From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: "Björn Töpel" <bjorn.topel@gmail.com>,
netdev@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net
Cc: "Björn Töpel" <bjorn.topel@gmail.com>,
bpf@vger.kernel.org, magnus.karlsson@gmail.com,
magnus.karlsson@intel.com, jonathan.lemon@gmail.com,
ecree@solarflare.com, thoiland@redhat.com,
andrii.nakryiko@gmail.com
Subject: Re: [PATCH bpf-next v3 0/6] Introduce the BPF dispatcher
Date: Mon, 09 Dec 2019 16:00:36 +0100 [thread overview]
Message-ID: <87h829ilwr.fsf@toke.dk> (raw)
In-Reply-To: <20191209135522.16576-1-bjorn.topel@gmail.com>
Björn Töpel <bjorn.topel@gmail.com> writes:
> Overview
> ========
>
> This is the 4th iteration of the series that introduces the BPF
> dispatcher, which is a mechanism to avoid indirect calls.
>
> The BPF dispatcher is a multi-way branch code generator, targeted for
> BPF programs. E.g. when an XDP program is executed via the
> bpf_prog_run_xdp(), it is invoked via an indirect call. With
> retpolines enabled, the indirect call has a substantial performance
> impact. The dispatcher is a mechanism that transform indirect calls to
> direct calls, and therefore avoids the retpoline. The dispatcher is
> generated using the BPF JIT, and relies on text poking provided by
> bpf_arch_text_poke().
>
> The dispatcher hijacks a trampoline function it via the __fentry__ nop
> of the trampoline. One dispatcher instance currently supports up to 48
> dispatch points. This can be extended in the future.
>
> In this series, only one dispatcher instance is supported, and the
> only user is XDP. The dispatcher is updated when an XDP program is
> attached/detached to/from a netdev. An alternative to this could have
> been to update the dispatcher at program load point, but as there are
> usually more XDP programs loaded than attached, so the latter was
> picked.
I like the new version where it's integrated into bpf_prog_run_xdp();
nice! :)
> The XDP dispatcher is always enabled, if available, because it helps
> even when retpolines are disabled. Please refer to the "Performance"
> section below.
Looking at those numbers, I think I would moderate "helps" to "doesn't
hurt" - a difference of less than 1ns is basically in the noise.
You mentioned in the earlier version that this would impact the time it
takes to attach an XDP program. Got any numbers for this?
-Toke
next prev parent reply other threads:[~2019-12-09 15:00 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-09 13:55 [PATCH bpf-next v3 0/6] Introduce the BPF dispatcher Björn Töpel
2019-12-09 13:55 ` [PATCH bpf-next v3 1/6] bpf: move trampoline JIT image allocation to a function Björn Töpel
2019-12-09 13:55 ` [PATCH bpf-next v3 2/6] bpf: introduce BPF dispatcher Björn Töpel
2019-12-10 5:50 ` Alexei Starovoitov
2019-12-10 5:54 ` Björn Töpel
2019-12-09 13:55 ` [PATCH bpf-next v3 3/6] bpf, xdp: start using the BPF dispatcher for XDP Björn Töpel
2019-12-09 13:55 ` [PATCH bpf-next v3 4/6] bpf: start using the BPF dispatcher in BPF_TEST_RUN Björn Töpel
2019-12-09 13:55 ` [PATCH bpf-next v3 5/6] selftests: bpf: add xdp_perf test Björn Töpel
2019-12-10 11:05 ` Jesper Dangaard Brouer
2019-12-10 11:56 ` Björn Töpel
2019-12-09 13:55 ` [PATCH bpf-next v3 6/6] bpf, x86: align dispatcher branch targets to 16B Björn Töpel
2019-12-09 15:00 ` Toke Høiland-Jørgensen [this message]
2019-12-09 17:42 ` [PATCH bpf-next v3 0/6] Introduce the BPF dispatcher Björn Töpel
2019-12-11 12:38 ` Björn Töpel
2019-12-11 13:17 ` Toke Høiland-Jørgensen
2019-12-09 17:00 ` Jesper Dangaard Brouer
2019-12-09 17:45 ` Björn Töpel
2019-12-09 19:50 ` Jesper Dangaard Brouer
2019-12-10 19:28 ` Samudrala, Sridhar
2019-12-10 20:04 ` Björn Töpel
2019-12-10 19:59 ` Björn Töpel
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