Please ignore this one, apologies for the noise. On Friday, 6 May 2022 9:57:54 AM AEST Alistair Popple wrote: > 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 >
