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 <[email protected]>
Reviewed-by: Russell King (Oracle) <[email protected]>
Signed-off-by: Christoph Lameter (Ampere) <[email protected]>
Acked-by: Mark Rutland <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[[email protected]: use 'select' instead of duplicating 'config CPUMASK_OFFSTACK']
Signed-off-by: Catalin Marinas <[email protected]>

Author: Christoph Lameter (Ampere)

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"