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

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Message-ID: <aQKLCuX5v5aO3fDa@slm.duckdns.org>
Date: Wed, 29 Oct 2025 11:45:46 -1000
From: Tejun Heo <tj@...nel.org>
To: Song Liu <song@...nel.org>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>,
	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

Hello,

On Wed, Oct 29, 2025 at 02:37:38PM -0700, Song Liu wrote:
> On Wed, Oct 29, 2025 at 2:27 PM Tejun Heo <tj@...nel.org> wrote:
> > Doesn't that assume that the programs are more or less stateless? Wouldn't
> > oom handlers want to track historical information, running averages, which
> > process expanded the most and so on?
> 
> Yes, this does mean the program needs to store data in some BPF maps.
> Do we have concern with the performance of BPF maps?

It's just a lot more awkward to do and I have a difficult time thinking up
reasons why one would need to do that. If you attach a single struct_ops
instance to one cgroup, you can use global variables, maps, arena to track
what's happening with the cgroup. If you share the same struct_ops across
multiple cgroups, each operation has to scope per-cgroup states. I can see
how that probably makes sense for sockets but cgroups aren't sockets. There
are a lot fewer cgroups and they are organized in a tree.

Thanks.

-- 
tejun