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Subject[PATCH] CONFIG_NR_CPUS, redux
From
Date
On Tue, 2002-06-11 at 02:27, Andrew Morton wrote:

> In which case, CONFIG_NR_CPUS is the only way to get the memory
> back...

Alright, so here we go. Andrew introduced this idea last week and here
is a revised, perhaps final, version. We have two goals:

(a) allow NR_CPUS to be set at configure time, to save
on memory and gain an unmeasurable boost in speed

(b) let each architecture define a default NR_CPUS, i.e.
64-bit architectures can have an NR_CPUS of 64

We do this fairly easily. In include/linux/threads.h:

#ifdef CONFIG_SMP
#define NR_CPUS CONFIG_NR_CPUS
#else
#define NR_CPUS 1
#endif

and in each arch/xxx/config.in add a configure entry, conditional on
CONFIG_SMP, for CONFIG_NR_CPUS with a proper default. If CONFIG_SMP is
set in defconfig, set CONFIG_NR_CPUS properly there, too.

Here are the defaults I picked:

CONFIG_NR_CPUS=32: i386, mips, parisc, ppc, sparc

CONFIG_NR_CPUS=64: alpha, ia64, mips64, ppc64, s390, s390x, sparc64, x86-64

No CONFIG_NR_CPUS: arm, cris, sh

Andrew has pointed out some architectures may need minor tweaks to work
with NR_CPUS < 32. He discovered and fixed a minor issue on i386...
other architectures, please verify non-standard options work. Also make
sure 64 works!

Patch is against 2.5.21, enjoy...

Robert Love

diff -urN linux-2.5.21/arch/alpha/config.in linux/arch/alpha/config.in
--- linux-2.5.21/arch/alpha/config.in Sat Jun 8 22:28:07 2002
+++ linux/arch/alpha/config.in Sun Jun 9 13:22:46 2002
@@ -232,6 +232,7 @@

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
diff -urN linux-2.5.21/arch/i386/Config.help linux/arch/i386/Config.help
--- linux-2.5.21/arch/i386/Config.help Sat Jun 8 22:27:21 2002
+++ linux/arch/i386/Config.help Sun Jun 9 13:13:02 2002
@@ -25,6 +25,14 @@

If you don't know what to do here, 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
+ mimimum 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
diff -urN linux-2.5.21/arch/i386/config.in linux/arch/i386/config.in
--- linux-2.5.21/arch/i386/config.in Sat Jun 8 22:28:47 2002
+++ linux/arch/i386/config.in Sun Jun 9 13:23:20 2002
@@ -185,8 +185,8 @@

bool 'Math emulation' CONFIG_MATH_EMULATION
bool 'MTRR (Memory Type Range Register) support' CONFIG_MTRR
-bool 'Symmetric multi-processing support' CONFIG_SMP
bool 'Preemptible Kernel' CONFIG_PREEMPT
+bool 'Symmetric multi-processing support' CONFIG_SMP
if [ "$CONFIG_SMP" != "y" ]; then
bool 'Local APIC support on uniprocessors' CONFIG_X86_UP_APIC
dep_bool 'IO-APIC support on uniprocessors' CONFIG_X86_UP_IOAPIC $CONFIG_X86_UP_APIC
@@ -197,6 +197,7 @@
define_bool CONFIG_X86_IO_APIC y
fi
else
+ int 'Maximum number of CPUs (2-32)' CONFIG_NR_CPUS 32
bool 'Multiquad NUMA system' CONFIG_MULTIQUAD
fi

diff -urN linux-2.5.21/arch/i386/defconfig linux/arch/i386/defconfig
--- linux-2.5.21/arch/i386/defconfig Sat Jun 8 22:27:26 2002
+++ linux/arch/i386/defconfig Sun Jun 9 13:21:32 2002
@@ -74,6 +74,7 @@
# CONFIG_PREEMPT is not set
# CONFIG_MULTIQUAD is not set
CONFIG_HAVE_DEC_LOCK=y
+CONFIG_NR_CPUS=32

#
# General options
diff -urN linux-2.5.21/arch/i386/kernel/smpboot.c linux/arch/i386/kernel/smpboot.c
--- linux-2.5.21/arch/i386/kernel/smpboot.c Sat Jun 8 22:29:52 2002
+++ linux/arch/i386/kernel/smpboot.c Sun Jun 9 13:13:02 2002
@@ -54,7 +54,7 @@
static int smp_b_stepping;

/* Setup configured maximum number of CPUs to activate */
-static int max_cpus = -1;
+static int max_cpus = NR_CPUS;

/* Total count of live CPUs */
int smp_num_cpus = 1;
@@ -1145,7 +1145,7 @@

if (!(phys_cpu_present_map & (1 << bit)))
continue;
- if ((max_cpus >= 0) && (max_cpus <= cpucount+1))
+ if (max_cpus <= cpucount+1)
continue;

do_boot_cpu(apicid);
diff -urN linux-2.5.21/arch/ia64/config.in linux/arch/ia64/config.in
--- linux-2.5.21/arch/ia64/config.in Sat Jun 8 22:30:07 2002
+++ linux/arch/ia64/config.in Sun Jun 9 13:24:00 2002
@@ -95,6 +95,10 @@
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

diff -urN linux-2.5.21/arch/ia64/defconfig linux/arch/ia64/defconfig
--- linux-2.5.21/arch/ia64/defconfig Sat Jun 8 22:27:16 2002
+++ linux/arch/ia64/defconfig Sun Jun 9 13:21:00 2002
@@ -61,6 +61,7 @@
CONFIG_EFI_VARS=y
CONFIG_BINFMT_ELF=y
# CONFIG_BINFMT_MISC is not set
+CONFIG_NR_CPUS=64

#
# ACPI Support
diff -urN linux-2.5.21/arch/mips/config.in linux/arch/mips/config.in
--- linux-2.5.21/arch/mips/config.in Sat Jun 8 22:30:41 2002
+++ linux/arch/mips/config.in Sun Jun 9 13:24:57 2002
@@ -500,6 +500,8 @@
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

diff -urN linux-2.5.21/arch/mips64/config.in linux/arch/mips64/config.in
--- linux-2.5.21/arch/mips64/config.in Sat Jun 8 22:27:29 2002
+++ linux/arch/mips64/config.in Sun Jun 9 13:25:12 2002
@@ -247,6 +247,8 @@
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

diff -urN linux-2.5.21/arch/mips64/defconfig linux/arch/mips64/defconfig
--- linux-2.5.21/arch/mips64/defconfig Sat Jun 8 22:27:52 2002
+++ linux/arch/mips64/defconfig Sun Jun 9 13:20:17 2002
@@ -32,6 +32,7 @@
# CONFIG_EISA is not set
# CONFIG_MCA is not set
# CONFIG_SBUS is not set
+CONFIG_NR_CPUS=64

#
# CPU selection
diff -urN linux-2.5.21/arch/parisc/config.in linux/arch/parisc/config.in
--- linux-2.5.21/arch/parisc/config.in Sat Jun 8 22:31:31 2002
+++ linux/arch/parisc/config.in Sun Jun 9 13:25:41 2002
@@ -18,6 +18,10 @@
# 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

diff -urN linux-2.5.21/arch/ppc/config.in linux/arch/ppc/config.in
--- linux-2.5.21/arch/ppc/config.in Sat Jun 8 22:26:28 2002
+++ linux/arch/ppc/config.in Sun Jun 9 13:26:29 2002
@@ -172,6 +172,7 @@
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
diff -urN linux-2.5.21/arch/ppc64/config.in linux/arch/ppc64/config.in
--- linux-2.5.21/arch/ppc64/config.in Sat Jun 8 22:27:32 2002
+++ linux/arch/ppc64/config.in Sun Jun 9 13:26:43 2002
@@ -21,6 +21,7 @@
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
fi
diff -urN linux-2.5.21/arch/ppc64/defconfig linux/arch/ppc64/defconfig
--- linux-2.5.21/arch/ppc64/defconfig Sat Jun 8 22:28:19 2002
+++ linux/arch/ppc64/defconfig Sun Jun 9 13:20:43 2002
@@ -38,6 +38,7 @@
CONFIG_IRQ_ALL_CPUS=y
# CONFIG_HMT is not set
# CONFIG_PREEMPT is not set
+CONFIG_NR_CPUS=64

#
# General setup
diff -urN linux-2.5.21/arch/s390/config.in linux/arch/s390/config.in
--- linux-2.5.21/arch/s390/config.in Sat Jun 8 22:28:13 2002
+++ linux/arch/s390/config.in Sun Jun 9 13:27:07 2002
@@ -20,6 +20,9 @@
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
endmenu

mainmenu_option next_comment
diff -urN linux-2.5.21/arch/s390/defconfig linux/arch/s390/defconfig
--- linux-2.5.21/arch/s390/defconfig Sat Jun 8 22:31:19 2002
+++ linux/arch/s390/defconfig Sun Jun 9 13:40:37 2002
@@ -35,6 +35,7 @@
#
CONFIG_SMP=y
CONFIG_MATHEMU=y
+CONFIG_NR_CPUS=64

#
# Base setup
diff -urN linux-2.5.21/arch/s390x/config.in linux/arch/s390x/config.in
--- linux-2.5.21/arch/s390x/config.in Sat Jun 8 22:27:21 2002
+++ linux/arch/s390x/config.in Sun Jun 9 13:40:14 2002
@@ -19,6 +19,9 @@
mainmenu_option next_comment
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
diff -urN linux-2.5.21/arch/s390x/defconfig linux/arch/s390x/defconfig
--- linux-2.5.21/arch/s390x/defconfig Sat Jun 8 22:26:26 2002
+++ linux/arch/s390x/defconfig Sun Jun 9 13:40:46 2002
@@ -36,6 +36,7 @@
CONFIG_SMP=y
CONFIG_S390_SUPPORT=y
CONFIG_BINFMT_ELF32=y
+CONFIG_NR_CPUS=64

#
# Base setup
diff -urN linux-2.5.21/arch/sparc/config.in linux/arch/sparc/config.in
--- linux-2.5.21/arch/sparc/config.in Sat Jun 8 22:26:59 2002
+++ linux/arch/sparc/config.in Sun Jun 9 13:27:41 2002
@@ -17,6 +17,10 @@

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

diff -urN linux-2.5.21/arch/sparc64/config.in linux/arch/sparc64/config.in
--- linux-2.5.21/arch/sparc64/config.in Sat Jun 8 22:28:44 2002
+++ linux/arch/sparc64/config.in Sun Jun 9 13:28:05 2002
@@ -17,6 +17,10 @@
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

diff -urN linux-2.5.21/arch/sparc64/defconfig linux/arch/sparc64/defconfig
--- linux-2.5.21/arch/sparc64/defconfig Sat Jun 8 22:28:49 2002
+++ linux/arch/sparc64/defconfig Sun Jun 9 13:21:45 2002
@@ -63,6 +63,7 @@
CONFIG_BINFMT_MISC=m
# CONFIG_SUNOS_EMUL is not set
CONFIG_SOLARIS_EMUL=m
+CONFIG_NR_CPUS=64

#
# Parallel port support
diff -urN linux-2.5.21/arch/x86_64/config.in linux/arch/x86_64/config.in
--- linux-2.5.21/arch/x86_64/config.in Sat Jun 8 22:28:13 2002
+++ linux/arch/x86_64/config.in Sun Jun 9 13:28:46 2002
@@ -45,8 +45,11 @@
#bool 'MTRR (Memory Type Range Register) support' CONFIG_MTRR
bool 'Symmetric multi-processing support' CONFIG_SMP
bool 'Preemptible Kernel' CONFIG_PREEMPT
-if [ "$CONFIG_SMP" = "y" -a "$CONFIG_X86_CMPXCHG" = "y" ]; then
- define_bool CONFIG_HAVE_DEC_LOCK y
+if [ "$CONFIG_SMP" = "y" ]; then
+ int 'Maximum number of CPUs (2-64)' CONFIG_NR_CPUS 64
+ if [ "$CONFIG_X86_CMPXCHG" = "y" ]; then
+ define_bool CONFIG_HAVE_DEC_LOCK y
+ fi
fi

define_bool CONFIG_X86_MCE y
diff -urN linux-2.5.21/arch/x86_64/defconfig linux/arch/x86_64/defconfig
--- linux-2.5.21/arch/x86_64/defconfig Sat Jun 8 22:30:52 2002
+++ linux/arch/x86_64/defconfig Sun Jun 9 13:20:30 2002
@@ -51,6 +51,7 @@
CONFIG_HAVE_DEC_LOCK=y
CONFIG_X86_MCE=y
# CONFIG_X86_MCE_NONFATAL is not set
+CONFIG_NR_CPUS=64

#
# General options
diff -urN linux-2.5.21/include/linux/threads.h linux/include/linux/threads.h
--- linux-2.5.21/include/linux/threads.h Sat Jun 8 22:26:49 2002
+++ linux/include/linux/threads.h Sun Jun 9 13:16:29 2002
@@ -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
-
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