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

[Hao Luo]

On Wed, Feb 23, 2022 at 5:10 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Wed, Feb 23, 2022 at 4:33 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> >
> > On Wed, Feb 23, 2022 at 4:11 PM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> > >
> > > On Wed, Feb 23, 2022 at 4:05 PM Hao Luo <haoluo@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> > > > ---
> > >
> > > LGTM. Can you share performance numbers before and after?
> > >
> > > Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > >
> >
> > 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.
>
> Ok, once it lands upstream, I'd really appreciate if you can retest
> and update us with numbers. Thanks!

Sure, will do that.