Re: cgroup specific sticky resources (was: Re: [PATCH bpf-next 0/5] bpf: BPF specific memory allocator.)

[Yosry Ahmed <yosryahmed@google.com>]

On Tue, Jul 19, 2022 at 12:16 PM Tejun Heo <tj@kernel.org> wrote:
>
> Hello,
>
> On Tue, Jul 19, 2022 at 11:46:41AM -0700, Mina Almasry wrote:
> > An interface like cgroup.sticky.[bpf/tmpfs/..] would work for us
> > similar to tmpfs memcg= mount option. I would maybe rename it to
> > cgroup.charge_for.[bpf/tmpfs/etc] or something.
>
> So, I'm not a fan because having this in cgroupfs would create the
> expectation that these resources can be moved across cgroups dynamically
> (and that's the only way the interface can be useful, right?). I'd much

Is there a reason why these resources cannot be moved across cgroups
dynamically? The only scenario I imagine is if you already have tmpfs
mounted and files charged to different cgroups, but once you attribute
tmpfs to one cgroup.charge_for.tmpfs (or sticky,..), I assume that we
can dynamically move the resources, right?

In fact, is there a reason why we can't move the tmpfs charges in that
scenario as well? When we move processes we loop their pages tables
and move pages and their stats, is there a reason why we wouldn't be
able to do this with tmpfs mounts or bpf maps as well?


> prefer something a lot more minimal - e.g. temporarily allow assuming an
> ancestor identity while creating a resource or sth along that line, and to
> add something like that, I think we need pretty strong arguments for why it
> can't be handled through cgroup layering in userspace.
>
> Thanks.
>
> --
> tejun