selftests/bpf: Add exponential backoff to map_update_retriable in test_maps

Using a fixed delay of 1 microsecond has proven flaky in slow CPU environment,
e.g. Github Actions CI system. This patch adds exponential backoff with a cap
of 50ms to reduce the flakiness of the test. Initial delay is chosen at random
in the range [0ms, 5ms).

Signed-off-by: Yucong Sun <fallentree@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210816175250.296110-1-fallentree@fb.com

tools/testing/selftests/bpf/test_maps.c

@@ -1396,15 +1396,22 @@ static void test_map_stress(void)
 #define DO_DELETE 0
 
 #define MAP_RETRIES 20
+#define MAX_DELAY_US 50000
+#define MIN_DELAY_RANGE_US 5000
 
 static int map_update_retriable(int map_fd, const void *key, const void *value,
 				int flags, int attempts)
 {
+	int delay = rand() % MIN_DELAY_RANGE_US;
+
 	while (bpf_map_update_elem(map_fd, key, value, flags)) {
 		if (!attempts || (errno != EAGAIN && errno != EBUSY))
 			return -errno;
 
-		usleep(1);
+		if (delay <= MAX_DELAY_US / 2)
+			delay *= 2;
+
+		usleep(delay);
 		attempts--;
 	}