On Thu, Dec 8, 2022 at 4:02 PM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > Stanislav Fomichev <sdf@xxxxxxxxxx> writes: > > > On Thu, Dec 8, 2022 at 2:59 PM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > >> > >> Stanislav Fomichev <sdf@xxxxxxxxxx> writes: > >> > >> > From: Toke Høiland-Jørgensen <toke@xxxxxxxxxx> > >> > > >> > Support RX hash and timestamp metadata kfuncs. We need to pass in the cqe > >> > pointer to the mlx5e_skb_from* functions so it can be retrieved from the > >> > XDP ctx to do this. > >> > >> So I finally managed to get enough ducks in row to actually benchmark > >> this. With the caveat that I suddenly can't get the timestamp support to > >> work (it was working in an earlier version, but now > >> timestamp_supported() just returns false). I'm not sure if this is an > >> issue with the enablement patch, or if I just haven't gotten the > >> hardware configured properly. I'll investigate some more, but figured > >> I'd post these results now: > >> > >> Baseline XDP_DROP: 25,678,262 pps / 38.94 ns/pkt > >> XDP_DROP + read metadata: 23,924,109 pps / 41.80 ns/pkt > >> Overhead: 1,754,153 pps / 2.86 ns/pkt > >> > >> As per the above, this is with calling three kfuncs/pkt > >> (metadata_supported(), rx_hash_supported() and rx_hash()). So that's > >> ~0.95 ns per function call, which is a bit less, but not far off from > >> the ~1.2 ns that I'm used to. The tests where I accidentally called the > >> default kfuncs cut off ~1.3 ns for one less kfunc call, so it's > >> definitely in that ballpark. > >> > >> I'm not doing anything with the data, just reading it into an on-stack > >> buffer, so this is the smallest possible delta from just getting the > >> data out of the driver. I did confirm that the call instructions are > >> still in the BPF program bytecode when it's dumped back out from the > >> kernel. > >> > >> -Toke > >> > > > > Oh, that's great, thanks for running the numbers! Will definitely > > reference them in v4! > > Presumably, we should be able to at least unroll most of the > > _supported callbacks if we want, they should be relatively easy; but > > the numbers look fine as is? > > Well, this is for one (and a half) piece of metadata. If we extrapolate > it adds up quickly. Say we add csum and vlan tags, say, and maybe > another callback to get the type of hash (l3/l4). Those would probably > be relevant for most packets in a fairly common setup. Extrapolating > from the ~1 ns/call figure, that's 8 ns/pkt, which is 20% of the > baseline of 39 ns. > > So in that sense I still think unrolling makes sense. At least for the > _supported() calls, as eating a whole function call just for that is > probably a bit much (which I think was also Jakub's point in a sibling > thread somewhere). imo the overhead is tiny enough that we can wait until generic 'kfunc inlining' infra is ready. We're planning to dual-compile some_kernel_file.c into native arch and into bpf arch. Then the verifier will automatically inline bpf asm of corresponding kfunc.
