Re: [RFC PATCH v2 13/17] sched: Add core wide task selection and scheduling.

[Aubrey Li]

On Mon, May 20, 2019 at 10:04 PM Vineeth Pillai
<vpillai@xxxxxxxxxxxxxxxx> wrote:
>
> > > The following patch improved my test cases.
> > > Welcome any comments.
> > >
> >
> > This is certainly better than violating the point of the core scheduler :)
> >
> > If I'm understanding this right what will happen in this case is instead
> > of using the idle process selected by the sibling we do the core scheduling
> > again. This may start with a newidle_balance which might bring over something
> > to run that matches what we want to put on the sibling. If that works then I
> > can see this helping.
> >
> > But I'd be a little concerned that we could end up thrashing. Once we do core
> > scheduling again here we'd force the sibling to resched and if we got a different
> > result which "helped" him pick idle we'd go around again.

Thrashing means more IPIs right? That's not what I observed, because idle task
has less chance onto CPU, rescheduling is reduced accordingly.

> > I think inherent in the concept of core scheduling (barring a perfectly aligned set
> > of jobs) is some extra idle time on siblings.

Yeah, I understand and agree with this, but 10-15% idle time on an overloaded
system makes me to try to figure out how this could happen and if we
can improve it.

> >
> >
> I was also thinking along the same lines. This change basically always
> tries to avoid idle and there by constantly interrupting the sibling.
> While this change might benefit a very small subset of workloads, it
> might introduce thrashing more often.

Thrashing is not observed under an overloaded case but may happen under a
light load or a mid load case, I need more investigation.

>
> One other reason you might be seeing performance improvement is
> because of the bugs that caused both siblings to go idle even though
> there are runnable and compatible threads in the queue. Most of the
> issues are fixed based on all the feedback received in v2. We have a
> github repo with the pre v3 changes here:
> https://github.com/digitalocean/linux-coresched/tree/coresched

Okay, thanks, it looks like the core functions pick_next_task() and pick_task()
have a lot of changes against v2. Need more brain power..

>
> Please try this and see how it compares with the vanilla v2. I think its
> time for a v3 now and we shall be posting it soon after some more
> testing and benchmarking.

Is there any potential change between pre v3 and v3? I prefer working
based on v3 so that everyone are on the same page.

Thanks,
-Aubrey