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

[liuyuntao (F)]

Gentle ping

On 2024/6/14 15:53, Yuntao Liu wrote:
> 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 at huawei.com>
> ---
>   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"