RFC: Memory Tiering Kernel Interfaces

Wei Xu
2022-04-30T02:11:01Z

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.

A tiering relationship between NUMA nodes in the form of demotion path
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 then builds the tiering hierarchy
tier-by-tier by establishing the per-node demotion targets based on
the distances between nodes.

The current memory tiering interface needs to be improved to address
several important use cases:

* The current tiering 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 the top tier.

* The current tiering 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 tiering 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 tiering hierarchy gets changed, even though no
memory node is added or removed. This can make the tiering
hierarchy much less stable.

* A higher tier node can only be demoted to selected nodes on the
next lower tier, not any other node from the next 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.

* There are no interfaces for the userspace to learn about the memory
tiering hierarchy in order to optimize its memory allocations.

I'd like to propose revised memory tiering kernel interfaces based on
the discussions in the threads:

- https://lore.kernel.org/lkml/20220425201728.5kzm4seu7rep7ndr@offworld/T/
- https://lore.kernel.org/linux-mm/20220426114300.00003ad8@Huawei.com/t/

Sysfs Interfaces
================

* /sys/devices/system/node/memory_tiers

Format: node list (one tier per line, in the tier order)

When read, list memory nodes by tiers.

When written (one tier per line), take the user-provided node-tier
assignment as the new tiering hierarchy and rebuild the per-node
demotion order. It is allowed to only override the top tiers, in
which cases, the kernel will establish the lower tiers automatically.

Kernel Representation
=====================

* nodemask_t node_states[N_TOPTIER_MEMORY]

Store all top-tier memory nodes.

* nodemask_t memory_tiers[MAX_TIERS]

Store memory nodes by tiers.

* struct demotion_nodes node_demotion[]

where: struct demotion_nodes { nodemask_t preferred; nodemask_t allowed; }

For a node N:

node_demotion[N].preferred lists all preferred demotion targets;

node_demotion[N].allowed lists all allowed demotion targets
(initialized to be all the nodes in the same demotion tier).

Tiering Hierarchy Initialization
================================

By default, all memory nodes are in the top tier (N_TOPTIER_MEMORY).

A device driver can remove its memory nodes from the top tier, e.g.
a dax driver can remove PMEM nodes from the top tier.

The kernel builds the memory tiering hierarchy and per-node demotion
order tier-by-tier starting from N_TOPTIER_MEMORY. For a node N, the
best distance nodes in the next lower tier are assigned to
node_demotion[N].preferred and all the nodes in the next lower tier
are assigned to node_demotion[N].allowed.

node_demotion[N].preferred can be empty if no preferred demotion node
is available for node N.

If the userspace overrides the tiers via the memory_tiers sysfs
interface, the kernel then only rebuilds the per-node demotion order
accordingly.

Memory tiering hierarchy is rebuilt upon hot-add or hot-remove of a
memory node, but is NOT rebuilt upon hot-add or hot-remove of a CPU
node.

Memory Allocation for Demotion
==============================

When allocating a new demotion target page, both a preferred node
and the allowed nodemask are provided to the allocation function.
The default kernel allocation fallback order is used to allocate the
page from the specified node and nodemask.

The memopolicy of cpuset, vma and owner task of the source page can
be set to refine the demotion nodemask, e.g. to prevent demotion or
select a particular allowed node as the demotion target.

Examples
========

* Example 1:
Node 0 & 1 are DRAM nodes, node 2 & 3 are PMEM nodes.

Node 0 has node 2 as the preferred demotion target and can also
fallback demotion to node 3.

Node 1 has node 3 as the preferred demotion target and can also
fallback demotion to node 2.

Set mempolicy to prevent cross-socket demotion and memory access,
e.g. cpuset.mems=0,2

node distances:
node 0 1 2 3
0 10 20 30 40
1 20 10 40 30
2 30 40 10 40
3 40 30 40 10

/sys/devices/system/node/memory_tiers
0-1
2-3

N_TOPTIER_MEMORY: 0-1

node_demotion[]:
0: [2], [2-3]
1: [3], [2-3]
2: [], []
3: [], []

* Example 2:
Node 0 & 1 are DRAM nodes.
Node 2 is a PMEM node and closer to node 0.

Node 0 has node 2 as the preferred and only demotion target.

Node 1 has no preferred demotion target, but can still demote
to node 2.

Set mempolicy to prevent cross-socket demotion and memory access,
e.g. cpuset.mems=0,2

node distances:
node 0 1 2
0 10 20 30
1 20 10 40
2 30 40 10

/sys/devices/system/node/memory_tiers
0-1
2

N_TOPTIER_MEMORY: 0-1

node_demotion[]:
0: [2], [2]
1: [], [2]
2: [], []

* Example 3:
Node 0 & 1 are DRAM nodes.
Node 2 is a PMEM node and has the same distance to node 0 & 1.

Node 0 has node 2 as the preferred and only demotion target.

Node 1 has node 2 as the preferred and only demotion target.

node distances:
node 0 1 2
0 10 20 30
1 20 10 30
2 30 30 10

/sys/devices/system/node/memory_tiers
0-1
2

N_TOPTIER_MEMORY: 0-1

node_demotion[]:
0: [2], [2]
1: [2], [2]
2: [], []

* Example 4:
Node 0 & 1 are DRAM nodes, Node 2 is a memory-only DRAM node.

All nodes are top-tier.

node distances:
node 0 1 2
0 10 20 30
1 20 10 30
2 30 30 10

/sys/devices/system/node/memory_tiers
0-2

N_TOPTIER_MEMORY: 0-2

node_demotion[]:
0: [], []
1: [], []
2: [], []

* Example 5:
Node 0 is a DRAM node with CPU.
Node 1 is a HBM node.
Node 2 is a PMEM node.

With userspace override, node 1 is the top tier and has node 0 as
the preferred and only demotion target.

Node 0 is in the second tier, tier 1, and has node 2 as the
preferred and only demotion target.

Node 2 is in the lowest tier, tier 2, and has no demotion targets.

node distances:
node 0 1 2
0 10 21 30
1 21 10 40
2 30 40 10

/sys/devices/system/node/memory_tiers (userspace override)
1
0
2

N_TOPTIER_MEMORY: 1

node_demotion[]:
0: [2], [2]
1: [0], [0]
2: [], []

-- Wei