[PATCH] CONFIG_NR_CPUS

By Robert Love, this patch sets the maximum number
of CPUs a kernel can support (From 2-32 on IA32)
The reasoning behind this is a space saving of
8KB per CPU.

arch/alpha/config.in

@@ -233,6 +233,7 @@ fi
 
 if [ "$CONFIG_SMP" = "y" ]; then
    define_bool CONFIG_HAVE_DEC_LOCK y
+   int  'Maximum number of CPUs (2-64)' CONFIG_NR_CPUS 64
 fi
 
 if [ "$CONFIG_EXPERIMENTAL" = "y" ]; then

arch/i386/Config.help

@@ -34,6 +34,14 @@ CONFIG_HUGETLB_PAGE
 
   Otherwise, say N.
 
+CONFIG_NR_CPUS
+  This allows you to specify the maximum number of CPUs which this
+  kernel will support.  The maximum supported value is 32 and the
+  minimum value which makes sense is 2.
+
+  This is purely to save memory - each supported CPU adds
+  approximately eight kilobytes to the kernel image.
+
 CONFIG_PREEMPT
   This option reduces the latency of the kernel when reacting to
   real-time or interactive events by allowing a low priority process to

arch/i386/config.in

@@ -168,6 +168,7 @@ if [ "$CONFIG_SMP" != "y" ]; then
       define_bool CONFIG_X86_IO_APIC y
    fi
 else
+  int  'Maximum number of CPUs (2-32)' CONFIG_NR_CPUS 32
   bool 'Multi-node NUMA system support' CONFIG_X86_NUMA
   if [ "$CONFIG_X86_NUMA" = "y" ]; then
      #Platform Choices

arch/i386/defconfig

@@ -76,6 +76,7 @@ CONFIG_NOHIGHMEM=y
 # CONFIG_MATH_EMULATION is not set
 CONFIG_MTRR=y
 CONFIG_HAVE_DEC_LOCK=y
+CONFIG_NR_CPUS=32
 
 #
 # Power management options (ACPI, APM)

arch/ia64/config.in

@@ -118,6 +118,10 @@ bool 'Performance monitor support' CONFIG_PERFMON
 tristate '/proc/pal support' CONFIG_IA64_PALINFO
 tristate '/proc/efi/vars support' CONFIG_EFI_VARS
 
+if [ "$CONFIG_SMP" = "y" ]; then
+   int  'Maximum number of CPUs (2-64)' CONFIG_NR_CPUS 64
+fi
+
 tristate 'Kernel support for ELF binaries' CONFIG_BINFMT_ELF
 tristate 'Kernel support for MISC binaries' CONFIG_BINFMT_MISC
 

arch/ia64/defconfig

@@ -61,6 +61,7 @@ CONFIG_IA64_PALINFO=y
 CONFIG_EFI_VARS=y
 CONFIG_BINFMT_ELF=y
 # CONFIG_BINFMT_MISC is not set
+CONFIG_NR_CPUS=64
 
 #
 # ACPI Support

arch/mips/config.in

@@ -491,6 +491,8 @@ fi
 bool 'Magic SysRq key' CONFIG_MAGIC_SYSRQ
 if [ "$CONFIG_SMP" != "y" ]; then
    bool 'Run uncached' CONFIG_MIPS_UNCACHED
+else
+   int  'Maximum number of CPUs (2-32)' CONFIG_NR_CPUS 32
 fi
 endmenu
 

arch/mips64/config.in

@@ -245,6 +245,8 @@ bool 'Remote GDB kernel debugging' CONFIG_REMOTE_DEBUG
 bool 'Magic SysRq key' CONFIG_MAGIC_SYSRQ
 if [ "$CONFIG_SMP" != "y" ]; then
    bool 'Run uncached' CONFIG_MIPS_UNCACHED
+else
+   int  'Maximum number of CPUs (2-64)' CONFIG_NR_CPUS 64
 fi
 endmenu
 

arch/mips64/defconfig

@@ -32,6 +32,7 @@ CONFIG_L1_CACHE_SHIFT=7
 # CONFIG_EISA is not set
 # CONFIG_MCA is not set
 # CONFIG_SBUS is not set
+CONFIG_NR_CPUS=64
 
 #
 # CPU selection

arch/parisc/config.in

@@ -19,6 +19,10 @@ comment 'General options'
 # bool 'Symmetric multi-processing support' CONFIG_SMP
 define_bool CONFIG_SMP n
 
+if [ "$CONFIG_SMP" = "y" ]; then
+   int  'Maximum number of CPUs (2-32)' CONFIG_NR_CPUS 32
+fi
+
 bool 'Kernel Debugger support' CONFIG_KWDB
 # define_bool CONFIG_KWDB n
 

arch/ppc/config.in

@@ -174,6 +174,7 @@ fi
 bool 'Symmetric multi-processing support' CONFIG_SMP
 if [ "$CONFIG_SMP" = "y" ]; then
    bool '  Distribute interrupts on all CPUs by default' CONFIG_IRQ_ALL_CPUS
+   int  'Maximum number of CPUs (2-32)' CONFIG_NR_CPUS 32
 fi
 if [ "$CONFIG_SMP" != "y" ]; then
    bool 'Preemptible Kernel' CONFIG_PREEMPT

arch/ppc64/config.in

@@ -21,6 +21,7 @@ define_bool CONFIG_PPC64 y
 bool 'Symmetric multi-processing support' CONFIG_SMP
 if [ "$CONFIG_SMP" = "y" ]; then
   bool '  Distribute interrupts on all CPUs by default' CONFIG_IRQ_ALL_CPUS
+  int  'Maximum number of CPUs (2-64)' CONFIG_NR_CPUS 64
   if [ "$CONFIG_PPC_PSERIES" = "y" ]; then
     bool '  Hardware multithreading' CONFIG_HMT
     bool '  Discontiguous Memory Support' CONFIG_DISCONTIGMEM

arch/ppc64/defconfig

@@ -40,6 +40,7 @@ CONFIG_IRQ_ALL_CPUS=y
 # CONFIG_DISCONTIGMEM is not set
 # CONFIG_PREEMPT is not set
 # CONFIG_RTAS_FLASH is not set
+CONFIG_NR_CPUS=64
 
 #
 # General setup

arch/s390/config.in

@@ -21,6 +21,9 @@ comment 'Base setup'
 comment 'Processor type and features'
 bool 'Symmetric multi-processing support' CONFIG_SMP
 bool 'IEEE FPU emulation' CONFIG_MATHEMU
+if [ "$CONFIG_SMP" = "y" ]; then
+   int  'Maximum number of CPUs (2-32)' CONFIG_NR_CPUS 32
+fi
 
 comment 'I/O subsystem configuration'
 bool 'Process warning machine checks' CONFIG_MACHCHK_WARNING

arch/s390/defconfig

@@ -39,6 +39,7 @@ CONFIG_KMOD=y
 #
 CONFIG_SMP=y
 CONFIG_MATHEMU=y
+CONFIG_NR_CPUS=64
 
 #
 # I/O subsystem configuration

arch/s390x/config.in

@@ -20,6 +20,9 @@ mainmenu_option next_comment
 comment 'Base setup'
 comment 'Processor type and features'
 bool 'Symmetric multi-processing support' CONFIG_SMP
+if [ "$CONFIG_SMP" = "y" ]; then
+   int  'Maximum number of CPUs (2-64)' CONFIG_NR_CPUS 64
+fi
 bool 'Kernel support for 31 bit emulation' CONFIG_S390_SUPPORT
 if [ "$CONFIG_S390_SUPPORT" = "y" ]; then
   tristate 'Kernel support for 31 bit ELF binaries' CONFIG_BINFMT_ELF32 

arch/s390x/defconfig

@@ -40,6 +40,7 @@ CONFIG_KMOD=y
 CONFIG_SMP=y
 CONFIG_S390_SUPPORT=y
 CONFIG_BINFMT_ELF32=y
+CONFIG_NR_CPUS=64
 
 #
 # I/O subsystem configuration

arch/sparc/config.in

@@ -19,6 +19,10 @@ define_bool CONFIG_HW_CONSOLE y
 
 bool 'Symmetric multi-processing support (does not work on sun4/sun4c)' CONFIG_SMP
 
+if [ "$CONFIG_SMP" = "y" ]; then
+   int  'Maximum number of CPUs (2-32)' CONFIG_NR_CPUS 32
+fi
+
 # Identify this as a Sparc32 build
 define_bool CONFIG_SPARC32 y
 

arch/sparc64/config.in

@@ -20,6 +20,10 @@ bool 'SPARC64 Huge TLB Page Support' CONFIG_HUGETLB_PAGE
 bool 'Symmetric multi-processing support' CONFIG_SMP
 bool 'Preemptible Kernel' CONFIG_PREEMPT
 
+if [ "$CONFIG_SMP" = "y" ]; then
+   int  'Maximum number of CPUs (2-64)' CONFIG_NR_CPUS 64
+fi
+
 # Identify this as a Sparc64 build
 define_bool CONFIG_SPARC64 y
 

arch/sparc64/defconfig

@@ -59,6 +59,7 @@ CONFIG_BINFMT_ELF=y
 CONFIG_BINFMT_MISC=m
 # CONFIG_SUNOS_EMUL is not set
 CONFIG_SOLARIS_EMUL=m
+CONFIG_NR_CPUS=64
 
 #
 # Parallel port support

arch/x86_64/config.in

@@ -54,6 +54,7 @@ if [ "$CONFIG_SMP" = "n" ]; then
 fi
 if [ "$CONFIG_SMP" = "y" ]; then
     define_bool CONFIG_HAVE_DEC_LOCK y
+    int  'Maximum number of CPUs (2-8)' CONFIG_NR_CPUS 8
 fi
 
 define_bool CONFIG_X86_MCE y

arch/x86_64/defconfig

@@ -51,6 +51,7 @@ CONFIG_SMP=y
 CONFIG_HAVE_DEC_LOCK=y
 CONFIG_X86_MCE=y
 # CONFIG_X86_MCE_NONFATAL is not set
+CONFIG_NR_CPUS=8
 
 #
 # Power management options

include/linux/threads.h

@@ -8,10 +8,16 @@
  * /proc/sys/kernel/threads-max.
  */
  
+/*
+ * 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.
+ */
 #ifdef CONFIG_SMP
-#define NR_CPUS	32		/* Max processors that can be running in SMP */
+#define NR_CPUS		CONFIG_NR_CPUS
 #else
-#define NR_CPUS 1
+#define NR_CPUS		1
 #endif
 
 #define MIN_THREADS_LEFT_FOR_ROOT 4