selftests: memcg: increase error tolerance of child memory.current check in test_memc...

selftests: memcg: increase error tolerance of child memory.current check in test_memcg_protection()

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/20250502010443.106022-3-longman@redhat.com
Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>