On Sun, 17 Jul 2022 at 20:17, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote: > > 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); It is also possible here, if we could not push into the map with a certain key it wouldn't be a PIFO. Please look at patch 16/17 for an example (test_xdp_pifo.c), it's just that the interface is different (bpf_redirect_map), the key has been expanded to 64 bits to accommodate such use cases. It is also possible in a future version of the patch to amortize the cost of taking the lock for each enqueue by doing batching, similar to what cpumap/devmap implementations do. > > (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.
