cgroups: refactor children cgroups in memcg tests

cgroups: refactor children cgroups in memcg tests

Patch series "Fix bugs in memcontroller cgroup tests", v2.

tools/testing/selftests/cgroup/test_memcontrol.c contains a set of
testcases which validate expected behavior of the cgroup memory
controller.  Roman Gushchin recently sent out a patchset that fixed a few
issues in the test.  This patchset continues that effort by fixing a few
more issues that were causing non-deterministic failures in the suite.
With this patchset, I'm unable to reproduce any more errors after running
the tests in a continuous loop for many iterations.  Before, I was able to
reproduce at least one of the errors fixed in this patchset with just one
or two runs.

This patch (of 5):

In test_memcg_min() and test_memcg_low(), there is an array of four
sibling cgroups.  All but one of these sibling groups does a 50MB
allocation, and the group that does no allocation is the third of four in
the array.  This is not a problem per se, but makes it a bit tricky to do
some assertions in test_memcg_low(), as we want to make assertions on the
siblings based on whether or not they performed allocations.  Having a
static index before which all groups have performed an allocation makes
this cleaner.

This patch therefore reorders the sibling groups so that the group that
performs no allocations is the last in the array.  A follow-on patch will
leverage this to fix a bug in the test that incorrectly asserts that a
sibling group that had performed an allocation, but only had protection
from its parent, will not observe any memory.events.low events during
reclaim.

Link: https://lkml.kernel.org/r/20220423155619.3669555-1-void@manifault.com
Link: https://lkml.kernel.org/r/20220423155619.3669555-2-void@manifault.com
Signed-off-by: David Vernet <void@manifault.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>