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

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Message-ID: <CAHzjS_vRrxTLR0kFJvm4R3kO0s8hyN5Onsr6XfbA3Peyasz+Cg@mail.gmail.com>
Date: Thu, 30 Oct 2025 23:14:24 -0700
From: Song Liu <song@...nel.org>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Amery Hung <ameryhung@...il.com>, 
	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>, Tejun Heo <tj@...nel.org>
Subject: Re: bpf_st_ops and cgroups. Was: [PATCH v2 02/23] bpf: initial
 support for attaching struct ops to cgroups

On Thu, Oct 30, 2025 at 4:24 PM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
>
> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
>
> > On Thu, Oct 30, 2025 at 12:06 PM Roman Gushchin
> > <roman.gushchin@...ux.dev> wrote:
> >>
> >> Ok, let me summarize the options we discussed here:
> >>
> >> 1) Make the attachment details (e.g. cgroup_id) the part of struct ops
> >> itself. The attachment is happening at the reg() time.
> >>
> >>   +: It's convenient for complex stateful struct ops'es, because a
> >>       single entity represents a combination of code and data.
> >>   -: No way to attach a single struct ops to multiple entities.
> >>
> >> This approach is used by Tejun for per-cgroup sched_ext prototype.
> >
> > It's wrong. It should adopt bpf_struct_ops_link_create() approach
> > and use attr->link_create.cgroup.relative_fd to attach.
>
> This is basically what I have in v2, but Andrii and Song suggested that
> I should use attr->link_create.target_fd instead.
>
> I have a slight preference towards attr->link_create.cgroup.relative_fd
> because it makes it clear that fd is a cgroup fd and potentially opens
> a possibility to e.g. attach struct_ops to individual tasks and
> cgroups, but I'm fine with both options.

relative_fd and relative_id have specific meaning. When multiple
programs are attached to the same object (cgroup, socket, etc.),
relative_fd and relative_id (together with BPF_F_BEFORE and
BPF_F_AFTER) are used to specify the order of execution.

>
> Also, as Song pointed out, fd==0 is in theory a valid target, so instead of
> using the "if (fd) {...}" check we might need a new flag. Idk if it
> really makes sense to complicate the code for it.
>
> Can we, please, decide on what's best here?

How about we add a new attach_type BPF_STRUCT_OPS_CGROUP?

Thanks,
Song