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

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Message-ID: <871pmle5ng.fsf@linux.dev>
Date: Wed, 29 Oct 2025 14:53:39 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Song Liu <song@...nel.org>
Cc: Tejun Heo <tj@...nel.org>,  Andrew Morton <akpm@...ux-foundation.org>,
  linux-kernel@...r.kernel.org,  Alexei Starovoitov <ast@...nel.org>,
  Suren Baghdasaryan <surenb@...gle.com>,  Michal Hocko
 <mhocko@...nel.org>,  Shakeel Butt <shakeel.butt@...ux.dev>,  Johannes
 Weiner <hannes@...xchg.org>,  Andrii Nakryiko <andrii@...nel.org>,  JP
 Kobryn <inwardvessel@...il.com>,  linux-mm@...ck.org,
  cgroups@...r.kernel.org,  bpf@...r.kernel.org,  Martin KaFai Lau
 <martin.lau@...nel.org>,  Kumar Kartikeya Dwivedi <memxor@...il.com>
Subject: Re: [PATCH v2 02/23] bpf: initial support for attaching struct ops
 to cgroups

Song Liu <song@...nel.org> writes:

> Hi Tejun,
>
> On Wed, Oct 29, 2025 at 1:36 PM Tejun Heo <tj@...nel.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.