[PATCH v7 1/2] selftests: memcg: Allow low event with no memory.low and memory_recursiveprot on

[Waiman Long <longman@redhat.com>]

The test_memcontrol selftest consistently fails its test_memcg_low
sub-test due to the fact that its 3rd test child cgroup which
have a memmory.low of 0 have low event count. This happens when
memory_recursiveprot mount option is enabled which is the default
setting used by systemd to mount cgroup2 filesystem.

Modify the test_memcontrol.c to allow non-zero low event count in this
particular case with memory_recursiveprot on.

With this patch applied, the test_memcg_low sub-test finishes
successfully without failure in most cases. Though both test_memcg_low
and test_memcg_min sub-tests may still fail occasionally if the
memory.current values fall outside of the expected ranges.

The 4th test child cgroup has no memory usage and so has an effective
low of 0. It has no low event count because the mem_cgroup_below_low()
check in shrink_node_memcgs() is skipped as mem_cgroup_below_min()
returns true. If we ever change mem_cgroup_below_min() in such a way
that it no longer skips the no usage case, we will have to add code to
explicitly skip it.

Suggested-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 tools/testing/selftests/cgroup/test_memcontrol.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index 16f5d74ae762..5a5dcbe57b56 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -380,10 +380,10 @@ static bool reclaim_until(const char *memcg, long goal);
  *
  * Then it checks actual memory usages and expects that:
  * A/B    memory.current ~= 50M
- * A/B/C  memory.current ~= 29M
- * A/B/D  memory.current ~= 21M
- * A/B/E  memory.current ~= 0
- * A/B/F  memory.current  = 0
+ * A/B/C  memory.current ~= 29M [memory.events:low > 0]
+ * A/B/D  memory.current ~= 21M [memory.events:low > 0]
+ * A/B/E  memory.current ~= 0   [memory.events:low == 0 if !memory_recursiveprot, > 0 otherwise]
+ * A/B/F  memory.current  = 0   [memory.events:low == 0]
  * (for origin of the numbers, see model in memcg_protection.m.)
  *
  * After that it tries to allocate more than there is
@@ -525,8 +525,14 @@ static int test_memcg_protection(const char *root, bool min)
 		goto cleanup;
 	}
 
+	/*
+	 * Child 2 has memory.low=0, but some low protection is still being
+	 * distributed down from its parent with memory.low=50M if cgroup2
+	 * memory_recursiveprot mount option is enabled. So the low event
+	 * count will be non-zero in this case.
+	 */
 	for (i = 0; i < ARRAY_SIZE(children); i++) {
-		int no_low_events_index = 1;
+		int no_low_events_index = has_recursiveprot ? 2 : 1;
 		long low, oom;
 
 		oom = cg_read_key_long(children[i], "memory.events", "oom ");
-- 
2.49.0