High kmalloc-32 slab cache consumption with 10k containers

[Bharata B Rao]

Hi,

When running 10000 (more-or-less-empty-)containers on a bare-metal Power9
server(160 CPUs, 2 NUMA nodes, 256G memory), it is seen that memory
consumption increases quite a lot (around 172G) when the containers are
running. Most of it comes from slab (149G) and within slab, the majority of
it comes from kmalloc-32 cache (102G)

The major allocator of kmalloc-32 slab cache happens to be the list_head
allocations of list_lru_one list. These lists are created whenever a
FS mount happens. Specially two such lists are registered by alloc_super(),
one for dentry and another for inode shrinker list. And these lists
are created for all possible NUMA nodes and for all given memcgs
(memcg_nr_cache_ids to be particular)

If,

A = Nr allocation request per mount: 2 (one for dentry and inode list)
B = Nr NUMA possible nodes
C = memcg_nr_cache_ids
D = size of each kmalloc-32 object: 32 bytes,

then for every mount, the amount of memory consumed by kmalloc-32 slab
cache for list_lru creation is A*B*C*D bytes.

Following factors contribute to the excessive allocations:

- Lists are created for possible NUMA nodes.
- memcg_nr_cache_ids grows in bulk (see memcg_alloc_cache_id() and additional
  list_lrus are created when it grows. Thus we end up creating list_lru_one
  list_heads even for those memcgs which are yet to be created.
  For example, when 10000 memcgs are created, memcg_nr_cache_ids reach
  a value of 12286.
- When a memcg goes offline, the list elements are drained to the parent
  memcg, but the list_head entry remains.
- The lists are destroyed only when the FS is unmounted. So list_heads
  for non-existing memcgs remain and continue to contribute to the
  kmalloc-32 allocation. This is presumably done for performance
  reason as they get reused when new memcgs are created, but they end up
  consuming slab memory until then.
- In case of containers, a few file systems get mounted and are specific
  to the container namespace and hence to a particular memcg, but we
  end up creating lists for all the memcgs.
  As an example, if 7 FS mounts are done for every container and when
  10k containers are created, we end up creating 2*7*12286 list_lru_one
  lists for each NUMA node. It appears that no elements will get added
  to other than 2*7=14 of them in the case of containers.

One straight forward way to prevent this excessive list_lru_one
allocations is to limit the list_lru_one creation only to the
relevant memcg. However I don't see an easy way to figure out
that relevant memcg from FS mount path (alloc_super())

As an alternative approach, I have this below hack that does lazy
list_lru creation. The memcg-specific list is created and initialized
only when there is a request to add an element to that particular
list. Though I am not sure about the full impact of this change
on the owners of the lists and also the performance impact of this,
the overall savings look good.

Used memory
		Before		During		After
W/o patch	23G		172G		40G
W/  patch	23G		69G		29G

Slab consumption
		Before		During		After
W/o patch	1.5G		149G		22G
W/  patch	1.5G		45G		10G

Number of kmalloc-32 allocations
		Before		During		After
W/o patch	178176		3442409472	388933632
W/  patch	190464		468992		468992

Any thoughts on other approaches to address this scenario and
any specific comments about the approach that I have taken is
appreciated. Meanwhile the patch looks like below: