[PATCH v2] perf/test: fix perf ftrace test on s390

[Thomas Richter <tmricht@linux.ibm.com>]

On s390 the perf test case ftrace sometimes fails as follows:

  # ./perf test ftrace
  79: perf ftrace tests    : FAILED!
  #

The failure depends on the kernel .config file. Some configurarions
always work fine, some do not.  The ftrace profile test mostly fails,
because the ring buffer was not large enough, and some lines
(especially the interesting ones with nanosleep in it) where dropped.

To achieve success for all our tested kernel configurations, enlarge
the buffer to store the traces complete without wrapping.
The default buffer size is too small  for all kernel configurations.
Set the buffer size of /sys/kernel/tracing/buffer_size_kb to 16 MB

Output after:
  # ./perf test ftrace
  79: perf ftrace tests     : Ok
  #

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Suggested-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: James Clark <james.clark@linaro.org>
---
 tools/perf/tests/shell/ftrace.sh | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/tools/perf/tests/shell/ftrace.sh b/tools/perf/tests/shell/ftrace.sh
index a6ee740f0d7e..6161a8bdc251 100755
--- a/tools/perf/tests/shell/ftrace.sh
+++ b/tools/perf/tests/shell/ftrace.sh
@@ -14,6 +14,11 @@ output=$(mktemp /tmp/__perf_test.ftrace.XXXXXX)
 
 cleanup() {
   rm -f "${output}"
+  if [ "$(uname -m)" = "s390x" ]
+  then
+	echo $ftrace_size > /sys/kernel/tracing/buffer_size_kb
+  fi
+
 
   trap - EXIT TERM INT
 }
@@ -80,6 +85,12 @@ test_ftrace_profile() {
     echo "perf ftrace profile test  [Success]"
 }
 
+if [ "$(uname -m)" = "s390x" ]
+then
+	ftrace_size=$(cat /sys/kernel/tracing/buffer_size_kb)
+	echo 16384 > /sys/kernel/tracing/buffer_size_kb
+fi
+
 test_ftrace_list
 test_ftrace_trace
 test_ftrace_latency
-- 
2.47.0