linux-kernel - Re: [PATCH] arm64: allow NR_CPUS=1 for non-SMP and adjust default accordingly

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Message-ID: <90864bf5-2edb-4901-9566-c89431c6d717@arm.com>
Date: Fri, 25 Jul 2025 07:46:08 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Suchit Karunakaran <suchitkarunakaran@...il.com>,
 catalin.marinas@....com, will@...nel.org,
 linux-arm-kernel@...ts.infradead.org
Cc: skhan@...uxfoundation.org, linux-kernel-mentees@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: allow NR_CPUS=1 for non-SMP and adjust default
 accordingly

On 24/07/25 10:26 PM, Suchit Karunakaran wrote:
> The arm64 Kconfig for NR_CPUS previously required a minimum of 2 CPUs.
> This patch changes the minimum allowed CPUs to 1, enabling single-core
> non-SMP configurations.
> 
> Additionally, the default value for NR_CPUS is now conditional:
> - Defaults to 1 if SMP is not selected (non-SMP systems)
> - Defaults to 512 if SMP is enabled

Just curious - what actually prompted this change ?

> 
> Signed-off-by: Suchit Karunakaran <suchitkarunakaran@...il.com>
> ---
>  arch/arm64/Kconfig | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 393d71124f5d..9b2970f7c5ec 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1528,9 +1528,10 @@ config SCHED_SMT
>  	  places. If unsure say N here.
>  
>  config NR_CPUS
> -	int "Maximum number of CPUs (2-4096)"
> -	range 2 4096
> -	default "512"
> +	int "Maximum number of CPUs (1-4096)"
> +	range 1 4096
> +	default "1" if !SMP
> +	default "512" if SMP
>  
>  config HOTPLUG_CPU
>  	bool "Support for hot-pluggable CPUs"