[PATCH] selftests: cgroup: fix test_kmem_memcg_deletion false positives

[Lucas Karpinski]

The test allocates dcache inside a cgroup, then destroys the cgroups and
then checks the sanity of numbers on the parent level. The reason it
fails is because dentries are freed with an RCU delay - a debugging
sleep shows that usage drops as expected shortly after.

Insert a 1s sleep after completing the cgroup creation/deletions. This
should be good enough, assuming that machines running those tests are
otherwise not very busy. This commit is directly inspired by Johannes
over at the link below.

Link: https://lore.kernel.org/all/20230801135632.1768830-1-hannes@xxxxxxxxxxx/

Signed-off-by: Lucas Karpinski <lkarpins@xxxxxxxxxx>
---
 tools/testing/selftests/cgroup/test_kmem.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/testing/selftests/cgroup/test_kmem.c b/tools/testing/selftests/cgroup/test_kmem.c
index 67cc0182058d..7ac384bbfdd5 100644
--- a/tools/testing/selftests/cgroup/test_kmem.c
+++ b/tools/testing/selftests/cgroup/test_kmem.c
@@ -183,6 +183,9 @@ static int test_kmem_memcg_deletion(const char *root)
 	if (cg_run_in_subcgroups(parent, alloc_kmem_smp, NULL, 100))
 		goto cleanup;
 
+	/* wait for RCU freeing */
+	sleep(1);
+
 	current = cg_read_long(parent, "memory.current");
 	slab = cg_read_key_long(parent, "memory.stat", "slab ");
 	anon = cg_read_key_long(parent, "memory.stat", "anon ");
-- 
2.41.0