[PATCH v2 2/7] memcg: Scale down MEMCG_CHARGE_BATCH with increase in PAGE_SIZE

[Waiman Long <longman@redhat.com>]

For a system with 4k page size, each percpu memcg_stock can hide up
to 256 kbytes of memory with the current MEMCG_CHARGE_BATCH value of
64. For another system with 64k page size, that becomes 4 Mbytes. This
hidden charges will affect the accurary of the memory.current value.

This MEMCG_CHARGE_BATCH value also controls how often should the
memcg vmstat values need flushing. As a result, the values reported
in memory.stat cgroup control files are less indicative of the actual
memory consumption of a particular memory cgroup when the page size
increases from 4k.

This problem can be illustrated by running the test_memcontrol
selftest. Running a 4k page size kernel on a 128-core arm64 system,
the test_memcg_current_peak test which allocates a 50M anonymous memory
passed. With a 64k page size kernel on the same system, however, the
same test failed because the "anon" attribute of memory.stat file might
report a size of 0 depending on the number of CPUs the system has.

To solve this inaccurate memory stats problem, we need to scale down
the amount of memory that can be hidden by reducing MEMCG_CHARGE_BATCH
when the page size increases. The same user application will likely
consume more memory on systems with larger page size and it is also
less efficient if we scale down MEMCG_CHARGE_BATCH by too much.  So I
believe a good compromise is to scale down MEMCG_CHARGE_BATCH by 2 for
16k page size and by 4 with 64k page size.

With that change, the test_memcg_current_peak test passed again with
the modified 64k page size kernel.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/memcontrol.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 70b685a85bf4..748cfd75d998 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -328,8 +328,14 @@ struct mem_cgroup {
  * size of first charge trial.
  * TODO: maybe necessary to use big numbers in big irons or dynamic based of the
  * workload.
+ *
+ * There are 3 common base page sizes - 4k, 16k & 64k. In order to limit the
+ * amount of memory that can be hidden in each percpu memcg_stock for a given
+ * memcg, we scale down MEMCG_CHARGE_BATCH by 2 for 16k and 4 for 64k.
  */
-#define MEMCG_CHARGE_BATCH 64U
+#define MEMCG_CHARGE_BATCH_BASE  64U
+#define MEMCG_CHARGE_BATCH_SHIFT ((PAGE_SHIFT <= 16) ? (PAGE_SHIFT - 12)/2 : 2)
+#define MEMCG_CHARGE_BATCH	 (MEMCG_CHARGE_BATCH_BASE >> MEMCG_CHARGE_BATCH_SHIFT)
 
 extern struct mem_cgroup *root_mem_cgroup;
 
-- 
2.53.0