[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 at 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 at 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 at rustcorp.com.au>
    Signed-off-by: Mike Travis <travis at 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.