perf test: Avoid list test blocking on writing to stdout

Python's json.tool will output the input json to stdout. Redirect to
/dev/null to avoid blocking on stdout writes.

Tested-by: James Clark <[email protected]>
Signed-off-by: Ian Rogers <[email protected]>
Cc: Colin Ian King <[email protected]>
Cc: Howard Chu <[email protected]>
Cc: Weilin Wang <[email protected]>
Cc: Ilya Leoshkevich <[email protected]>
Cc: Thomas Richter <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Dapeng Mi <[email protected]>
Cc: Athira Jajeev <[email protected]>
Cc: Michael Petlan <[email protected]>
Cc: Veronika Molnarova <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Namhyung Kim <[email protected]>

Author: captain5050

tools/perf/tests/shell/list.sh

@@ -24,8 +24,11 @@ trap trap_cleanup EXIT TERM INT
 
 test_list_json() {
   echo "Json output test"
+  # Generate perf list json output into list_output file.
   perf list -j -o "${list_output}"
-  $PYTHON -m json.tool "${list_output}"
+  # Validate the json using python, redirect the json copy to /dev/null as
+  # otherwise the test may block writing to stdout.
+  $PYTHON -m json.tool "${list_output}" /dev/null
   echo "Json output test [Success]"
 }