linux-kernel - Re: [PATCH v1 01/14] mm: introduce bpf struct ops for OOM handling

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Message-ID: <87qzwnxgfr.fsf@linux.dev>
Date: Wed, 03 Sep 2025 16:30:16 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Tejun Heo <tj@...nel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,  Martin KaFai Lau
 <martin.lau@...ux.dev>,  Kumar Kartikeya Dwivedi <memxor@...il.com>,
  linux-mm <linux-mm@...ck.org>,  bpf <bpf@...r.kernel.org>,  Suren
 Baghdasaryan <surenb@...gle.com>,  Johannes Weiner <hannes@...xchg.org>,
  Michal Hocko <mhocko@...e.com>,  David Rientjes <rientjes@...gle.com>,
  Matt Bobrowski <mattbobrowski@...gle.com>,  Song Liu <song@...nel.org>,
  Alexei Starovoitov <ast@...nel.org>,  Andrew Morton
 <akpm@...ux-foundation.org>,  LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 01/14] mm: introduce bpf struct ops for OOM handling

Tejun Heo <tj@...nel.org> writes:

> Hello, Roman. How are you?

Hi Tejun! Thank you for the links...

>
> On Tue, Sep 02, 2025 at 10:31:33AM -0700, Roman Gushchin wrote:
> ...
>> Btw, what's the right way to attach struct ops to a cgroup, if there is
>> one? Add a cgroup_id field to the struct and use it in the .reg()
>> callback? Or there is something better?
>
> So, I'm trying to do something similar with sched_ext. Right now, I only
> have a very rough prototype (I can attach multiple schedulers with warnings
> and they even can schedule for several seconds before melting down).
> However, the basic pieces should may still be useful. The branch is:
>
>  git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git scx-hier-prototype
>
> There are several pieces:
>
> - cgroup recently grew lifetime notifiers that you can hook in there to
>   receive on/offline events. This is useful for initializing per-cgroup
>   fields and cleaning up when cgroup dies:
>
>   https://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git/tree/kernel/sched/ext.c?h=scx-hier-prototype#n5469

This is neat, I might use this for the psi struct ops to give a user a
chance to create new trigger(s) if a new cgroup is created.

>
> - I'm passing in cgroup_id as an optional field in struct_ops and then in
>   enable path, look up the matching cgroup, verify it can attach there and
>   insert and update data structures accordingly:
>
>   https://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git/tree/kernel/sched/ext.c?h=scx-hier-prototype#n5280

Yeah, we discussed this option with Martin up in this thread. It doesn't
look as the best possible solution, but maybe the best we have at the moment.

Ideally, I want something like this:

void test_oom(void)
{
	struct test_oom *skel;
	int err, cgroup_fd;

        cgroup_fd = open(...);
        if (cgroup_fd < 0)
		goto cleanup;

	skel = test_oom__open_and_load();
        if (!skel)
		goto cleanup;

	err = test_oom__attach_cgroup(skel, cgroup_fd);
	if (CHECK_FAIL(err))
		goto cleanup;