[PATCH v11 0/8] mm/demotion: Memory tiers and demotion

Aneesh Kumar K.V
2022-07-28T19:05:52Z

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
==========================

Linux kernel presents memory devices as NUMA nodes and each memory device is of
a specific type. The memory type of a device is represented by its abstract
distance. A memory tier corresponds to a range of abstract distance. This allows
for classifying memory devices with a specific performance range into a memory
tier.

By default, all memory nodes are assigned to the default tier with
abstract distance 512.

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 v10:
* rename performance level to abstract distance
* Thanks to all the good feedback from Huang, Ying <ying.huang@intel.com>.
Updated the patchset to cover most of the review feedback.

Changes from v9:
* Use performance level for initializing memory tiers.

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: Add hotplug callbacks to handle new numa node onlined
mm/demotion/dax/kmem: Set node's abstract distance to
MEMTIER_ADISTANCE_PMEM
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 | 9 +
include/linux/memory-tiers.h | 79 +++++
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 | 586 +++++++++++++++++++++++++++++++++++
mm/migrate.c | 453 +--------------------------
mm/mprotect.c | 1 +
mm/vmscan.c | 59 +++-
mm/vmstat.c | 4 -
12 files changed, 725 insertions(+), 491 deletions(-)
create mode 100644 include/linux/memory-tiers.h
create mode 100644 mm/memory-tiers.c

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2.37.1