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

[Song Liu]

On Thu, Oct 30, 2025 at 3:42 PM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote:
[...]
>
> The link can be detached (struct_ops's unreg) by the user space.
>
> The link can also be detached from the subsystem (cgroup) here.
> It was requested by scx:
> https://lore.kernel.org/all/20240530065946.979330-7-thinker.li@xxxxxxxxx/
>
> Not sure if scx has started using it.

I see. The user space can poll the link fd, and get notified when the
cgroup is removed.

> >
> >> 3) Move the attachment out of .reg() scope entirely. reg() will register
> >> the implementation system-wide and then some 3rd-party interface
> >> (e.g. cgroupfs) should be used to select the implementation.
> >>
> >>    +: ?
> >>    -: New hard-coded interfaces might be required to enable bpf-driven
> >>       kernel customization. The "attachment" code is not shared between
> >>       various struct ops cases.
> >>       Implementing stateful struct ops'es is harder and requires passing
> >>       an additional argument (some sort of "self") to all callbacks.
> >>
> >> This approach works well for cases when there is already a selection
> >> of implementations (e.g. tcp congestion mechanisms), and bpf is adding
> >> another one.
> >
> > Another benefit of 3) is that it allows loading an OOM controller in a
> > kernel module, just like loading a file system in a kernel module. This
> > is possible with 3) because we paid the cost of adding a new select
> > attach interface.
> >
> > A semi-separate topic, option 2) enables attaching a BPF program
> > to a kernel object (a cgroup here, but could be something else). This
> > is an interesting idea, and we may find it useful in other cases (attach
> > a BPF program to a task_struct, etc.).
>
> Does it have plan for a pure kernel module oom implementation?
> I think the link-to-cgrp support here does not necessary stop the
> later write to cgroupfs support if a kernel module oom is indeed needed
> in the future.

I am not aware of use cases to write OOM handlers in modules. Also
agreed that adding attach to cgroup link doesn't stop us from using
modules in the future.

Thanks,
Song