Re: [PATCH bpf-next v2] bpf: Cache the last valid build_id.

From: Hao Luo <haoluo@google.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Song Liu <songliubraving@fb.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Blake Jones <blakejones@google.com>, bpf <bpf@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	Greg Thelen <gthelen@google.com>
Subject: Re: [PATCH bpf-next v2] bpf: Cache the last valid build_id.
Date: Wed, 23 Feb 2022 16:33:37 -0800	[thread overview]
Message-ID: <CA+khW7h4bL3qUst4nDy6LDmx73xVA_ch=PLc=o=v2iJNbGn21A@mail.gmail.com> (raw)
In-Reply-To: <CAEf4Bzb44WR2LiYchxB5JZ=Jdie6FEEi90mh=SCv07v4h4W11w@mail.gmail.com>

On Wed, Feb 23, 2022 at 4:11 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Wed, Feb 23, 2022 at 4:05 PM Hao Luo <haoluo@google.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@google.com>
> > Signed-off-by: Hao Luo <haoluo@google.com>
> > ---
>
> LGTM. Can you share performance numbers before and after?
>
> Acked-by: Andrii Nakryiko <andrii@kernel.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.