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 12:38 PM Tejun Heo <tj@kernel.org> wrote:
>
> On Tue, Jul 19, 2022 at 12:30:17PM -0700, Yosry Ahmed wrote:
> > 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?
>
> Nothing is impossible but nothing is free as well. Moving charges around
> traditionally caused a lot of headaches in the past and never became
> reliable. There are inherent trade-offs here. You can make things more
> dynamic usually by making hot paths more expensive or doing some
> synchronization dancing which tends to be pretty hairy. People generally
> don't wanna make hot paths slower, so we tend to end up with something
> twisted which unfortunately turns out to be a headache in the long term.
>
> In general, I'd rather keep resource associations as static as possible.
> It's okay if we do something neat inside the kernel but if we create
> userspace expectation that resources can be moved around dynamically, we'll
> be stuck with that for a long time likely forfeiting future simplification /
> optimization opportunities.
>
> So, that's gonna be a fairly strong nack from my end.

Fair enough :)

Thanks for elaborating your point of view and the challenges that come
with this!

>
> Thanks.
>
> --
> tejun