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.

This issue was originally fixed by commit cdc69458a5f3 ("cgroup: account for memory_recursiveprot in test_memcg_low()"). It was later reverted by commit 1d09069f5313 ("selftests: memcg: expect no low events in unprotected sibling") expecting the memory reclaim code would be fixed. However, it turns out the unprotected cgroup may still have some residual effective memory.low protection depending on the memory.low settings in its parent and its siblings. As a result, low events may still be triggered.

One way to fix the test failure is to revert the revert commit. However, Michal suggested that it might be better to ignore the low event count with memory_recursiveprot enabled as low event may or may not happen depending on the actual test configuration.

Modify the test_memcontrol.c to ignore low event in the 3rd child cgroup with memory_recursiveprot on.

The 4th 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.

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: Michal Koutný mkoutny@suse.com Signed-off-by: Waiman Long longman@redhat.com --- .../testing/selftests/cgroup/test_memcontrol.c | 18 ++++++++++++++---- 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c index 16f5d74ae762..58602c1831f1 100644 --- a/tools/testing/selftests/cgroup/test_memcontrol.c +++ b/tools/testing/selftests/cgroup/test_memcontrol.c @@ -380,10 +380,11 @@ 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, + * undefined 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 @@ -525,7 +526,14 @@ static int test_memcg_protection(const char *root, bool min) goto cleanup; }

+ /* + * Child 2 has memory.low=0, but some low protection may still be + * distributed down from its parent with memory.low=50M if cgroup2 + * memory_recursiveprot mount option is enabled. Ignore the low + * event count in this case. + */ for (i = 0; i < ARRAY_SIZE(children); i++) { + int ignore_low_events_index = has_recursiveprot ? 2 : -1; int no_low_events_index = 1; long low, oom;

@@ -534,6 +542,8 @@ static int test_memcg_protection(const char *root, bool min)

if (oom) goto cleanup; + if (i == ignore_low_events_index) + continue; if (i <= no_low_events_index && low <= 0) goto cleanup; if (i > no_low_events_index && low)

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:

17 not ok 1 test_memcg_min 22 not ok 2 test_memcg_low

After applying this patch, there were no test failure for test_memcg_min and test_memcg_low in 100 test runs. However, these tests may still fail once in a while if the memory usage goes beyond the newly extended range.

Signed-off-by: Waiman Long longman@redhat.com --- 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 58602c1831f1..d6534d7301a2 100644 --- a/tools/testing/selftests/cgroup/test_memcontrol.c +++ b/tools/testing/selftests/cgroup/test_memcontrol.c @@ -496,10 +496,10 @@ 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)