[PATCH] RISC-V: Dynamically allocate cpumasks and further increase range and default value of NR_CPUS

[Yuntao Liu]

Currently default NR_CPUS is 64 for riscv64, since the latest QEMU virt
machine supports up to 512 CPUS, so set default NR_CPUS 512 for riscv64.

Under the promotion of RISC-V International and related chip
manufacturers, RISC-V has also begun to enter the server market, which
demands higher performance. Other major architectures (such as ARM64,
x86_64, MIPS, etc) already have a higher range, so further increase
this range up to 4096 for riscv64.

Due to the fact that increasing NR_CPUS enlarges the size of cpumasks,
there is a concern that this could significantly impact stack usage,
especially for code that allocates cpumasks on the stack. To address
this, we have the option to enable CPUMASK_OFFSTACK, which prevents
cpumasks from being allocated on the stack. we choose to enable this
feature only when NR_CPUS is greater than 512, why 512, since then
the kernel size with offstack is smaller.

vmlinux size comparison(difference to vmlinux_onstack_NR_CPUS baseline):

NR_CPUS     256         512         1024        2048        4096
onstack     19814536    19840760    19880584    19969672    20141704
offstack    19819144    19840936    19880480    19968544    20135456
difference  +0.023%     +0.001%     -0.001%     -0.001      -0.031%
is_smaller  n           n           y           y           y

Signed-off-by: Yuntao Liu <liuyuntao12@xxxxxxxxxx>
---
 arch/riscv/Kconfig | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 0525ee2d63c7..5960713b3bf9 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -77,6 +77,7 @@ config RISCV
 	select CLINT_TIMER if RISCV_M_MODE
 	select CLONE_BACKWARDS
 	select COMMON_CLK
+	select CPUMASK_OFFSTACK if NR_CPUS > 512
 	select CPU_PM if CPU_IDLE || HIBERNATION || SUSPEND
 	select EDAC_SUPPORT
 	select FRAME_POINTER if PERF_EVENTS || (FUNCTION_TRACER && !DYNAMIC_FTRACE)
@@ -428,11 +429,11 @@ config SCHED_MC
 config NR_CPUS
 	int "Maximum number of CPUs (2-512)"
 	depends on SMP
-	range 2 512 if !RISCV_SBI_V01
+	range 2 4096 if !RISCV_SBI_V01
 	range 2 32 if RISCV_SBI_V01 && 32BIT
 	range 2 64 if RISCV_SBI_V01 && 64BIT
 	default "32" if 32BIT
-	default "64" if 64BIT
+	default "512" if 64BIT
 
 config HOTPLUG_CPU
 	bool "Support for hot-pluggable CPUs"
-- 
2.34.1