| Message ID | 20250407162316.1434714-3-longman@redhat.com |
|---|---|
| State | Superseded |
| Headers | show |
| Series | memcg: Fix test_memcg_min/low test failures | expand |
On Mon, Apr 07, 2025 at 12:23:16PM -0400, Waiman Long <longman@redhat.com> wrote: > 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. I like Roman's suggestion of nr_cpus dependence but I assume your variations were still on the same system, weren't they? Is it fair to say that reclaim is chaotic [1]? I wonder what may cause variations between separate runs of the test. Would it help to `echo 3 >drop_caches` before each run to have more stable initial conditions? (Not sure if it's OK in selftests.) <del>Or sleep 0.5s to settle rstat flushing?</del> No, page_counter's don't suffer that but stock MEMCG_CHARGE_BATCH in percpu stocks. So maybe drain the stock so that counters are precise after the test? (Either by executing a dummy memcg on each CPU or via some debugging API.) Michal [1] https://en.wikipedia.org/wiki/Chaos_theory#Chaotic_dynamics
On 4/11/25 1:22 PM, Michal Koutný wrote: > On Mon, Apr 07, 2025 at 12:23:16PM -0400, Waiman Long <longman@redhat.com> wrote: >> 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. > I like Roman's suggestion of nr_cpus dependence but I assume your > variations were still on the same system, weren't they? > Is it fair to say that reclaim is chaotic [1]? I wonder what may cause > variations between separate runs of the test. Yes, the variation I saw was on the same system with multiple runs. The memory.current values are read by the time the parent cgroup memory usage reaches near the target 50M, but how much memory are remaining in each child varies from run-to-run. You can say that it is somewhat chaotic. > > Would it help to `echo 3 >drop_caches` before each run to have more > stable initial conditions? (Not sure if it's OK in selftests.) I don't know, we may have to try it out. However, I doubt it will have an effect. > > <del>Or sleep 0.5s to settle rstat flushing?</del> No, page_counter's > don't suffer that but stock MEMCG_CHARGE_BATCH in percpu stocks. > So maybe drain the stock so that counters are precise after the test? > (Either by executing a dummy memcg on each CPU or via some debugging > API.) The test itself is already sleeping up to 5 times in 1s interval to wait until the parent memory usage is settled down. Cheers, Longman
diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c index bab826b6b7b0..8f4f2479650e 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)
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. Signed-off-by: Waiman Long <longman@redhat.com> --- tools/testing/selftests/cgroup/test_memcontrol.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)