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

[Yafang Shao]

On Fri, Oct 31, 2025 at 7:30 AM Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote:
>
> Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes:
>
> > On Thu, Oct 30, 2025 at 12:06 PM Roman Gushchin
> > <roman.gushchin@xxxxxxxxx> 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.

I recall that Linus has reminded the BPF subsystem not to use `if
(fd)` to check for a valid fd. We should avoid repeating this mistake.
The proper solution is to add a new flag to indicate whether a fd is
valid.

> Idk if it
> really makes sense to complicate the code for it.
>
> Can we, please, decide on what's best here?
>

It seems the only way for us to learn is through practice—even if that
means making mistakes first ;-)

I can imagine a key benefit of a single struct-ops-to-multiple-cgroups
model is the ability to pre-load all required policies. This allows
users the flexibility to attach them on demand, while completely
avoiding the complex lifecycle management of individual links—a major
practical pain point.

-- 
Regards
Yafang