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

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Message-ID: <aQJ61wC0mvzc7qIU@slm.duckdns.org>
Date: Wed, 29 Oct 2025 10:36:39 -1000
From: Tejun Heo <tj@...nel.org>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: 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>,
	Song Liu <song@...nel.org>,
	Kumar Kartikeya Dwivedi <memxor@...il.com>
Subject: Re: [PATCH v2 02/23] bpf: initial support for attaching struct ops
 to cgroups

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.

Thanks.

-- 
tejun