Re: [External] Re: [PATCH] cpuset: introduce non-blocking cpuset.mems setting option

[Tejun Heo <tj@kernel.org>]

On Wed, May 21, 2025 at 10:35:57AM +0800, Zhongkun He wrote:
> On Tue, May 20, 2025 at 9:35 PM Waiman Long <llong@redhat.com> wrote:
> >
> > On 5/19/25 11:15 PM, Zhongkun He wrote:
> > > Setting the cpuset.mems in cgroup v2 can trigger memory
> > > migrate in cpuset. This behavior is fine for newly created
> > > cgroups but it can cause issues for the existing cgroups.
> > > In our scenario, modifying the cpuset.mems setting during
> > > peak times frequently leads to noticeable service latency
> > > or stuttering.
> > >
> > > It is important to have a consistent set of behavior for
> > > both cpus and memory. But it does cause issues at times,
> > > so we would hope to have a flexible option.
> > >
> > > This idea is from the non-blocking limit setting option in
> > > memory control.
> > >
> > > https://lore.kernel.org/all/20250506232833.3109790-1-shakeel.butt@linux.dev/
> > >
> > > Signed-off-by: Zhongkun He <hezhongkun.hzk@bytedance.com>
> > > ---
> > >   Documentation/admin-guide/cgroup-v2.rst |  7 +++++++
> > >   kernel/cgroup/cpuset.c                  | 11 +++++++++++
> > >   2 files changed, 18 insertions(+)
> > >
> > > diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> > > index 1a16ce68a4d7..d9e8e2a770af 100644
> > > --- a/Documentation/admin-guide/cgroup-v2.rst
> > > +++ b/Documentation/admin-guide/cgroup-v2.rst
> > > @@ -2408,6 +2408,13 @@ Cpuset Interface Files
> > >       a need to change "cpuset.mems" with active tasks, it shouldn't
> > >       be done frequently.
> > >
> > > +     If cpuset.mems is opened with O_NONBLOCK then the migration is
> > > +     bypassed. This is useful for admin processes that need to adjust
> > > +     the cpuset.mems dynamically without blocking. However, there is
> > > +     a risk that previously allocated pages are not within the new
> > > +     cpuset.mems range, which may be altered by move_pages syscall or
> > > +     numa_balance.

I don't think this is a good idea. O_NONBLOCK means "don't wait", not "skip
this".

Thanks.

-- 
tejun