[PATCH] riscv: Raise default NR_CPUS for 64BIT to 256

[PATCH] riscv: Raise default NR_CPUS for 64BIT to 256

[Vivian Wang]

SpacemiT has already produced a 80-core RVA23 RISC-V server [1], and
going further back, the dual-socket SG2042-based Sophgo Pisces has 128
cores (although that had some issues achieving mainline support).
Therefore, an NR_CPUS of 64 is not enough.

Raise default NR_CPUS to 256 for 64BIT (when !RISCV_SBI_V01, since very
old firmware can't support more than 64 cores). The number was picked as
a power of two that is at least double the known max. I believe this
should be the right balance between not wasting too much memory and not
having to touch this too often.

Ubuntu has already been shipping NR_CPUS=512 for riscv64. We have also
been testing NR_CPUS=256 internally at ISCAS and found negligible
performance impact and no ill effects.

Reported-by: Lufei Zheng <lufei.zheng@spacemit.com>
Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1140651 # [1]
Suggested-by: Han Gao <gaohan@iscas.ac.cn>
Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>
---
 arch/riscv/Kconfig | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 3f0a647218e4..c0a6992933e4 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -454,7 +454,8 @@ config NR_CPUS
 	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 "64" if RISCV_SBI_V01 && 64BIT
+	default "256" if !RISCV_SBI_V01 && 64BIT
 
 config HOTPLUG_CPU
 	bool "Support for hot-pluggable CPUs"

---
base-commit: ab9de95c9cf952332ab79453b4b5d1bfca8e514f
change-id: 20260625-riscv-more-nr-cpus-8158dbb4640c

Best regards,
--  
Vivian Wang <wangruikang@iscas.ac.cn>

Re: [PATCH] riscv: Raise default NR_CPUS for 64BIT to 256

[Paul Walmsley]

On Thu, 25 Jun 2026, Vivian Wang wrote:

> SpacemiT has already produced a 80-core RVA23 RISC-V server [1], and
> going further back, the dual-socket SG2042-based Sophgo Pisces has 128
> cores (although that had some issues achieving mainline support).
> Therefore, an NR_CPUS of 64 is not enough.
> 
> Raise default NR_CPUS to 256 for 64BIT (when !RISCV_SBI_V01, since very
> old firmware can't support more than 64 cores). The number was picked as
> a power of two that is at least double the known max. I believe this
> should be the right balance between not wasting too much memory and not
> having to touch this too often.
> 
> Ubuntu has already been shipping NR_CPUS=512 for riscv64. We have also
> been testing NR_CPUS=256 internally at ISCAS and found negligible
> performance impact and no ill effects.
> 
> Reported-by: Lufei Zheng <lufei.zheng@spacemit.com>
> Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1140651 # [1]
> Suggested-by: Han Gao <gaohan@iscas.ac.cn>
> Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>

Thanks, queued for v7.2-rc.


- Paul