linux-kernel - [PATCH] selftests: cgroup: fix test_kmem

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Date:   Tue,  1 Aug 2023 09:56:32 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Michal Hocko <mhocko@...e.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] selftests: cgroup: fix test_kmem_basic false positives

This test fails routinely in our prod testing environment, and I can
reproduce it locally as well.

The test allocates dcache inside a cgroup, then drops the memory limit
and checks that usage drops correspondingly. 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 dropping the limit. This should be good
enough, assuming that machines running those tests are otherwise not
very busy.

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
 tools/testing/selftests/cgroup/test_kmem.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/testing/selftests/cgroup/test_kmem.c b/tools/testing/selftests/cgroup/test_kmem.c
index 258ddc565deb..1b2cec9d18a4 100644
--- a/tools/testing/selftests/cgroup/test_kmem.c
+++ b/tools/testing/selftests/cgroup/test_kmem.c
@@ -70,6 +70,10 @@ static int test_kmem_basic(const char *root)
 		goto cleanup;

 	cg_write(cg, "memory.high", "1M");
+
+	/* wait for RCU freeing */
+	sleep(1);
+
 	slab1 = cg_read_key_long(cg, "memory.stat", "slab ");
 	if (slab1 <= 0)
 		goto cleanup;
-- 
2.41.0