netdev - Re: FW: [PATCH bpf-next 2/4] xsk: allow AF_XDP sockets to receive packets directly from a queue

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Date:   Wed, 9 Oct 2019 18:06:53 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
Cc:     "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Netdev <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        "Herbert, Tom" <tom.herbert@...el.com>
Subject: Re: FW: [PATCH bpf-next 2/4] xsk: allow AF_XDP sockets to receive
 packets directly from a queue

On Wed, Oct 9, 2019 at 12:12 PM Samudrala, Sridhar
<sridhar.samudrala@...el.com> wrote:
> >>     34.57%  xdpsock          xdpsock           [.] main
> >>     17.19%  ksoftirqd/1      [kernel.vmlinux]  [k] ___bpf_prog_run
> >>     13.12%  xdpsock          [kernel.vmlinux]  [k] ___bpf_prog_run
> >
> > That must be a bad joke.
> > The whole patch set is based on comparing native code to interpreter?!
> > It's pretty awesome that interpreter is only 1.5x slower than native x86.
> > Just turn the JIT on.
>
> Thanks Alexei for pointing out that i didn't have JIT on.
> When i turn it on, the performance improvement is a more modest 1.5x
> with rxdrop and 1.2x with l2fwd.

I want to see perf reports back to back with proper performance analysis.

> >
> > Obvious Nack to the patch set.
> >
>
> Will update the patchset with the right performance data and address
> feedback from Bjorn.
> Hope you are not totally against direct XDP approach as it does provide
> value when an AF_XDP socket is bound to a queue and a HW filter can
> direct packets targeted for that queue.

I used to be in favor of providing "prog bypass" for af_xdp,
because of anecdotal evidence that it will make af_xdp faster.
Now seeing the numbers and the way they were collected
I'm against such bypass.
I want to see hard proof that trivial bpf prog is actually slowing things down
before reviewing any new patch sets.