[PATCH 5/5] selftests: cgroup: Deflake test_memcg_sock() in test_memcontrol.c.

[Kuniyuki Iwashima <kuniyu@google.com>]

With MEMCG_SOCK batched in per-CPU memcg_stock, test_memcg_sock()
can fail in two places:

1. In tcp_client(), reading memory.stat only flushes rstat
   synchronously when vmstats->stats_updates exceeds
   MEMCG_CHARGE_BATCH * num_online_cpus().  Because mod_memcg_state()
   is now called less frequently, 16 read() iterations (0x10) on a
   64-CPU system can finish in a few milliseconds before
   stats_updates reaches the 4096-page threshold or the 2-second
   periodic flusher (FLUSH_TIME) runs, leaving "sock " at 0.

2. After the TCP connection is closed, up to MEMCG_CHARGE_BATCH - 1
   pages of MEMCG_SOCK delta can remain cached in per-CPU
   stock->nr_sock[] (e.g. when the initial charge published +n via
   try_charge_memcg() and mod_memcg_state(), while the uncharge was
   absorbed into stock->nr_sock[] as -n without overflowing
   stock->nr_pages[]).  Since the periodic rstat flusher does not
   drain memcg_stock, "sock " in memory.stat can stay non-zero across
   cg_read_key_long_poll().

Let's increase retries in tcp_client() from 0x10 to 0x40 and write
"0" to memory.high before polling memory.stat at the end of
test_memcg_sock() to trigger drain_all_stock() and flush
stock->nr_sock[].

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 tools/testing/selftests/cgroup/test_memcontrol.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index 0ed82347044e..1311aa5bc522 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -1333,7 +1333,7 @@ static int tcp_client(const char *cgroup, unsigned short port)
 	const char server[] = "localhost";
 	struct addrinfo *ai;
 	char servport[6];
-	int retries = 0x10; /* nice round number */
+	int retries = 0x40; /* nice round number */
 	int sk, ret;
 	long allocated;
 
@@ -1447,6 +1447,13 @@ static int test_memcg_sock(const char *root)
 	if (cg_read_long(memcg, "memory.current") < 0)
 		goto cleanup;
 
+	/*
+	 * Drain per-CPU memcg_stock so cached MEMCG_SOCK deltas in
+	 * stock->nr_sock[] are flushed to vmstats.
+	 */
+	if (cg_write(memcg, "memory.high", "0"))
+		goto cleanup;
+
 	/*
 	 * memory.stat is updated asynchronously via the memcg rstat
 	 * flushing worker, which runs periodically (every 2 seconds,
-- 
2.56.0.212.g129ede20d9-goog