Re: [LSF/MM/BPF TOPIC] bpf qdisc

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Kumar Kartikeya Dwivedi <memxor@gmail.com> writes:

> On Mon, 26 Feb 2024 at 19:04, Amery Hung <ameryhung@gmail.com> wrote:
>>
>> Hi all,
>>
>> I would like to discuss bpf qdisc in the BPF track. As we now try to
>> support bpf qdisc using struct_ops, we found some limitations of
>> bpf/struct_ops. While some have been discussed briefly on the mailing
>> list, we can discuss in more detail to make struct_ops a more
>> generic/palatable approach to replace kernel functions.
>>
>> In addition, I would like to discuss supporting adding kernel objects
>> to bpf_list/rbtree, which may have performance benefits in some
>> applications and can improve the programming experience. The current
>> bpf fq in the RFC has a 6% throughput loss compared to the native
>> counterpart due to memory allocation in enqueue() to store skb kptr.
>> With a POC I wrote that allows adding skb to bpf_list, the throughput
>> becomes comparable. We can discuss the approach and other potential
>> use cases.
>>
>
> When discussing this with Toke (Cc'd) long ago for the XDP queueing
> patch set, we discussed the same thing, in that the sk_buff already
> has space for a list or rbnode due to it getting queued in other
> layers (TCP OoO queue, qdiscs, etc.) so it would make sense to teach
> the verifier that it is a valid bpf_list_node and bpf_rb_node and
> allow inserting it as an element into a BPF list or rbtree. Back then
> we didn't add that as the posting only used the PIFO map.
>
> I think not only sk_buff, you can do a similar thing with xdp_buff as
> well.

Yeah, I agree that allowing skbs to be inserted directly into a BPF
rbtree would make a lot of sense if it can be done safely. I am less
sure about xdp_frames, mostly for performance reasons, but if it does
turn out to be useful whichever mechanism we add for skbs should be
fairly straight forward to reuse.

> The verifier side changes should be fairly minimal, just allowing the
> use of a known kernel type as the contained object in a list or
> rbtree, and the field pointing to this allowlisted list or rbnode.

I think one additional concern here is how we ensure that an skb has
been correctly removed from any rbtrees it sits in before it is being
transmitted to another part of the stack?

-Toke