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: <87bjloht28.fsf@linux.dev>
Date: Thu, 30 Oct 2025 16:24:15 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: 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

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.

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?