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

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Message-ID: <CALOAHbBW0jM=WZG7BM3Lh=xHBV7M585j85xMzpWAtWdrkRa+nQ@mail.gmail.com>
Date: Thu, 30 Oct 2025 14:33:29 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Tejun Heo <tj@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, Song Liu <song@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, LKML <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 <linux-mm@...ck.org>, 
	"open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>, bpf <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

On Thu, Oct 30, 2025 at 8:16 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Wed, Oct 29, 2025 at 5:03 PM Tejun Heo <tj@...nel.org> wrote:
> >
> > Oh, if there are other mechanisms to enforce boundaries, it's not a problem,
> > but I can almost guarantee as the framework grows, there will be needs for
> > kfuncs to identify and verify the callers and handlers communicating with
> > each other along the hierarchy requiring recursive calls.
>
> tbh I think it's a combination of sched_ext_ops and bpf infra problem.
> All of the scx ops are missing "this" pointer which would have
> been there if it was a C++ class.
> And "this" should be pointing to an instance of class.
> If sched-ext progs are attached to different cgroups, then
> every attachment would have been a different instance and
> different "this".
> Then all kfuncs would effectively be declared as helper
> methods within a class. In this case within "struct sched_ext_ops"
> as functions that ops callback can call but they will
> also have implicit "this" that points back to a particular instance.
>
> Special aux__prog and prog_assoc are not exactly pretty
> workarounds for lack of "this".
>

I also share the concern that supporting the attachment of a single
struct-ops to multiple cgroups appears over-engineered for the current
needs. Given that we do not anticipate a large number of cgroup
attachments in real-world use, implementing such a generalized
mechanism now seems premature. We can always introduce this
functionality later in a backward-compatible manner if concrete use
cases emerge.

That said, if we still decide to move forward with this approach, I
would suggest merging this patch as a standalone change. Doing so
would allow my BPF-THP series to build upon the same mechanism.

-- 
Regards
Yafang