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

[Hao Luo]

On Fri, Feb 25, 2022 at 12:43 PM Pasha Tatashin
<pasha.tatashin@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
>
> Reviewed-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
>

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