From 0499a78369adacec1af29340b71ff8dd375b4697 Mon Sep 17 00:00:00 2001 From: "Christoph Lameter (Ampere)" Date: Wed, 6 Mar 2024 17:45:04 -0800 Subject: [PATCH] ARM64: Dynamically allocate cpumasks and increase supported CPUs to 512 Currently defconfig selects NR_CPUS=256, but some vendors (e.g. Ampere Computing) are planning to ship systems with 512 CPUs. So that all CPUs on these systems can be used with defconfig, we'd like to bump NR_CPUS to 512. Therefore this patch increases the default NR_CPUS from 256 to 512. As increasing NR_CPUS will increase the size of cpumasks, there's a fear that this might have a significant impact on stack usage due to code which places cpumasks on the stack. To mitigate that concern, we can select CPUMASK_OFFSTACK. As that doesn't seem to be a problem today with NR_CPUS=256, we only select this when NR_CPUS > 256. CPUMASK_OFFSTACK configures the cpumasks in the kernel to be dynamically allocated. This was used in the X86 architecture in the past to enable support for larger CPU configurations up to 8k cpus. With that is becomes possible to dynamically size the allocation of the cpu bitmaps depending on the quantity of processors detected on bootup. Memory used for cpumasks will increase if the kernel is run on a machine with more cores. Further increases may be needed if ARM processor vendors start supporting more processors. Given the current inflationary trends in core counts from multiple processor manufacturers this may occur. There are minor regressions for hackbench. The kernel data size for 512 cpus is smaller with offstack than with onstack. Benchmark results using hackbench average over 10 runs of hackbench -s 512 -l 2000 -g 15 -f 25 -P on Altra 80 Core Support for 256 CPUs on stack. Baseline 7.8564 sec Support for 512 CUs on stack. 7.8713 sec + 0.18% 512 CPUS offstack 7.8916 sec + 0.44% Kernel size comparison: text data filename Difference to onstack256 baseline 25755648 9589248 vmlinuz-6.8.0-rc4-onstack256 25755648 9607680 vmlinuz-6.8.0-rc4-onstack512 +0.19% 25755648 9603584 vmlinuz-6.8.0-rc4-offstack512 +0.14% Tested-by: Eric Mackay Reviewed-by: Russell King (Oracle) Signed-off-by: Christoph Lameter (Ampere) Acked-by: Mark Rutland Link: https://lore.kernel.org/r/37099a57-b655-3b3a-56d0-5f7fbd49d7db@gentwo.org [catalin.marinas@arm.com: use 'select' instead of duplicating 'config CPUMASK_OFFSTACK'] Signed-off-by: Catalin Marinas --- arch/arm64/Kconfig | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index b73d702e1e301..cdb307584d31f 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -120,6 +120,7 @@ config ARM64 select CLONE_BACKWARDS select COMMON_CLK select CPU_PM if (SUSPEND || CPU_IDLE) + select CPUMASK_OFFSTACK if NR_CPUS > 256 select CRC32 select DCACHE_WORD_ACCESS select DYNAMIC_FTRACE if FUNCTION_TRACER @@ -1424,7 +1425,7 @@ config SCHED_SMT config NR_CPUS int "Maximum number of CPUs (2-4096)" range 2 4096 - default "256" + default "512" config HOTPLUG_CPU bool "Support for hot-pluggable CPUs"