[PATCH] selftests: cgroup: make test_memcg_sock robust against delayed sock stats

[Guopeng Zhang <zhangguopeng@kylinos.cn>]

test_memcg_sock() currently requires that memory.stat's "sock " counter
is exactly zero immediately after the TCP server exits. On a busy system
this assumption is too strict:

  - Socket memory may be freed with a small delay (e.g. RCU callbacks).
  - memcg statistics are updated asynchronously via the rstat flushing
    worker, so the "sock " value in memory.stat can stay non-zero for a
    short period of time even after all socket memory has been uncharged.

As a result, test_memcg_sock() can intermittently fail even though socket
memory accounting is working correctly.

Make the test more robust by polling memory.stat for the "sock " counter
and allowing it some time to drop to zero instead of checking it only
once. If the counter does not become zero within the timeout, the test
still fails as before.

On my test system, running test_memcontrol 50 times produced:

  - Before this patch:  6/50 runs passed.
  - After this patch:  50/50 runs passed.

Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
 .../selftests/cgroup/test_memcontrol.c        | 24 ++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index 4e1647568c5b..86d9981cddd8 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -1384,6 +1384,8 @@ static int test_memcg_sock(const char *root)
 	int bind_retries = 5, ret = KSFT_FAIL, pid, err;
 	unsigned short port;
 	char *memcg;
+	long sock_post = -1;
+	int i, retries = 30;
 
 	memcg = cg_name(root, "memcg_test");
 	if (!memcg)
@@ -1432,7 +1434,27 @@ static int test_memcg_sock(const char *root)
 	if (cg_read_long(memcg, "memory.current") < 0)
 		goto cleanup;
 
-	if (cg_read_key_long(memcg, "memory.stat", "sock "))
+	/*
+	 * memory.stat is updated asynchronously via the memcg rstat
+	 * flushing worker, so the "sock " counter may stay non-zero
+	 * for a short period of time after the TCP connection is
+	 * closed and all socket memory has been uncharged.
+	 *
+	 * Poll memory.stat for up to 3 seconds and require that the
+	 * "sock " counter eventually drops to zero.
+	 */
+	for (i = 0; i < retries; i++) {
+		sock_post = cg_read_key_long(memcg, "memory.stat", "sock ");
+		if (sock_post < 0)
+			goto cleanup;
+
+		if (!sock_post)
+			break;
+
+		usleep(100 * 1000); /* 100ms */
+	}
+
+	if (sock_post)
 		goto cleanup;
 
 	ret = KSFT_PASS;
-- 
2.25.1