From 1e1cd49ded597a7cc89f774ab3f42e22ff24fd57 Mon Sep 17 00:00:00 2001 From: Kai Huang Date: Mon, 9 Mar 2026 11:23:13 +1300 Subject: [PATCH] ACPI: NUMA: Only parse CFMWS at boot when CXL_ACPI is on On CXL platforms, the Static Resource Affinity Table (SRAT) may not cover memory affinity information for all the CXL memory regions. Since each CXL memory region is enumerated via a CXL Fixed Memory Window Structure (CFMWS), during early boot the kernel parses the CFMWS tables to find all CXL memory regions and sets a NUMA node for each of them. This memory affinity information of CXL memory regions is later used by the CXL ACPI driver. The CFMWS table doesn't provide the memory affinity information either. Currently the kernel assigns a 'faked' NUMA node for each CXL memory region, starting from the next node of the highest node that is enumerated via the SRAT. This can potentially increase the maximum NUMA node ID of the platform ('nr_node_ids') a lot. E.g., on a GNR platform with 4 NUMA nodes and 18 CFMWS tables, this bumps the 'nr_node_ids' to 22. Increasing the 'nr_node_ids' has side effects. For instance, it is widely used by the kernel for "highest possible NUMA node" based memory allocations. It also impacts userspace ABIs, e.g., some NUMA memory related system calls such as 'get_mempolicy' which requires 'maxnode' not being smaller than the 'nr_node_ids'. Currently parsing CFMWS tables and assigning faked NUMA node at boot is done unconditionally. However, if the CXL ACPI driver is not enabled, there will be no user of such memory affinity information of CXL memory regions. Change to only parsing the CFMWS tables at boot when CXL_ACPI is enabled in Kconfig to avoid the unnecessary cost of bumping up 'nr_node_ids'. E.g., on the aforementioned GNR platform, the "Slab" in /proc/meminfo is reduced with this change (when CXL_ACPI is off): w/ this change w/o Slab 900488 kB 923660 kB Signed-off-by: Kai Huang Reviewed-by: Jonathan Cameron Reviewed-by: Gregory Price Reviewed-by: Alison Schofield Link: https://patch.msgid.link/20260308222313.14014-1-kai.huang@intel.com Signed-off-by: Dave Jiang --- drivers/acpi/numa/srat.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/drivers/acpi/numa/srat.c b/drivers/acpi/numa/srat.c index aa87ee1583a4e..62d4a8df0b8c1 100644 --- a/drivers/acpi/numa/srat.c +++ b/drivers/acpi/numa/srat.c @@ -654,8 +654,11 @@ int __init acpi_numa_init(void) } last_real_pxm = fake_pxm; fake_pxm++; - acpi_table_parse_cedt(ACPI_CEDT_TYPE_CFMWS, acpi_parse_cfmws, - &fake_pxm); + + /* No need to expand numa nodes if CXL is disabled */ + if (IS_ENABLED(CONFIG_CXL_ACPI)) + acpi_table_parse_cedt(ACPI_CEDT_TYPE_CFMWS, acpi_parse_cfmws, + &fake_pxm); if (cnt < 0) return cnt; -- 2.47.3