On Fri, Mar 1, 2024 at 6:08 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > Amery Hung <ameryhung@xxxxxxxxx> writes: > > > On Wed, Feb 28, 2024 at 6:36 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > >> > >> Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> writes: > >> > >> > On Mon, 26 Feb 2024 at 19:04, Amery Hung <ameryhung@xxxxxxxxx> 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? > > > > I think one solution is to disallow shared ownership of skb in > > multiple lists or rbtrees. That is, users should not be able to > > acquire the reference of an skb from the ctx more than once in > > ".enqueue" or using bpf_refcount_acquire(). > > Can the verifier enforce this, even across multiple enqueue/dequeue > calls? Not sure if acquiring a refcount ensures that the rbtree entry > has been cleared? > > Basically, I'm worried about a dequeue() op that does something like: > > skb = rbtree_head(); > // skb->rbnode is not cleared > return skb; // stack will keep processing it > > I'm a little fuzzy on how the bpf rbtree stuff works, though, so maybe > the verifier is already ensuring that a node cannot be read from a tree > without being properly cleared from it? > I see what you are saying now, and thanks Kumar for the clarification! I was thinking about how to prevent an skb from being added to lists and rbtrees at the same time, since list and rbnode share the same space. Hence the suggestion. > -Toke >
