Hello Wei and Ying, Please find my comments below based on a discussion with Jonathan. On Fri, 27 May 2022 10:58:39 +0800 Ying Huang <ying.huang@xxxxxxxxx> wrote: > On Thu, 2022-05-26 at 14:22 -0700, Wei Xu wrote: > > Changes since v2 > > ================ > > * Updated the design and examples to use "rank" instead of device ID > > to determine the order between memory tiers for better > > flexibility. > > > > Overview > > ======== > > > > 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. > > > > I'd like to propose revised memory tier 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/ > > - > > https://lore.kernel.org/linux-mm/867bc216386eb6cbf54648f23e5825830f5b922e.camel@xxxxxxxxx/T/ > > - > > https://lore.kernel.org/linux-mm/d6314cfe1c7898a6680bed1e7cc93b0ab93e3155.camel@xxxxxxxxx/T/ > > > > > > High-level Design Ideas > > ======================= > > > > * Define memory tiers explicitly, not implicitly. > > > > * Memory tiers are defined based on hardware capabilities of memory > > nodes, not their relative node distances between each other. > > > > * The tier assignment of each node is independent from each other. > > Moving a node from one tier to another tier doesn't affect the > > tier assignment of any other node. > > > > * The node-tier association is stable. A node can be reassigned to a > > different tier only under the specific conditions that don't block > > future tier-based memory cgroup accounting. > > > > * A node can demote its pages to any nodes of any lower tiers. The > > demotion target node selection follows the allocation fallback > > order of the source node, which is built based on node distances. > > The demotion targets are also restricted to only the nodes from the > > tiers lower than the source node. We no longer need to maintain a > > separate per-node demotion order (node_demotion[]). > > > > > > Sysfs Interfaces > > ================ > > > > * /sys/devices/system/memtier/ > > > > This is the directory containing the information about memory > > tiers. > > > > Each memory tier has its own subdirectory. > > > > The order of memory tiers is determined by their rank values, not > > by their memtier device names. > > > > - /sys/devices/system/memtier/possible > > > > Format: ordered list of "memtier(rank)" > > Example: 0(64), 1(128), 2(192) > > > > Read-only. When read, list all available memory tiers and their > > associated ranks, ordered by the rank values (from the highest > > tier to the lowest tier). > > I like the idea of "possible" file. And I think we can show default > tier too. That is, if "1(128)" is the default tier (tier with DRAM), > then the list can be, > > " > 0/64 [1/128] 2/192 > " > > To make it more easier to be parsed by shell, I will prefer something > like, > > " > 0 64 > 1 128 default > 2 192 > " > > But one line format is OK for me too. > I wonder if there's a good argument to have this "possible" file at all? My thinking is that, 1) all the details can be scripted at user-level by reading memtierN/nodeN, offloading some work from the kernel side, and 2) the format/numbers are confusing anyway; it could get tricky when/if tier device IDs are similar to ranks. The other thing is whether we should have a file called "default" containing the default tier value for the user to read? > > > > * /sys/devices/system/memtier/memtierN/ > > > > This is the directory containing the information about a > > particular memory tier, memtierN, where N is the memtier device ID > > (e.g. 0, 1). > > > > The memtier device ID number itself is just an identifier and has > > no special meaning, i.e. memtier device ID numbers do not determine > > the order of memory tiers. > > > > - /sys/devices/system/memtier/memtierN/rank > > > > Format: int > > Example: 100 > > > > Read-only. When read, list the "rank" value associated with > > memtierN. > > > > "Rank" is an opaque value. Its absolute value doesn't have any > > special meaning. But the rank values of different memtiers can > > be compared with each other to determine the memory tier order. > > For example, if we have 3 memtiers: memtier0, memtier1, > > memiter2, and their rank values are 10, 20, 15, then the memory > > tier order is: memtier0 -> memtier2 -> memtier1, where memtier0 is > > the highest tier and memtier1 is the lowest tier. > > > > The rank value of each memtier should be unique. > > > > - /sys/devices/system/memtier/memtierN/nodelist > > > > Format: node_list > > Example: 1-2 > > > > Read-only. When read, list the memory nodes in the specified > > tier. > > > > If a memory tier has no memory nodes, the kernel can hide the > > sysfs directory of this memory tier, though the tier itself can > > still be visible from /sys/devices/system/memtier/possible. > > Is there a good reason why the kernel needs to hide this directory? > > * /sys/devices/system/node/nodeN/memtier > > > > where N = 0, 1, ... > > > > Format: int or empty > > Example: 1 > > > > When read, list the device ID of the memory tier that the node > > belongs to. Its value is empty for a CPU-only NUMA node. > > > > When written, the kernel moves the node into the specified memory > > tier if the move is allowed. The tier assignment of all other > > nodes are not affected. > > Who decides if the move is allowed or not? Might need to explicitly mention that? > > Initially, we can make this interface read-only. > > > > > > Kernel Representation > > ===================== > > > > * All memory tiering code is guarded by CONFIG_TIERED_MEMORY. > > > > * #define MAX_MEMORY_TIERS 3 > > > > Support 3 memory tiers for now. This can be a kconfig option. > > > > * #define MEMORY_DEFAULT_TIER_DEVICE 1 > > > > The default tier device that a memory node is assigned to. > > > > * struct memtier_dev { > > nodemask_t nodelist; > > int rank; > > int tier; > > } memtier_devices[MAX_MEMORY_TIERS] > > > > Store memory tiers by device IDs. > > > > * struct memtier_dev *memory_tier(int tier) > > > > Returns the memtier device for a given memory tier. > > Might need to define the case where there's no memory tier device for a specific tier number. For example, we can return NULL or an error code when an invalid tier number is passed (e.g., -1 for CPU-only nodes). > > * int node_tier_dev_map[MAX_NUMNODES] > > > > Map a node to its tier device ID.. > > > > For each CPU-only node c, node_tier_dev_map[c] = -1. > > > > > > Memory Tier Initialization > > ========================== > > > > By default, all memory nodes are assigned to the default tier > > (MEMORY_DEFAULT_TIER_DEVICE). The default tier device has a rank > > value in the middle of the possible rank value range (e.g. 127 if > > the range is [0..255]). > > > > 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. > > Is "up/down" here still relative after the rank addition? > > 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. > > > > > > Memory Tier Reassignment > > ======================== > > > > After a memory node is hot-removed, it can be hot-added back to a > > different memory tier. This is useful for supporting dynamically > > provisioned CXL.mem NUMA nodes, which may connect to different > > memory devices across hot-plug events. Such tier changes should > > be compatible with tier-based memory accounting. > > > > The userspace may also reassign an existing online memory node to a > > different tier. However, this should only be allowed when no pages > > are allocated from the memory node or when there are no non-root > > memory cgroups (e.g. during the system boot). This restriction is > > important for keeping memory tier hierarchy stable enough for > > tier-based memory cgroup accounting. > > One way to do this is hot-remove all memory of a node, change its > memtier, then hot-add its memory. > > Best Regards, > Huang, Ying > > > Hot-adding/removing CPUs doesn't affect memory tier hierarchy. > > > > > > Memory Allocation for Demotion > > ============================== > > > > To allocate a new page as the demotion target for a page, the kernel > > calls the allocation function (__alloc_pages_nodemask) with the > > source page node as the preferred node and the union of all lower > > tier nodes as the allowed nodemask. The actual target node > > selection then follows the allocation fallback order that the > > kernel has already defined. > > > > The pseudo code looks like: > > > > targets = NODE_MASK_NONE; > > src_nid = page_to_nid(page); > > src_tier = memtier_devices[node_tier_dev_map[src_nid]].tier; > > for (i = src_tier + 1; i < MAX_MEMORY_TIERS; i++) > > nodes_or(targets, targets, memory_tier(i)->nodelist); > > new_page = __alloc_pages_nodemask(gfp, order, src_nid, targets); > > > > The memopolicy of cpuset, vma and owner task of the source page can > > be set to refine the demotion target nodemask, e.g. to prevent > > demotion or select a particular allowed node as the demotion target. > > > > > > Memory Allocation for Promotion > > =============================== > > > > The page allocation for promotion is similar to demotion, except > > that (1) the target nodemask uses the promotion tiers, (2) the > > preferred node can be the accessing CPU node, not the source page > > node. > > > > > > Examples > > ======== > > > > * Example 1: > > > > Node 0 & 1 are DRAM nodes, node 2 & 3 are PMEM nodes. > > > > 20 > > Node 0 (DRAM) ---- Node 1 (DRAM) > > | \ / | > > | 30 40 X 40 | 30 > > | / \ | > > Node 2 (PMEM) ---- Node 3 (PMEM) > > 40 > > > > 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 > > > > $ cat /sys/devices/system/memtier/possible > > 0(64), 1(128), 2(192) > > > > $ grep '' /sys/devices/system/memtier/memtier*/rank > > /sys/devices/system/memtier/memtier1/rank:128 > > /sys/devices/system/memtier/memtier2/rank:192 > > > > $ grep '' /sys/devices/system/memtier/memtier*/nodelist > > /sys/devices/system/memtier/memtier1/nodelist:0-1 > > /sys/devices/system/memtier/memtier2/nodelist:2-3 > > > > $ grep '' /sys/devices/system/node/node*/memtier > > /sys/devices/system/node/node0/memtier:1 > > /sys/devices/system/node/node1/memtier:1 > > /sys/devices/system/node/node2/memtier:2 > > /sys/devices/system/node/node3/memtier:2 > > > > Demotion fallback order: > > node 0: 2, 3 > > node 1: 3, 2 > > node 2: empty > > node 3: empty > > > > To prevent cross-socket demotion and memory access, the user can set > > mempolicy, e.g. cpuset.mems=0,2. > > > > > > * Example 2: > > > > Node 0 & 1 are DRAM nodes. > > Node 2 is a PMEM node and closer to node 0. > > > > 20 > > Node 0 (DRAM) ---- Node 1 (DRAM) > > | / > > | 30 / 40 > > | / > > Node 2 (PMEM) > > > > node distances: > > node 0 1 2 > > 0 10 20 30 > > 1 20 10 40 > > 2 30 40 10 > > > > $ cat /sys/devices/system/memtier/possible > > 0(64), 1(128), 2(192) > > > > $ grep '' /sys/devices/system/memtier/memtier*/rank > > /sys/devices/system/memtier/memtier1/rank:128 > > /sys/devices/system/memtier/memtier2/rank:192 > > > > $ grep '' /sys/devices/system/memtier/memtier*/nodelist > > /sys/devices/system/memtier/memtier1/nodelist:0-1 > > /sys/devices/system/memtier/memtier2/nodelist:2 > > > > $ grep '' /sys/devices/system/node/node*/memtier > > /sys/devices/system/node/node0/memtier:1 > > /sys/devices/system/node/node1/memtier:1 > > /sys/devices/system/node/node2/memtier:2 > > > > Demotion fallback order: > > node 0: 2 > > node 1: 2 > > node 2: empty > > > > > > * Example 3: > > > > Node 0 & 1 are DRAM nodes, Node 2 is a memory-only DRAM node. > > np: PMEM instead of memory-only DRAM? > > All nodes are in the same tier. > > > > 20 > > Node 0 (DRAM) ---- Node 1 (DRAM) > > \ / > > \ 30 / 30 > > \ / > > Node 2 (PMEM) > > > > node distances: > > node 0 1 2 > > 0 10 20 30 > > 1 20 10 30 > > 2 30 30 10 > > > > $ cat /sys/devices/system/memtier/possible > > 0(64), 1(128), 2(192) > > > > $ grep '' /sys/devices/system/memtier/memtier*/rank > > /sys/devices/system/memtier/memtier1/rank:128 > > > > $ grep '' /sys/devices/system/memtier/memtier*/nodelist > > /sys/devices/system/memtier/memtier1/nodelist:0-2 > > > > $ grep '' /sys/devices/system/node/node*/memtier > > /sys/devices/system/node/node0/memtier:1 > > /sys/devices/system/node/node1/memtier:1 > > /sys/devices/system/node/node2/memtier:1 > > > > Demotion fallback order: > > node 0: empty > > node 1: empty > > node 2: empty > > > > > > * Example 4: > > > > Node 0 is a DRAM node with CPU. > > Node 1 is a PMEM node. > > Node 2 is a GPU node. > > > > 50 > > Node 0 (DRAM) ---- Node 2 (GPU) > > \ / > > \ 30 / 60 > > \ / > > Node 1 (PMEM) > > > > node distances: > > node 0 1 2 > > 0 10 30 50 > > 1 30 10 60 > > 2 50 60 10 > > > > $ cat /sys/devices/system/memtier/possible > > 0(64), 1(128), 2(192) > > > > $ grep '' /sys/devices/system/memtier/memtier*/rank > > /sys/devices/system/memtier/memtier0/rank:64 > > /sys/devices/system/memtier/memtier1/rank:128 > > /sys/devices/system/memtier/memtier2/rank:192 > > > > $ grep '' /sys/devices/system/memtier/memtier*/nodelist > > /sys/devices/system/memtier/memtier0/nodelist:2 > > /sys/devices/system/memtier/memtier1/nodelist:0 > > /sys/devices/system/memtier/memtier2/nodelist:1 > > > > $ grep '' /sys/devices/system/node/node*/memtier > > /sys/devices/system/node/node0/memtier:1 > > /sys/devices/system/node/node1/memtier:2 > > /sys/devices/system/node/node2/memtier:0 > > > > Demotion fallback order: > > node 0: 1 > > node 1: empty > > node 2: 0, 1 > > > > > > * Example 5: > > > > Node 0 is a DRAM node with CPU. > > Node 1 is a GPU node. > > Node 2 is a PMEM node. > > Node 3 is a large, slow DRAM node without CPU. > > > > 100 > > Node 0 (DRAM) ---- Node 1 (GPU) > > / | / | > > /40 |30 120 / | 110 > > | | / | > > | Node 2 (PMEM) ---- / > > | \ / > > \ 80 \ / > > ------- Node 3 (Slow DRAM) > > > > node distances: > > node 0 1 2 3 > > 0 10 100 30 40 > > 1 100 10 120 110 > > 2 30 120 10 80 > > 3 40 110 80 10 > > > > MAX_MEMORY_TIERS=4 (memtier3 is a memory tier added later). > > > > $ cat /sys/devices/system/memtier/possible > > 0(64), 1(128), 3(160), 2(192) > > > > $ grep '' /sys/devices/system/memtier/memtier*/rank > > /sys/devices/system/memtier/memtier0/rank:64 > > /sys/devices/system/memtier/memtier1/rank:128 > > /sys/devices/system/memtier/memtier2/rank:192 > > /sys/devices/system/memtier/memtier3/rank:160 > > > > $ grep '' /sys/devices/system/memtier/memtier*/nodelist > > /sys/devices/system/memtier/memtier0/nodelist:1 > > /sys/devices/system/memtier/memtier1/nodelist:0 > > /sys/devices/system/memtier/memtier2/nodelist:2 > > /sys/devices/system/memtier/memtier3/nodelist:3 > > > > $ grep '' /sys/devices/system/node/node*/memtier > > /sys/devices/system/node/node0/memtier:1 > > /sys/devices/system/node/node1/memtier:0 > > /sys/devices/system/node/node2/memtier:2 > > /sys/devices/system/node/node3/memtier:3 > > > > Demotion fallback order: > > node 0: 2, 3 > > node 1: 0, 3, 2 > > node 2: empty > > node 3: 2 > >
