linux-kernel - Re: [PATCH bpf-next v2] bpf: Cache the last valid build

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Message-ID: <CA+khW7h4bL3qUst4nDy6LDmx73xVA_ch=PLc=o=v2iJNbGn21A@mail.gmail.com>
Date:   Wed, 23 Feb 2022 16:33:37 -0800
From:   Hao Luo <haoluo@...gle.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Song Liu <songliubraving@...com>,
        Namhyung Kim <namhyung@...nel.org>,
        Blake Jones <blakejones@...gle.com>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH bpf-next v2] bpf: Cache the last valid build_id.

On Wed, Feb 23, 2022 at 4:11 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Wed, Feb 23, 2022 at 4:05 PM Hao Luo <haoluo@...gle.com> wrote:
> >
> > For binaries that are statically linked, consecutive stack frames are
> > likely to be in the same VMA and therefore have the same build id.
> > As an optimization for this case, we can cache the previous frame's
> > VMA, if the new frame has the same VMA as the previous one, reuse the
> > previous one's build id. We are holding the MM locks as reader across
> > the entire loop, so we don't need to worry about VMA going away.
> >
> > Tested through "stacktrace_build_id" and "stacktrace_build_id_nmi" in
> > test_progs.
> >
> > Suggested-by: Greg Thelen <gthelen@...gle.com>
> > Signed-off-by: Hao Luo <haoluo@...gle.com>
> > ---
>
> LGTM. Can you share performance numbers before and after?
>
> Acked-by: Andrii Nakryiko <andrii@...nel.org>
>

Thanks Andrii.

On a real-world workload, we observed that 66% of cpu cycles in
__bpf_get_stackid() were spent on build_id_parse() and find_vma().
This was before.

We haven't evaluated the performance with this patch yet. This
optimization seems straightforward, so we plan to upstream it first
and then retest.