Re: [PATCH 1/2] perf test: Fix LBR test by ignoring idle task

[Ian Rogers <irogers@google.com>]

On Fri, May 9, 2025 at 2:30 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> I found 'perf record LBR tests' failing due to empty branch stacks.
>
>   $ perf test -v LBR
>   ...
>   LBR system wide any branch test
>   Lowering default frequency rate from 4000 to 1000.
>   Please consider tweaking /proc/sys/kernel/perf_event_max_sample_rate.
>   [ perf record: Woken up 8 times to write data ]
>   [ perf record: Captured and wrote 3.142 MB /tmp/__perf_test.perf.data.dgSBl (3572 samples) ]
>   LBR system wide any branch test: 3572 samples
>   LBR system wide any branch test [Failed empty br stack ratio exceed 2%: 3%]
>   LBR system wide any call test
>   Lowering default frequency rate from 4000 to 1000.
>   Please consider tweaking /proc/sys/kernel/perf_event_max_sample_rate.
>   [ perf record: Woken up 8 times to write data ]
>   [ perf record: Captured and wrote 3.337 MB /tmp/__perf_test.perf.data.dgSBl (3967 samples) ]
>   LBR system wide any call test: 3967 samples
>   LBR system wide any call test [Failed empty br stack ratio exceed 2%: 9%]
>   ...
>
> The failing cases were in system-wide mode and I realized that the
> samples were from the idle tasks (swapper).  I suspect going to/from
> idle state may affect the LBR contents.
>
> If we can skip empty branch stacks from the idle tasks, the failure
> should go away.  I can see the following output in perf report -D.
>
>   $ perf report -D | grep -m5 -A3 'branch stack: nr:0'
>   ...
>   --
>   ... branch stack: nr:0
>    ... thread: swapper:0
>    ...... dso: /proc/kcore
>
>   --
>   ... branch stack: nr:0
>    ... thread: swapper:0
>    ...... dso: /proc/kcore
>
>   --
>   ... branch stack: nr:0
>    ... thread: DefaultEventMan:10282
>    ...... dso: /proc/kcore
>
>   --
>   ... branch stack: nr:0
>    ... thread: swapper:0
>    ...... dso: /proc/kcore
>
>   --
>   ... branch stack: nr:0
>    ... thread: swapper:0
>    ...... dso: /proc/kcore
>
>   $ perf report -D | grep -c 'branch stack: nr:0'
>   145
>
>   $ perf report -D | grep -A3 'branch stack: nr:0' | grep thread | grep -c swapper
>   i36
>
>   $ perf report -D | grep -A3 'branch stack: nr:0' | grep thread | grep -cv swapper
>   9
>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>

Reviewed-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
>  tools/perf/tests/shell/record_lbr.sh | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/perf/tests/shell/record_lbr.sh b/tools/perf/tests/shell/record_lbr.sh
> index 8d750ee631f877fd..afad02d0180e023c 100755
> --- a/tools/perf/tests/shell/record_lbr.sh
> +++ b/tools/perf/tests/shell/record_lbr.sh
> @@ -93,7 +93,7 @@ lbr_test() {
>      return
>    fi
>
> -  zero_nr=$(echo "$out" | grep -c 'branch stack: nr:0' || true)
> +  zero_nr=$(echo "$out" | grep -A3 'branch stack: nr:0' | grep thread | grep -cv swapper || true)
>    r=$(($zero_nr * 100 / $bs_nr))
>    if [ $r -gt $threshold ]; then
>      echo "$test [Failed empty br stack ratio exceed $threshold%: $r%]"
> --
> 2.49.0.1015.ga840276032-goog
>