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

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Date:   Thu, 10 Mar 2022 17:16:10 -0800
From:   Hao Luo <haoluo@...gle.com>
To:     Pasha Tatashin <pasha.tatashin@...een.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@...r.kernel.org,
        linux-kernel@...r.kernel.org, Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH bpf-next v2] bpf: Cache the last valid build_id.

On Fri, Feb 25, 2022 at 12:43 PM Pasha Tatashin
<pasha.tatashin@...een.com> wrote:
>
> On 2/23/22 19:05, Hao Luo 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>
>
> Reviewed-by: Pasha Tatashin <pasha.tatashin@...een.com>
>

An update with performance numbers. Thanks to Blake Jones for
collecting the stats:

In a production workload, with BPF probes sampling stack trace, we see
the following changes:

 - stack_map_get_build_id_offset() is taking 70% of the time of
__bpf_get_stackid(); it was 80% before.

 - find_get_page() and find_vma() together are taking 75% of the time
of stack_map_get_build_id_offset(); it was 83% before.

Note the call chain is

__bpf_get_stackid()
  -> stack_map_get_build_id_offset()
    -> find_get_page()
    -> find_vma()

> Thanks,
> Pasha