Re: [PATCH v2 02/23] bpf: initial support for attaching struct ops to cgroups

Song Liu
2025-10-29T21:18:12Z

Hi Tejun,

On Wed, Oct 29, 2025 at 1:36 PM Tejun Heo <tj@kernel.org> wrote:
>
> On Wed, Oct 29, 2025 at 01:25:52PM -0700, Roman Gushchin wrote:
> > > BTW, for sched_ext sub-sched support, I'm just adding cgroup_id to
> > > struct_ops, which seems to work fine. It'd be nice to align on the same
> > > approach. What are the benefits of doing this through fd?
> >
> > Then you can attach a single struct ops to multiple cgroups (or Idk
> > sockets or processes or some other objects in the future).
> > And IMO it's just a more generic solution.
>
> I'm not very convinced that sharing a single struct_ops instance across
> multiple cgroups would be all that useful. If you map this to normal
> userspace programs, a given struct_ops instance is package of code and all
> the global data (maps). ie. it's not like running the same program multiple
> times against different targets. It's more akin to running a single program
> instance which can handle multiple targets.
>
> Maybe that's useful in some cases, but that program would have to explicitly
> distinguish the cgroups that it's attached to. I have a hard time imagining
> use cases where a single struct_ops has to service multiple disjoint cgroups
> in the hierarchy and it ends up stepping outside of the usual operation
> model of cgroups - commonality being expressed through the hierarchical
> structure.

How about we pass a pointer to mem_cgroup (and/or related pointers)
to all the callbacks in the struct_ops? AFAICT, in-kernel _ops structures like
struct file_operations and struct tcp_congestion_ops use this method. And
we can actually implement struct tcp_congestion_ops in BPF. With the
struct tcp_congestion_ops model, the struct_ops map and the struct_ops
link are both shared among multiple instances (sockets).

With this model, the system admin with root access can load a bunch of
available oom handlers, and users in their container can pick a preferred
oom handler for the sub cgroup. AFAICT, the users in the container can
pick the proper OOM handler without CAP_BPF. Does this sound useful
for some cases?

Thanks,
Song