RFC Manage the top tier memory in a tiered memory

Traditionally, all memory is DRAM. Some DRAM might be closer/faster than others NUMA wise, but a byte of media has about the same cost whether it is close or far. But, with new memory tiers such as Persistent Memory (PMEM). there is a choice between fast/expensive DRAM and slow/cheap PMEM.

The fast/expensive memory lives in the top tier of the memory hierachy.

Previously, the patchset
[PATCH 00/10] [v7] Migrate Pages in lieu of discard
https://lore.kernel.org/linux-mm/[email protected]/
provides a mechanism to demote cold pages from DRAM node into PMEM.

And the patchset
[PATCH 0/6] [RFC v6] NUMA balancing: optimize memory placement for memory tiering system
https://lore.kernel.org/linux-mm/[email protected]/
provides a mechanism to promote hot pages in PMEM to the DRAM node leveraging autonuma.

The two patchsets together keep the hot pages in DRAM and colder pages in PMEM.

To make fine grain cgroup based management of the precious top tier DRAM memory possible, this patchset adds a few new features:
1. Provides memory monitors on the amount of top tier memory used per cgroup and by the system as a whole.
2. Applies soft limits on the top tier memory each cgroup uses
3. Enables kswapd to demote top tier pages from cgroup with excess top tier memory usages.

This allows us to provision different amount of top tier memory to each cgroup according to the cgroup's latency need.

The patchset is based on cgroup v1 interface. One shortcoming of the v1 interface is the limit on the cgroup is a soft limit, so a cgroup can exceed the limit quite a bit before reclaim before page demotion reins it in.

We are also working on a cgroup v2 interface control interface that will will have a max limit on the top tier memory per cgroup but requires much additional logic to fall back and allocate from non top tier memory when a cgroup reaches the maximum limit. This simpler cgroup v1 implementation with all its warts is used to illustrate the concept of cgroup based top tier memory management and serves as a starting point of discussions.

The soft limit and soft reclaim logic in this patchset will be similar for what we would do for a cgroup v2 interface when we reach the high watermark for top tier usage in a cgroup v2 interface.

This patchset is applied on top of
[PATCH 00/10] [v7] Migrate Pages in lieu of discard
and
[PATCH 0/6] [RFC v6] NUMA balancing: optimize memory placement for memory tiering system

It is part of a larger patchset. You can play with the complete set of patches using the tree:
https://git.kernel.org/pub/scm/linux/kernel/git/vishal/tiering.git/log/?h=tiering-0.71

Tim Chen (11):
mm: Define top tier memory node mask
mm: Add soft memory limit for mem cgroup
mm: Account the top tier memory usage per cgroup
mm: Report top tier memory usage in sysfs
mm: Add soft_limit_top_tier tree for mem cgroup
mm: Handle top tier memory in cgroup soft limit memory tree utilities
mm: Account the total top tier memory in use
mm: Add toptier option for mem_cgroup_soft_limit_reclaim()
mm: Use kswapd to demote pages when toptier memory is tight
mm: Set toptier_scale_factor via sysctl
mm: Wakeup kswapd if toptier memory need soft reclaim

 Documentation/admin-guide/sysctl/vm.rst |  12 +
 drivers/base/node.c                     |   2 +
 include/linux/memcontrol.h              |  20 +-
 include/linux/mm.h                      |   4 +
 include/linux/mmzone.h                  |   7 +
 include/linux/nodemask.h                |   1 +
 include/linux/vmstat.h                  |  18 ++
 kernel/sysctl.c                         |  10 +
 mm/memcontrol.c                         | 303 +++++++++++++++++++-----
 mm/memory_hotplug.c                     |   3 +
 mm/migrate.c                            |   1 +
 mm/page_alloc.c                         |  36 ++-
 mm/vmscan.c                             |  73 +++++-
 mm/vmstat.c                             |  22 +-
 14 files changed, 444 insertions(+), 68 deletions(-)

2.20.1