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

Mike Galbraith
2024-11-12T14:24:14Z

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".

> 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.

> 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.

-Mike