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

[Roman Gushchin <roman.gushchin@linux.dev>]

Song Liu <song@kernel.org> writes:

> 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).

+1 to this.
I agree it might be debatable when it comes to cgroups, but when it comes to
sockets or similar objects, having a separate struct ops per object
isn't really an option.