though. Rendering
> them transparent in idle_cpu() did NOT wreck the progression, so
> maaaybe could help your regression.
>

You mean something like:

if (rq->nr_running > rq->h_nr_delayed)
return 0;

in idle_cpu() instead of the straight rq->nr_running check? I don't
have the h_nr_delayed stuff yet but can look for it. I'm not sure
that will help with the delayees being sticky. But I can try to try
that if I'm understanding you right.

I'll try to dig into it some more regardless.

> > I wonder if the last_wakee stuff could be leveraged here (an idle thought,
> > so to speak). Haven't looked closely enough.
>
> If you mean heuristics, the less of those we have, the better off we
> are.. they _always_ find a way to embed their teeth in your backside.
>

Sure, I get that. But when you have a trade-off like this being "smarter"
about when to do the dequeue might help. But yes, that could go wrong.

I'm not a fan of knobs either but we could do your patch with the feat
and default it off.

Cheers,
Phil

> -Mike
>

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