[PATCH v2 2/2] selftests: memcg: Increase error tolerance of child memory.current check in test_memcg_protection()

[Waiman Long]

The test_memcg_protection() function is used for the test_memcg_min and
test_memcg_low sub-tests. This function generates a set of parent/child
cgroups like:

  parent:  memory.min/low = 50M
  child 0: memory.min/low = 75M,  memory.current = 50M
  child 1: memory.min/low = 25M,  memory.current = 50M
  child 2: memory.min/low = 0,    memory.current = 50M

After applying memory pressure, the function expects the following
actual memory usages.

  parent:  memory.current ~= 50M
  child 0: memory.current ~= 29M
  child 1: memory.current ~= 21M
  child 2: memory.current ~= 0

In reality, the actual memory usages can differ quite a bit from the
expected values. It uses an error tolerance of 10% with the values_close()
helper.

Both the test_memcg_min and test_memcg_low sub-tests can fail
sporadically because the actual memory usage exceeds the 10% error
tolerance. Below are a sample of the usage data of the tests runs
that fail.

  Child   Actual usage    Expected usage    %err
  -----   ------------    --------------    ----
    1       16990208         22020096      -12.9%
    1       17252352         22020096      -12.1%
    0       37699584         30408704      +10.7%
    1       14368768         22020096      -21.0%
    1       16871424         22020096      -13.2%

The current 10% error tolerenace might be right at the time
test_memcontrol.c was first introduced in v4.18 kernel, but memory
reclaim have certainly evolved quite a bit since then which may result
in a bit more run-to-run variation than previously expected.

Increase the error tolerance to 15% for child 0 and 20% for child 1 to
minimize the chance of this type of failure. The tolerance is bigger
for child 1 because an upswing in child 0 corresponds to a smaller
%err than a similar downswing in child 1 due to the way %err is used
in values_close().

Before this patch, a 100 test runs of test_memcontrol produced the
following results:

     19 not ok 3 test_memcg_min
     13 not ok 4 test_memcg_low

After applying this patch, there were no test failure for test_memcg_min
and test_memcg_low in 100 test runs.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 tools/testing/selftests/cgroup/test_memcontrol.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index 16f5d74ae762..f442c0c3f5a7 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -495,10 +495,10 @@ static int test_memcg_protection(const char *root, bool min)
 	for (i = 0; i < ARRAY_SIZE(children); i++)
 		c[i] = cg_read_long(children[i], "memory.current");
 
-	if (!values_close(c[0], MB(29), 10))
+	if (!values_close(c[0], MB(29), 15))
 		goto cleanup;
 
-	if (!values_close(c[1], MB(21), 10))
+	if (!values_close(c[1], MB(21), 20))
 		goto cleanup;
 
 	if (c[3] != 0)
-- 
2.48.1