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

Roman Gushchin
2025-10-30T23:15:10Z

Martin KaFai Lau <martin.lau@linux.dev> writes:

> On 10/30/25 2:34 PM, Song Liu wrote:
>> Hi Roman,
>> On Thu, Oct 30, 2025 at 12:07 PM Roman Gushchin
>> <roman.gushchin@linux.dev> wrote:
>> [...]
>>>> In TCP congestion control and BPF qdisc's model:
>>>>
>>>> During link_create, both adds the struct_ops to a list, and the
>>>> struct_ops can be indexed by name. The struct_ops are not "active" by
>>>> this time.
>>>> Then, each has their own interface to 'apply' the struct_ops to a
>>>> socket or queue: setsockopt() or netlink.
>>>>
>>>> But maybe cgroup-related struct_ops are different.
>>>
>>> Both tcp congestion and qdisk cases are somewhat different because
>>> there already is a way to select between multiple implementations, bpf
>>> just adds another one. In the oom case, it's not true. As of today,
>>> there is only one (global) oom killer. Of course we can create
>>> interfaces to allow a user make a choice. But the question is do we want
>>> to create such interface for the oom case specifically (and later for
>>> each new case separately), or there is a place for some generalization?
>> Agreed that this approach requires a separate mechanism to attach
>> the struct_ops to an entity.
>>
>>> Ok, let me summarize the options we discussed here:
>> Thanks for the summary!
>>
>>>
>>> 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.
>>>
>>> 2) Make the attachment details a part of bpf_link creation. The
>>> attachment is still happening at the reg() time.
>>>
>>> +: A single struct ops can be attached to multiple entities.
>>> -: Implementing stateful struct ops'es is harder and requires passing
>>> an additional argument (some sort of "self") to all callbacks.
>>> I'm using this approach in the bpf oom proposal.
>>>
>> I think both 1) and 2) have the following issue. With cgroup_id in
>> struct_ops or the link, the cgroup_id works more like a filter. The
>> cgroup doesn't hold any reference to the struct_ops. The bpf link
>> holds the reference to the struct_ops, so we need to keep the
>> the link alive, either by keeping an active fd, or by pinning the
>> link to bpffs. When the cgroup is removed, we need to clean up
>> the bpf link separately.
>
> 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@gmail.com/
>
> Not sure if scx has started using it.
>
>>
>>> 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.).

Yep, task_struct is an attractive target for something like mm-related
policies (THP, NUMA, memory tiers etc).

>
> Does it have plan for a pure kernel module oom implementation?

I highly doubt.

> 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.
>
> imo, cgroup-bpf has a eco-system around it, so it is sort of special. bpf user
> has expectation on how a bpf prog is attached to a cgroup. The introspection,
> auto detachment from the cgroup when the link is gone...etc.
>
> If link-to-cgrp is used, I prefer (2). Stay with one way to attach
> to a cgrp. It is also consistent with the current way of attaching a single
> bpf prog to a cgroup. It is now attaching a map/set of bpf prog to a cgroup.
> The individual struct_ops implementation can decide if it should
> allow a struct_ops be attached multiple times.

+1