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). -Toke
