[LTP] [PATCH] sched_football: synchronize with kickoff flag to reduce skew

[Li Wang]

Previously, thread synchronization in sched_football only relied on a
thread_barrier. This ensured that all threads were created before the
referee started the game, but did not fully prevent offense threads from
getting a scheduling opportunity before defense threads were migrated,
leading to occasional non-zero final ball positions on kvm or debug kernels.

This patch introduces an explicit `kickoff_flag`:

* All player threads (offense, defense, fans) wait at the barrier and
  then spin until the referee sets `kickoff_flag`. This reduces kernel
  scheduling skew, as threads only proceed once the referee explicitly
  signals the kickoff.

* The referee now:
  - Waits at the barrier.
  - Clears the ball position.
  - Sets `kickoff_flag` to start the ball.

* Game termination is also slightly reordered (by Cyril):
  - Final ball position is read before `game_over` is set,
    avoiding a race where the ball could still move right after
    defense threads stop.

* Only test on RT-kernels.

This makes startup sequencing more deterministic while still allowing
some nondeterminism, which is intentional for testing scheduler
behavior under load.

Signed-off-by: Li Wang <liwang@redhat.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
Cc: Andrea Cervesato <andrea.cervesato@suse.com>
---
 .../func/sched_football/sched_football.c      | 27 ++++++++++++++-----
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/testcases/realtime/func/sched_football/sched_football.c b/testcases/realtime/func/sched_football/sched_football.c
index 0617bdb87..97ac9d413 100644
--- a/testcases/realtime/func/sched_football/sched_football.c
+++ b/testcases/realtime/func/sched_football/sched_football.c
@@ -44,6 +44,7 @@
 static tst_atomic_t the_ball;
 static int players_per_team = 0;
 static int game_length = DEF_GAME_LENGTH;
+static tst_atomic_t kickoff_flag;
 static tst_atomic_t game_over;
 
 static char *str_game_length;
@@ -55,6 +56,9 @@ void *thread_fan(void *arg LTP_ATTRIBUTE_UNUSED)
 {
 	prctl(PR_SET_NAME, "crazy_fan", 0, 0, 0);
 	pthread_barrier_wait(&start_barrier);
+	while (!tst_atomic_load(&kickoff_flag))
+		;
+
 	/*occasionally wake up and run across the field */
 	while (!tst_atomic_load(&game_over)) {
 		struct timespec start, stop;
@@ -80,6 +84,9 @@ void *thread_defense(void *arg LTP_ATTRIBUTE_UNUSED)
 {
 	prctl(PR_SET_NAME, "defense", 0, 0, 0);
 	pthread_barrier_wait(&start_barrier);
+	while (!tst_atomic_load(&kickoff_flag))
+		;
+
 	/*keep the ball from being moved */
 	while (!tst_atomic_load(&game_over)) {
 	}
@@ -92,6 +99,9 @@ void *thread_offense(void *arg LTP_ATTRIBUTE_UNUSED)
 {
 	prctl(PR_SET_NAME, "offense", 0, 0, 0);
 	pthread_barrier_wait(&start_barrier);
+	while (!tst_atomic_load(&kickoff_flag))
+		;
+
 	while (!tst_atomic_load(&game_over)) {
 		tst_atomic_add_return(1, &the_ball); /* move the ball ahead one yard */
 	}
@@ -115,9 +125,10 @@ void referee(int game_length)
 	now = start;
 
 	/* Start the game! */
-	tst_atomic_store(0, &the_ball);
-	pthread_barrier_wait(&start_barrier);
 	atrace_marker_write("sched_football", "Game_started!");
+	pthread_barrier_wait(&start_barrier);
+	tst_atomic_store(0, &the_ball);
+	tst_atomic_store(1, &kickoff_flag);
 
 	/* Watch the game */
 	while ((now.tv_sec - start.tv_sec) < game_length) {
@@ -125,14 +136,14 @@ void referee(int game_length)
 		gettimeofday(&now, NULL);
 	}
 
-	/* Stop the game! */
-	tst_atomic_store(1, &game_over);
-	atrace_marker_write("sched_football", "Game_Over!");
-
 	/* Blow the whistle */
 	final_ball = tst_atomic_load(&the_ball);
 	tst_res(TINFO, "Final ball position: %d", final_ball);
 
+	/* Stop the game! */
+	tst_atomic_store(1, &game_over);
+	atrace_marker_write("sched_football", "Game_Over!");
+
 	TST_EXP_EXPR(final_ball == 0);
 }
 
@@ -154,6 +165,7 @@ static void do_test(void)
 	/* We're the ref, so set our priority right */
 	param.sched_priority = sched_get_priority_min(SCHED_FIFO) + 80;
 	sched_setscheduler(0, SCHED_FIFO, &param);
+	tst_atomic_store(0, &kickoff_flag);
 
 	/*
 	 * Start the offense
@@ -186,6 +198,9 @@ static void do_test(void)
 
 static void do_setup(void)
 {
+	if (!tst_check_preempt_rt())
+		tst_brk(TCONF, "Test requires real-time kernel");
+
 	if (tst_parse_int(str_game_length, &game_length, 1, INT_MAX))
 		tst_brk(TBROK, "Invalid game length '%s'", str_game_length);
 
-- 
2.51.0