[Devel] [PATCH vz10] ms/selftests/sched_ext: flush stdout before test to avoid log spam

[Eva Kurchatova]

From: Emil Tsalapatis <etsal at meta.com>

The sched_ext selftests runner runs each test in the same process,
with each test possibly forking multiple times. When the main runner
has not flushed its stdout, the children inherit the buffered output
for previous tests and emit it during exit. This causes log spam.

Make sure stdout/stderr is fully flushed before each test.

Cc: Ihor Solodrai <ihor.solodrai at linux.dev>
Signed-off-by: Emil Tsalapatis <emil at etsalapatis.com>
Signed-off-by: Tejun Heo <tj at kernel.org>
Started by hand, the runner writes 12 MB for its 23 tests, the preamble
of each repeated about five thousand times, against 8 KB with this
applied. run_kselftest.sh starts every test under stdbuf --output=L,
so a run through it never shows the duplication.

(cherry picked from commit 579a3297b268f0281644ead7ff574a2b4bc64d3c)

https://virtuozzo.atlassian.net/browse/VSTOR-143426
Feature: fix selftests
Signed-off-by: Eva Kurchatova <eva.kurchatova at virtuozzo.com>
---
 tools/testing/selftests/sched_ext/runner.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/tools/testing/selftests/sched_ext/runner.c b/tools/testing/selftests/sched_ext/runner.c
index aa2d7d32dda9..5748d2c69903 100644
--- a/tools/testing/selftests/sched_ext/runner.c
+++ b/tools/testing/selftests/sched_ext/runner.c
@@ -46,6 +46,14 @@ static void print_test_preamble(const struct scx_test *test, bool quiet)
 	if (!quiet)
 		printf("DESCRIPTION: %s\n", test->description);
 	printf("OUTPUT:\n");
+
+	/*
+	 * The tests may fork with the preamble buffered
+	 * in the children's stdout. Flush before the test
+	 * to avoid printing the message multiple times.
+	 */
+	fflush(stdout);
+	fflush(stderr);
 }
 
 static const char *status_to_result(enum scx_test_status status)
-- 
2.55.0