[PATCH v3 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()

[Waiman Long <longman@redhat.com>]

The test_memcontrol selftest consistently fails its test_memcg_low
sub-test due to the fact that two of its test child cgroups which
have a memmory.low of 0 or an effective memory.low of 0 still have low
events generated for them since mem_cgroup_below_low() use the ">="
operator when comparing to elow.

The two failed use cases are as follows:

1) memory.low is set to 0, but low events can still be triggered and
   so the cgroup may have a non-zero low event count. I doubt users are
   looking for that as they didn't set memory.low at all.

2) memory.low is set to a non-zero value but the cgroup has no task in
   it so that it has an effective low value of 0. Again it may have a
   non-zero low event count if memory reclaim happens. This is probably
   not a result expected by the users and it is really doubtful that
   users will check an empty cgroup with no task in it and expecting
   some non-zero event counts.

In the first case, even though memory.low isn't set, it may still have
some low protection if memory.low is set in the parent. So low event may
still be recorded. The test_memcontrol.c test has to be modified to
account for that.

For the second case, it really doesn't make sense to have non-zero low
event if the cgroup has 0 usage. So we need to skip this corner case
in shrink_node_memcgs().

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.

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 mm/vmscan.c                                      | 4 ++++
 tools/testing/selftests/cgroup/test_memcontrol.c | 7 ++++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b620d74b0f66..2a2957b9dc99 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -5963,6 +5963,10 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 
 		mem_cgroup_calculate_protection(target_memcg, memcg);
 
+		/* Skip memcg with no usage */
+		if (!page_counter_read(&memcg->memory))
+			continue;
+
 		if (mem_cgroup_below_min(target_memcg, memcg)) {
 			/*
 			 * Hard protection.
diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index 16f5d74ae762..bab826b6b7b0 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -525,8 +525,13 @@ 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. So the low
+	 * event count will be non-zero.
+	 */
 	for (i = 0; i < ARRAY_SIZE(children); i++) {
-		int no_low_events_index = 1;
+		int no_low_events_index = 2;
 		long low, oom;
 
 		oom = cg_read_key_long(children[i], "memory.events", "oom ");
-- 
2.48.1