Re: RFC: Memory Tiering Kernel Interfaces

[Alistair Popple]

Wei Xu <weixugc@xxxxxxxxxx> writes:

> 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@xxxxxxxxxx/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