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

Reviewed-by: Ian Rogers <[email protected]>
Signed-off-by: Namhyung Kim <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: namhyung

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%]"