Re: [RFC][PATCH 03/16] sched: Wrap rq::lock access

[Julien Desfossez]

On Fri, Mar 22, 2019 at 9:34 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, Mar 21, 2019 at 05:20:17PM -0400, Julien Desfossez wrote:
> > On further investigation, we could see that the contention is mostly in
> the
> > way rq locks are taken. With this patchset, we lock the whole core if
> > cpu.tag is set for at least one cgroup. Due to this, __schedule() is
> more or
> > less serialized for the core and that attributes to the performance loss
> > that we are seeing. We also saw that newidle_balance() takes considerably
> > long time in load_balance() due to the rq spinlock contention. Do you
> think
> > it would help if the core-wide locking was only performed when absolutely
> > needed ?
>
> Something like that could be done, but then you end up with 2 locks,
> something which I was hoping to avoid.
>
> Basically you keep rq->lock as it exists today, but add something like
> rq->core->core_lock, you then have to take that second lock (nested
> under rq->lock) for every scheduling action involving a tagged task.
>
> It makes things complicatd though; because now my head hurts thikning
> about pick_next_task().
>
> (this can obviously do away with the whole rq->lock wrappery)
>
> Also, completely untested..

We tried it and it dies within 30ms of enabling the tag on 2 VMs :-)
Now after trying to debug this my head hurts as well !

We'll continue trying to figure this out, but if you want to take a look,
the full dmesg is here: https://paste.debian.net/plainh/0b8f87f3

Thanks,

Julien