[PATCH v1 2/2] selftests/sched: Retry PR_SCHED_CORE_GET on EBUSY

[Hui Su]

During early boot, the pointer hash key may not be initialized when
PR_SCHED_CORE_GET is called. With the preceding kernel fix, GET reports
EBUSY until the pointer hash key becomes available.

Retry the operation for up to one second when it returns EBUSY. Report
other errors immediately, instead of describing every failed GET as an
unsupported core scheduling system.

cs_prctl_test passes on x86_64 QEMU with normal random initialization.
A targeted early-boot probe also confirmed that the fixed kernel
returns EBUSY while pointer hashing is unavailable.

Signed-off-by: Hui Su <sh_def@xxxxxxx>
---
 tools/testing/selftests/sched/cs_prctl_test.c | 27 ++++++++++++++-----
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/tools/testing/selftests/sched/cs_prctl_test.c b/tools/testing/selftests/sched/cs_prctl_test.c
index 52d97fae4dbd..dcb661cace5e 100644
--- a/tools/testing/selftests/sched/cs_prctl_test.c
+++ b/tools/testing/selftests/sched/cs_prctl_test.c
@@ -51,6 +51,8 @@ static pid_t gettid(void)
 
 #define MAX_PROCESSES 128
 #define MAX_THREADS   128
+#define CORE_COOKIE_RETRIES	100
+#define CORE_COOKIE_RETRY_US	10000
 
 static const char USAGE[] = "cs_prctl_test [options]\n"
 "    options:\n"
@@ -112,16 +114,27 @@ static void handle_usage(int rc, char *msg)
 static unsigned long get_cs_cookie(int pid)
 {
 	unsigned long long cookie;
-	int ret;
+	int i, ret, err = 0;
+
+	for (i = 0; i < CORE_COOKIE_RETRIES; i++) {
+		ret = prctl(PR_SCHED_CORE, PR_SCHED_CORE_GET, pid, PIDTYPE_PID,
+			    (unsigned long)&cookie);
+		if (!ret)
+			return cookie;
+
+		err = errno;
+		if (err != EBUSY)
+			break;
 
-	ret = prctl(PR_SCHED_CORE, PR_SCHED_CORE_GET, pid, PIDTYPE_PID,
-		    (unsigned long)&cookie);
-	if (ret) {
-		printf("Not a core sched system\n");
-		return -1UL;
+		usleep(CORE_COOKIE_RETRY_US);
 	}
 
-	return cookie;
+	if (err == EBUSY)
+		printf("Timed out waiting for core sched cookie\n");
+	else
+		printf("Failed to get core sched cookie: %s\n", strerror(err));
+
+	return -1UL;
 }
 
 static int child_func_thread(void __attribute__((unused))*arg)
-- 
2.55.0