selftests: cgroup: make test_memcg_sock robust against delayed sock stats

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.

Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Suggested-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: Guopeng Zhang

tools/testing/selftests/cgroup/test_memcontrol.c

@@ -21,6 +21,8 @@
 #include "kselftest.h"
 #include "cgroup_util.h"
 
+#define MEMCG_SOCKSTAT_WAIT_RETRIES        30
+
 static bool has_localevents;
 static bool has_recursiveprot;
 
@@ -1384,6 +1386,7 @@ 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;
 
 	memcg = cg_name(root, "memcg_test");
 	if (!memcg)
@@ -1432,7 +1435,22 @@ 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.
+	 */
+	sock_post = cg_read_key_long_poll(memcg, "memory.stat", "sock ", 0,
+					 MEMCG_SOCKSTAT_WAIT_RETRIES,
+					 DEFAULT_WAIT_INTERVAL_US);
+	if (sock_post)
 		goto cleanup;
 
 	ret = KSFT_PASS;