netdev - Re: [RFC PATCH bpf-next 2/4] bpf: introduce BPF dispatcher

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Message-ID: <CAJ+HfNhaOj+V7JuLb-SCAMf=7BudcE-C4EZAQrzT6P_NGpwvsw@mail.gmail.com>
Date:   Thu, 14 Nov 2019 12:21:27 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Edward Cree <ecree@...arflare.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Björn Töpel <bjorn.topel@...el.com>,
        bpf <bpf@...r.kernel.org>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH bpf-next 2/4] bpf: introduce BPF dispatcher

On Thu, 14 Nov 2019 at 11:19, Edward Cree <ecree@...arflare.com> wrote:
>
> On 14/11/2019 06:29, Björn Töpel wrote:
[...]
> > My rationale was that this mechanism would almost exclusively be used
> > by physical HW NICs using XDP. My hunch was that the number of netdevs
> > would be ~4, and typically less using XDP, so a more sophisticated
> > mechanism didn't really make sense IMO.
> That seems reasonable in most cases, although I can imagine systems with
>  a couple of four-port boards being a thing.  I suppose the netdevs are
>  likely to all have the same XDP prog, though, and if I'm reading your
>  code right it seems they'd share a slot in that case.
>

Yup, correct!

> > However, your approach is more
> > generic and doesn't require any arch specific work. What was the push
> > back for your work?
> Mainly that I couldn't demonstrate a performance benefit from the few
>  call sites I annotated, and others working in the area felt that
>  manual annotation wouldn't scale — Nadav Amit had a different approach
>  [2] that used a GCC plugin to apply a dispatcher on an opt-out basis
>  to all the indirect calls in the kernel; the discussion on that got
>  bogged down in interactions between text patching and perf tracing
>  which all went *waaaay* over my head.  AFAICT the static_call series I
>  was depending on never got merged, and I'm not sure if anyone's still
>  working on it.
>

Again, thanks for the pointers. PeterZ is (hopefully) still working on
the static_call stuff [3]. The static_call_inline would be a good fit
here, and maybe even using static_call as a patchpad/dispatcher like
you did is a better route. I will checkout Nadav's work!


Björn

[3] https://lore.kernel.org/lkml/20191007090225.44108711.6@infradead.org/#r

> -Ed
>
> [2] https://lkml.org/lkml/2018/12/31/19