On Thu, Aug 06, 2026 at 05:09:31PM +0900, Rakie Kim wrote: > Package-aware weighted interleave places a task's weighted-interleave > pages on the NUMA nodes of its local package, so that interleave traffic > does not have to cross the interconnect to another package. This keeps > each node's weight aligned with the bandwidth the task actually gets > from it, so effective bandwidth holds up on a system that has more than > one package. (A package is a CPU socket together with the memory > attached to it.) > > Changes from RFC: > https://lore.kernel.org/all/[email protected]/ > - Added an opt-in sysfs toggle (off by default) and a read-only sysfs > view of the package topology > - Added topology validation with a clean fallback to plain weighted > interleave on unsupported topologies > - Hardened the allocation, device-teardown, and node-hotplug paths
Please put change logs under the cover letter :) in mm we put the cover letter in the actual upstream commit so it's better to keep separate for reviewers. I see the RFC was from march and tied to an LSF session I think? While I don't want to be too pedantic, I think you should only really un-RFC in a situation where you have a good sense that the relevant maintainers are happy with the _concept_. Looking at the RFC thread it's not clear that David was OK with this on the mm side, though I see you got some feedback from Jonathan on the CXL driver side. So I wonder whether next respin this should be re-RFC'd unless you get clear feedback that we want to go in this direction? > > Weighted interleave places pages on nodes in proportion to per-node > weights that are set from each node's bandwidth. Within one package the > weight given to a node matches the bandwidth a task sees from it. Across > packages it no longer does: a memory node's physical bandwidth is fixed, > but the bandwidth a task effectively sees depends on which package its > CPU is in, because memory reached from another package, over the > interconnect between them, is slower than the same memory reached within > the package. The weights are set once from device bandwidth and applied > the same way wherever the task runs, so a node in another package is > given a weight higher than the bandwidth it can deliver to that task. > The kernel has no package abstraction and does not record which package > a node belongs to, so it cannot tell which node pairs are separated by > the interconnect. Please please - break up huge paragraphs like this :) It's 2026 so I have to mention that if you've used AI to assist with writing it (which is fine) please do a pass over it manually to curb AI's tendency to be overly verbose + definitely try to break up paragraphs into smaller charts at least :) > > node0 node1 > +-------+ +-------+ > | CPU 0 |---------| CPU 1 | > +-------+ +-------+ > | DRAM0 | | DRAM1 | > +---+---+ +---+---+ > | | > +---+---+ +---+---+ > | CXL 0 | | CXL 1 | > +-------+ +-------+ > node2 node3 ...though I _love_ ASCII diagrams so this is great ;) > > The numbers below are illustrative single-stream bandwidths (GB/s). > Local DRAM sustains 300 and local CXL 150; any path that crosses to > another package, over the interconnect, is capped at 100, so a node in > another package delivers 100 whether it is DRAM or CXL. Note that local > CXL (150) is still faster than any node in another package (100). The > effective bandwidth each CPU sees is therefore: > > node0 node1 node2 node3 > from CPU 0: 300 100 150 100 > from CPU 1: 100 300 100 150 > > Since a single per-node weight cannot encode the interconnect penalty, > a reasonable set of global weights is taken from local device bandwidth > (local DRAM : local CXL = 300 : 150 = 2 : 1): node0=2 node1=2 node2=1 > node3=1. Also great that you provide the receipts on actual observed real-world numbers that's great. NUMA isn't my area so I can't comment here on the technical details but thanks for providing this :) > > Applied the same way to every source, these weights give the map: > > node0 node1 node2 node3 > global: 2 2 1 1 > > A task on CPU 0 gives node1 - remote DRAM, effective 100 - the same > weight 2 as its own local node0 at 300. Worse, node1 is weighted above > node2, the task's local CXL at effective 150, even though node2 is the > faster of the two. The flat weights rank a slower interconnect-bound > node above a faster local one, which is exactly backwards. > > This series makes weighted interleave package-aware. When it is on, > weighted interleave prefers the task's current package: while the > package's nodes have room, the task's pages are spread across them by > weight, so allocations stay off the interconnect. The rest of the > policy nodemask is used when the local package cannot serve the request > - when a node in it is under pressure and the page allocator falls back > along the zonelist, or when the policy nodemask happens to exclude every > node of the current package, in which case the package spanned by the > policy's own nodes is used instead. The nodes considered are always > within the policy nodemask, which mempolicy already narrows to the > task's cpuset, so cpusets and the task nodemask stay in control. > > node0 node1 node2 node3 > from CPU 0: 2 0 1 0 > from CPU 1: 0 2 0 1 > > A task on CPU 0 now places pages on node0 (weight 2) and node2 > (weight 1) at 2:1, which matches their effective bandwidth of 300:150; > a task on CPU 1 places on node1 and node3 the same way. Placement > follows the bandwidth each task actually sees, NUMA locality is > preserved, and interleave traffic stays off the interconnect. > > To make this possible the kernel needs a notion of which nodes share a > package. The NUMA distance model offers only relative latencies and no > structural grouping, which is especially limiting for CXL memory nodes > that come online without an explicit package association. > > The series adds a package-aware topology layer that groups CPU and > memory-only nodes into a "memory package", built from the physical > package ids firmware reports and, for a memory-only node, an initiator > CPU node or SLIT distances. A package can contain more than one CPU node > or more than one memory-only node, so the layer maps a package to a set > of nodes rather than to a single node or a single CXL device. > > The feature is off by default and opt-in through a sysfs toggle. The > package topology itself is exposed read-only under > /sys/devices/system/package/; there is deliberately no writable > override, since a machine whose firmware describes its topology > incorrectly should be fixed in firmware. On a topology that does not > have the symmetric shape the placement relies on, enabling is refused > and any active mode degrades cleanly to the original flat behavior. > > Measured results: > > System Configuration: > - Processor: Dual-Socket Intel Xeon 6980P (Granite Rapids) > > 1) Throughput (System Bandwidth) > - DRAM Only: 966 GB/s > - Weighted Interleave: 903 GB/s (7% decrease compared to DRAM Only) > - Package-Aware Weighted Interleave: 1329 GB/s (1.33 TB/s) > (38% increase compared to DRAM Only, > 47% increase compared to Weighted Interleave) > > 2) Loaded Latency (Under High Bandwidth) > - DRAM Only: 544 ns > - Weighted Interleave: 545 ns > - Package-Aware Weighted Interleave: 436 ns > (20% reduction compared to both) > > A small CXL driver change registers a CXL memory node into its package > as the node comes online, using the initiator the driver resolves for > the region; this is where the package layer gets the CPU-side > association that plain NUMA distance does not carry. > > The memory_package layer offers a broader interface for grouping and > querying package topology - usable by memory tiering as well - and > package-aware weighted interleave uses the subset it needs. > > [PATCH 1/4] mm/numa: introduce nearest_nodes_nodemask() > Add a NUMA helper that returns every node sharing the minimum distance > from a source node. > > [PATCH 2/4] mm/memory-tiers: package-aware topology management > Group NUMA nodes into memory packages from firmware topology data, > expose the grouping read-only under /sys/devices/system/package/, and > validate the symmetric shape that package-aware placement relies on. > > [PATCH 3/4] mm/memory-tiers: register CXL nodes to packages > Bind a CXL memory node to a package using an initiator CPU node. > > [PATCH 4/4] mm/mempolicy: package-aware weighted interleave > Prefer the current package for weighted interleave node selection, > behind an opt-in package_mode sysfs toggle that is off by default. > > Rakie Kim (4): > mm/numa: introduce nearest_nodes_nodemask() > mm/memory-tiers: introduce package-aware topology management for NUMA > nodes > mm/memory-tiers: register CXL nodes to memory packages via initiator > mm/mempolicy: enhance weighted interleave with package-aware locality > > .../ABI/testing/sysfs-devices-system-package | 35 + > ...fs-kernel-mm-mempolicy-weighted-interleave | 17 + > drivers/cxl/core/region.c | 54 + > drivers/cxl/cxl.h | 1 + > drivers/dax/kmem.c | 3 + > include/linux/memory-tiers.h | 113 ++ > include/linux/numa.h | 11 + > mm/memory-tiers.c | 1009 +++++++++++++++++ > mm/mempolicy.c | 200 +++- > 9 files changed, 1439 insertions(+), 4 deletions(-) > create mode 100644 Documentation/ABI/testing/sysfs-devices-system-package > > > base-commit: 8cd9520d35a6c38db6567e97dd93b1f11f185dc6 > -- > 2.25.1 > -- Cheers, Lorenzo
