[PATCH v2] 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. The timeout is set to 3 seconds to cover the periodic
rstat flush interval (FLUSH_TIME = 2*HZ by default) plus some
scheduling slack. 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.

Suggested-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
v2:
 - Mention the periodic rstat flush interval (FLUSH_TIME = 2*HZ) in
   the comment and clarify the rationale for the 3s timeout.
 - Replace the hard-coded retry count and wait interval with macros
   to avoid magic numbers and make the 3s timeout calculation explicit.
---
 .../selftests/cgroup/test_memcontrol.c        | 30 ++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index 4e1647568c5b..7bea656658a2 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -24,6 +24,9 @@
 static bool has_localevents;
 static bool has_recursiveprot;
 
+#define MEMCG_SOCKSTAT_WAIT_RETRIES        30              /* 3s total */
+#define MEMCG_SOCKSTAT_WAIT_INTERVAL_US    (100 * 1000)    /* 100 ms */
+
 int get_temp_fd(void)
 {
 	return open(".", O_TMPFILE | O_RDWR | O_EXCL);
@@ -1384,6 +1387,8 @@ 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;
 
 	memcg = cg_name(root, "memcg_test");
 	if (!memcg)
@@ -1432,7 +1437,30 @@ 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, which runs periodically (every 2 seconds,
+	 * see FLUSH_TIME). On a busy system, 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 (~FLUSH_TIME plus some
+	 * scheduling slack) and require that the "sock " counter
+	 * eventually drops to zero.
+	 */
+	for (i = 0; i < MEMCG_SOCKSTAT_WAIT_RETRIES; i++) {
+		sock_post = cg_read_key_long(memcg, "memory.stat", "sock ");
+		if (sock_post < 0)
+			goto cleanup;
+
+		if (!sock_post)
+			break;
+
+		usleep(MEMCG_SOCKSTAT_WAIT_INTERVAL_US);
+	}
+
+	if (sock_post)
 		goto cleanup;
 
 	ret = KSFT_PASS;
-- 
2.25.1