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 <eric.mackay@xxxxxxxxxx> Reviewed-by: Russell King (Oracle) <rmk+kernel@xxxxxxxxxxxxxxx> Signed-off-by: Christoph Lameter (Ampere) <cl@xxxxxxxxx> --- Original post: https://www.spinics.net/lists/linux-mm/msg369701.html V2: https://lkml.org/lkml/2024/2/7/505 V1->V2 - Keep quotation marks - Remove whiltespace damage - Add tested by V2->V3: - Add test results - Rework descriptions arch/arm64/Kconfig | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index aa7c1d435139..4e767dede47d 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -1427,7 +1427,21 @@ config SCHED_SMT config NR_CPUS int "Maximum number of CPUs (2-4096)" range 2 4096 - default "256" + default "512" + +# +# Determines the placement of cpumasks. +# +# With CPUMASK_OFFSTACK the cpumasks are dynamically allocated. +# Useful for machines with lots of core because it avoids increasing +# the size of many of the data structures in the kernel. +# +# If this is off then the cpumasks have a static sizes and are +# embedded within data structures. +# + config CPUMASK_OFFSTACK + def_bool y + depends on NR_CPUS > 256 config HOTPLUG_CPU bool "Support for hot-pluggable CPUs" -- 2.39.2
