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. * There are no interfaces for the userspace to learn about the memory tier hierarchy in order to optimize its memory allocations. This patch series make the creation of memory tiers explicit under the control of userspace or device driver. Memory Tier Initialization ========================== By default, all memory nodes are assigned to the default tier (1). The default tier device has a rank value (200). A device driver can move up or down its memory nodes from the default tier. For example, PMEM can move down its memory nodes below the default tier, whereas GPU can move up 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. Memory Allocation for Demotion ============================== This patch series keep the demotion target page allocation logic same. The demotion page allocation pick the closest NUMA node in the next lower tier to the current NUMA node allocating pages from. This will be later improved to use the same page allocation strategy using fallback list. Sysfs Interface: ------------- Listing current list of memory tiers and rank details: :/sys/devices/system/memtier$ ls default_rank max_tier memtier1 power uevent :/sys/devices/system/memtier$ cat default_rank 200 :/sys/devices/system/memtier$ cat max_tier 3 :/sys/devices/system/memtier$ Per node memory tier details: For a cpu only NUMA node: :/sys/devices/system/node# cat node0/memtier :/sys/devices/system/node# echo 1 > node0/memtier :/sys/devices/system/node# cat node0/memtier :/sys/devices/system/node# For a NUMA node with memory: :/sys/devices/system/node# cat node1/memtier 1 :/sys/devices/system/node# ls ../memtier/ default_rank max_tier memtier1 power uevent :/sys/devices/system/node# echo 2 > node1/memtier :/sys/devices/system/node# :/sys/devices/system/node# ls ../memtier/ default_rank max_tier memtier1 memtier2 power uevent :/sys/devices/system/node# cat node1/memtier 2 :/sys/devices/system/node# :/sys/devices/system/node# cat ../memtier/memtier2/rank 300 :/sys/devices/system/node# :/sys/devices/system/node# cat ../memtier/memtier1/rank 200 :/sys/devices/system/node# Removing a NUMA node from demotion: :/sys/devices/system/node# cat node1/memtier 2 :/sys/devices/system/node# echo none > node1/memtier :/sys/devices/system/node# :/sys/devices/system/node# cat node1/memtier :/sys/devices/system/node# :/sys/devices/system/node# ls ../memtier/ default_rank max_tier memtier1 power uevent :/sys/devices/system/node# The above also resulted in removal of memtier2 which was created in the earlier step. Changelog ---------- 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 (2): mm/demotion: Add support to associate rank with memory tier mm/demotion: Add support for removing node from demotion memory tiers Jagdish Gediya (5): mm/demotion: Add support for explicit memory tiers mm/demotion: Expose per node memory tier to sysfs mm/demotion: Build demotion targets based on explicit memory tiers mm/demotion/dax/kmem: Set node's memory tier to MEMORY_TIER_PMEM mm/demotion: Demote pages according to allocation fallback order drivers/base/node.c | 43 +++ drivers/dax/kmem.c | 4 + include/linux/migrate.h | 39 ++- mm/Kconfig | 11 + mm/migrate.c | 756 ++++++++++++++++++++++++++-------------- mm/vmscan.c | 38 +- mm/vmstat.c | 5 - 7 files changed, 590 insertions(+), 306 deletions(-) -- 2.36.1
