+ selftests-memcg-increase-error-tolerance-of-child-memorycurrent-check-in-test_memcg_protection.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

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The patch titled
     Subject: selftests: memcg: Increase error tolerance of child memory.current check in test_memcg_protection()
has been added to the -mm mm-new branch.  Its filename is
     selftests-memcg-increase-error-tolerance-of-child-memorycurrent-check-in-test_memcg_protection.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/selftests-memcg-increase-error-tolerance-of-child-memorycurrent-check-in-test_memcg_protection.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Waiman Long <longman@redhat.com>
Subject: selftests: memcg: Increase error tolerance of child memory.current check in test_memcg_protection()
Date: Tue, 15 Apr 2025 17:04:15 -0400

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.

Link: https://lkml.kernel.org/r/20250415210415.13414-3-longman@redhat.com
Signed-off-by: Waiman Long <longman@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 tools/testing/selftests/cgroup/test_memcontrol.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/tools/testing/selftests/cgroup/test_memcontrol.c~selftests-memcg-increase-error-tolerance-of-child-memorycurrent-check-in-test_memcg_protection
+++ a/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -495,10 +495,10 @@ static int test_memcg_protection(const c
 	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)
_

Patches currently in -mm which might be from longman@redhat.com are

selftests-memcg-allow-low-event-with-no-memorylow-and-memory_recursiveprot-on.patch
selftests-memcg-increase-error-tolerance-of-child-memorycurrent-check-in-test_memcg_protection.patch