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 MEMCG_CHARGE_BATCH value also controls how often should the memcg vmstat values need flushing. As a result, the values reported in various memory cgroup control files are even 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 <[email protected]> --- 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
