[xdp-hints] Re: [PATCH bpf-next v3 11/12] mlx5: Support RX XDP metadata

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:

> On Thu, Dec 8, 2022 at 4:02 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>> Stanislav Fomichev <sdf@google.com> writes:
>>
>> > On Thu, Dec 8, 2022 at 2:59 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>> >>
>> >> Stanislav Fomichev <sdf@google.com> writes:
>> >>
>> >> > From: Toke Høiland-Jørgensen <toke@redhat.com>
>> >> >
>> >> > 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.

Is that "planning" or "actively working on"? Just trying to get a sense
of the time frames here, as this sounds neat, but also something that
could potentially require quite a bit of fiddling with the build system
to get to work? :)

-Toke