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

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

[Suchit Karunakaran]

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

Signed-off-by: Suchit Karunakaran <suchitkarunakaran@gmail.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"
-- 
2.50.1

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

[Anshuman Khandual]

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@gmail.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"

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

[Suchit K]

On Fri, 25 Jul 2025 at 07:46, Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
> 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 ?
>

Hi, sorry I think I should've explained it in the commit message. So,
in linux/include/linux/threads.h, there's this fixme comment:
/*
 * Maximum supported processors.  Setting this smaller saves quite a
 * bit of memory.  Use nr_cpu_ids instead of this except for static bitmaps.
 */
#ifndef CONFIG_NR_CPUS
/* FIXME: This should be fixed in the arch's Kconfig */
#define CONFIG_NR_CPUS 1
#endif

And the original commit is as follows:

commit 278d1ed65e25d80af7c3a112d707b3f70516ddb4
Author: Rusty Russell <rusty@rustcorp.com.au>
Date:   Tue Dec 30 09:05:12 2008 +1030

    cpumask: make CONFIG_NR_CPUS always valid.

    Impact: cleanup

    Currently we have NR_CPUS, which is 1 on UP, and CONFIG_NR_CPUS on
    SMP.  If we make CONFIG_NR_CPUS always valid (and always 1 on !SMP),
    we can skip the middleman.

    This also allows us to find and check all the unaudited NR_CPUS usage
    as we prepare for v. large NR_CPUS.

    To avoid breaking every arch, we cheat and do this for the moment
    in the header if the arch doesn't.

    Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
    Signed-off-by: Mike Travis <travis@sgi.com>

diff --git a/include/linux/threads.h b/include/linux/threads.h
index 38d1a5d6568e..052b12bec8bd 100644
--- a/include/linux/threads.h
+++ b/include/linux/threads.h
@@ -8,17 +8,17 @@
  */

 /*
- * Maximum supported processors that can run under SMP.  This value is
- * set via configure setting.  The maximum is equal to the size of the
- * bitmasks used on that platform, i.e. 32 or 64.  Setting this smaller
- * saves quite a bit of memory.
+ * Maximum supported processors.  Setting this smaller saves quite a
+ * bit of memory.  Use nr_cpu_ids instead of this except for static bitmaps.
  */
-#ifdef CONFIG_SMP
-#define NR_CPUS CONFIG_NR_CPUS
-#else
-#define NR_CPUS 1
+#ifndef CONFIG_NR_CPUS
+/* FIXME: This should be fixed in the arch's Kconfig */
+#define CONFIG_NR_CPUS 1
 #endif

+/* Places which use this should consider cpumask_var_t. */
+#define NR_CPUS CONFIG_NR_CPUS
+
 #define MIN_THREADS_LEFT_FOR_ROOT 4

 /*

This motivated me to do this. Please let me know if I misunderstood
something or I'm doing something incorrectly.

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

[Catalin Marinas]

On Thu, Jul 24, 2025 at 10:26:39PM +0530, 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.

Do you have such single-core system?

>  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

It's been some time since we forced CONFIG_SMP always on for arm64.

-- 
Catalin

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

[Suchit K]

On Fri, 25 Jul 2025 at 14:06, Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Thu, Jul 24, 2025 at 10:26:39PM +0530, 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.
>
> Do you have such single-core system?

No, I don't have a single core system. I've just used single core VMs
using QEMU, etc, but I don't know if it's equivalent.

>
> >  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
>
> It's been some time since we forced CONFIG_SMP always on for arm64.
>

I'm sorry, I wasn't aware of it. Thanks for the clarification.