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

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Date:   Thu, 14 Nov 2019 10:18:55 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     Björn Töpel <bjorn.topel@...il.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>
Subject: Re: [RFC PATCH bpf-next 2/4] bpf: introduce BPF dispatcher

On 14/11/2019 06:29, Björn Töpel wrote:
> On Wed, 13 Nov 2019 at 22:41, Edward Cree <ecree@...arflare.com> wrote:
>> On 13/11/2019 20:47, Björn Töpel wrote:
>> The first-come-first-served model for dispatcher slots might mean that
>>  a low-traffic user ends up getting priority while a higher-traffic
>>  user is stuck with the retpoline fallback.  Have you considered using
>>  a learning mechanism, like in my dynamic call RFC [1] earlier this
>>  year?  (Though I'm sure a better learning mechanism than the one I
>>  used there could be devised.)
> 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.

> 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.

-Ed

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