[PATCH v2 6/6] selftests: cgroup: Deflake test_memcg_sock() in test_memcontrol.c.

[Kuniyuki Iwashima <kuniyu@google.com>]

Now, MEMCG_SOCK stats updates are batched in per-CPU
memcg_stock->nr_sock[] up to MEMCG_CHARGE_BATCH (64) pages.

After closing the TCP connection, test_memcg_sock() runs
alloc_anon() to force an rstat flush and verifies that "sock"
in memory.stat drops to 0.

However, alloc_anon() only charges and uncharges anonymous pages
(nr_sock == 0), which trims stock->nr_pages[] without draining
stock->nr_sock[].

If a residual negative delta (-63 to -1 pages) remains cached
in stock->nr_sock[] when the socket closes, "sock" in memory.stat
stays positive even after rstat is flushed.

Let's write "0" and then "max" to memory.high in the flush_rounds
loop to trigger drain_all_stock() and flush stock->nr_sock[] into
per-CPU vmstats.

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

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index bb673012dcf0..62cb0278168d 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -1483,6 +1483,14 @@ static int test_memcg_sock(const char *root)
 
 	/* the uncharge is done, but a flush still walking the CPUs can hide it from a read */
 	while (flush_rounds--) {
+		/*
+		 * 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") ||
+		    cg_write(memcg, "memory.high", "max"))
+			goto cleanup;
+
 		if (cg_run(memcg, alloc_anon, (void *)MEMCG_FLUSH_BYTES))
 			goto cleanup;
 
-- 
2.56.0.313.g2479322e68-goog