On Wed, Jul 13, 2022 at 11:52:07PM +0200, Toke Høiland-Jørgensen wrote: > Stanislav Fomichev <sdf@xxxxxxxxxx> writes: > > > On Wed, Jul 13, 2022 at 4:14 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > >> > >> Packet forwarding is an important use case for XDP, which offers > >> significant performance improvements compared to forwarding using the > >> regular networking stack. However, XDP currently offers no mechanism to > >> delay, queue or schedule packets, which limits the practical uses for > >> XDP-based forwarding to those where the capacity of input and output links > >> always match each other (i.e., no rate transitions or many-to-one > >> forwarding). It also prevents an XDP-based router from doing any kind of > >> traffic shaping or reordering to enforce policy. > >> > >> This series represents a first RFC of our attempt to remedy this lack. The > >> code in these patches is functional, but needs additional testing and > >> polishing before being considered for merging. I'm posting it here as an > >> RFC to get some early feedback on the API and overall design of the > >> feature. > >> > >> DESIGN > >> > >> The design consists of three components: A new map type for storing XDP > >> frames, a new 'dequeue' program type that will run in the TX softirq to > >> provide the stack with packets to transmit, and a set of helpers to dequeue > >> packets from the map, optionally drop them, and to schedule an interface > >> for transmission. > >> > >> The new map type is modelled on the PIFO data structure proposed in the > >> literature[0][1]. It represents a priority queue where packets can be > >> enqueued in any priority, but is always dequeued from the head. From the > >> XDP side, the map is simply used as a target for the bpf_redirect_map() > >> helper, where the target index is the desired priority. > > > > I have the same question I asked on the series from Cong: > > Any considerations for existing carousel/edt-like models? > > Well, the reason for the addition in patch 5 (continuously increasing > priorities) is exactly to be able to implement EDT-like behaviour, where > the priority is used as time units to clock out packets. Are you sure? I seriouly doubt your patch can do this at all... Since your patch relies on bpf_map_push_elem(), which has no room for 'key' hence you reuse 'flags' but you also reserve 4 bits there... How could tstamp be packed with 4 reserved bits?? To answer Stanislav's question, this is how my code could handle EDT: // BPF_CALL_3(bpf_skb_map_push, struct bpf_map *, map, struct sk_buff *, skb, u64, key) skb->tstamp = XXX; bpf_skb_map_push(map, skb, skb->tstamp); (Please refer another reply from me for how to get the min when poping, which is essentially just a popular interview coding problem.) Actually, if we look into the in-kernel EDT implementation (net/sched/sch_etf.c), it is also based on rbtree rather than PIFO. ;-) Thanks.
