[v9,0/8] mm/demotion: Memory tiers and demotion

Message

Aneesh Kumar K.V July 14, 2022, 4:53 a.m. UTC

The current kernel has the basic memory tiering support: Inactive
pages on a higher tier NUMA node can be migrated (demoted) to a lower
tier NUMA node to make room for new allocations on the higher tier
NUMA node.  Frequently accessed pages on a lower tier NUMA node can be
migrated (promoted) to a higher tier NUMA node to improve the
performance.

In the current kernel, memory tiers are defined implicitly via a
demotion path relationship between NUMA nodes, which is created during
the kernel initialization and updated when a NUMA node is hot-added or
hot-removed.  The current implementation puts all nodes with CPU into
the top tier, and builds the tier hierarchy tier-by-tier by establishing
the per-node demotion targets based on the distances between nodes.

This current memory tier kernel interface needs to be improved for
several important use cases:

* The current tier initialization code always initializes
  each memory-only NUMA node into a lower tier.  But a memory-only
  NUMA node may have a high performance memory device (e.g. a DRAM
  device attached via CXL.mem or a DRAM-backed memory-only node on
  a virtual machine) and should be put into a higher tier.

* The current tier hierarchy always puts CPU nodes into the top
  tier. But on a system with HBM (e.g. GPU memory) devices, these
  memory-only HBM NUMA nodes should be in the top tier, and DRAM nodes
  with CPUs are better to be placed into the next lower tier.

* Also because the current tier hierarchy always puts CPU nodes
  into the top tier, when a CPU is hot-added (or hot-removed) and
  triggers a memory node from CPU-less into a CPU node (or vice
  versa), the memory tier hierarchy gets changed, even though no
  memory node is added or removed.  This can make the tier
  hierarchy unstable and make it difficult to support tier-based
  memory accounting.

* A higher tier node can only be demoted to selected nodes on the
  next lower tier as defined by the demotion path, not any other
  node from any lower tier.  This strict, hard-coded demotion order
  does not work in all use cases (e.g. some use cases may want to
  allow cross-socket demotion to another node in the same demotion
  tier as a fallback when the preferred demotion node is out of
  space), and has resulted in the feature request for an interface to
  override the system-wide, per-node demotion order from the
  userspace.  This demotion order is also inconsistent with the page
  allocation fallback order when all the nodes in a higher tier are
  out of space: The page allocation can fall back to any node from
  any lower tier, whereas the demotion order doesn't allow that.

This patch series make the creation of memory tiers explicit under
the control of device driver.

Memory Tier Initialization
==========================

By default, all memory nodes are assigned to the default tier with
tier ID value 200.

A device driver can move its memory nodes from the default
tier.  For example, PMEM can move its memory nodes below the
default tier, whereas GPU can move its memory nodes above the
default tier.

The kernel initialization code makes the decision on which exact tier
a memory node should be assigned to based on the requests from the
device drivers as well as the memory device hardware information
provided by the firmware.

Hot-adding/removing CPUs doesn't affect memory tier hierarchy.

Changes from v8:
* Drop the sysfs interface patches and related documentation changes.

Changes from v7:
* Fix kernel crash with demotion.
* Improve documentation.

Changes from v6:
* Drop the usage of rank.
* Address other review feedback.

Changes from v5:
* Remove patch supporting N_MEMORY node removal from memory tiers. memory tiers
  are going to be used for features other than demotion. Hence keep all N_MEMORY
  nodes in memory tiers irrespective of whether they want to participate in promotion or demotion.
* Add NODE_DATA->memtier
* Rearrage patches to add sysfs files later.
* Add support to create memory tiers from userspace.
* Address other review feedback.


Changes from v4:
* Address review feedback.
* Reverse the meaning of "rank": higher rank value means higher tier.
* Add "/sys/devices/system/memtier/default_tier".
* Add node_is_toptier

v4:
Add support for explicit memory tiers and ranks.

v3:
- Modify patch 1 subject to make it more specific
- Remove /sys/kernel/mm/numa/demotion_targets interface, use
  /sys/devices/system/node/demotion_targets instead and make
  it writable to override node_states[N_DEMOTION_TARGETS].
- Add support to view per node demotion targets via sysfs

v2:
In v1, only 1st patch of this patch series was sent, which was
implemented to avoid some of the limitations on the demotion
target sharing, however for certain numa topology, the demotion
targets found by that patch was not most optimal, so 1st patch
in this series is modified according to suggestions from Huang
and Baolin. Different examples of demotion list comparasion
between existing implementation and changed implementation can
be found in the commit message of 1st patch.



Aneesh Kumar K.V (7):
  mm/demotion: Add support for explicit memory tiers
  mm/demotion: Move memory demotion related code
  mm/demotion/dax/kmem: Set node's memory tier to MEMORY_TIER_PMEM
  mm/demotion: Add hotplug callbacks to handle new numa node onlined
  mm/demotion: Build demotion targets based on explicit memory tiers
  mm/demotion: Add pg_data_t member to track node memory tier details
  mm/demotion: Update node_is_toptier to work with memory tiers

Jagdish Gediya (1):
  mm/demotion: Demote pages according to allocation fallback order

 drivers/dax/kmem.c           |   6 +-
 include/linux/memory-tiers.h |  59 ++++
 include/linux/migrate.h      |  15 -
 include/linux/mmzone.h       |   3 +
 include/linux/node.h         |   5 -
 mm/Makefile                  |   1 +
 mm/huge_memory.c             |   1 +
 mm/memory-tiers.c            | 653 +++++++++++++++++++++++++++++++++++
 mm/migrate.c                 | 453 +-----------------------
 mm/mprotect.c                |   1 +
 mm/vmscan.c                  |  59 +++-
 mm/vmstat.c                  |   4 -
 12 files changed, 768 insertions(+), 492 deletions(-)
 create mode 100644 include/linux/memory-tiers.h
 create mode 100644 mm/memory-tiers.c