linux-kernel - [PATCH 1/2] perf test: Fix LBR test by ignoring idle task

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Message-ID: <20250509213017.204343-1-namhyung@kernel.org>
Date: Fri,  9 May 2025 14:30:16 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Kan Liang <kan.liang@...ux.intel.com>
Cc: Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-perf-users@...r.kernel.org
Subject: [PATCH 1/2] perf test: Fix LBR test by ignoring idle task

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@...nel.org>
---
 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