Re: MM global locks as core counts quadruple

[Namhyung Kim <namhyung@google.com>]

Hello,

On Fri, Jun 21, 2024 at 12:10 PM Tejun Heo <tj@kernel.org> wrote:
>
> Hello,
>
> On Thu, Jun 20, 2024 at 05:35:45PM -0700, David Rientjes wrote:
> > As core counts are rapidly expanding over the next four years, Namhyung
> > and I were looking at global locks that we're already seeing high
> > contention on even today.
> >
> > Some of these are not MM specific:
> >  - cgroup_mutex
>
> Our machines are getting bigger but we aren't creating and destroying
> cgroups frequently enough for this to matter. But yeah, I can see how this
> can be a problem.
>
> >  - cgroup_threadgroup_rwsem
>
> This one shouldn't matter at all in setups where new cgroups are populated
> with CLONE_INTO_CGROUP and not migrated further. The lock isn't grabbed in
> such usage pattern, which should be the vast majority already, I think. Are
> you guys migrating tasks a lot or not using CLONE_INTO_CGROUP?

I'm afraid there are still some use cases in Google that migrate processes
and/or threads between cgroups. :(

Thanks,
Namhyung