Subject: [PATCH RFC] Simplify the Linux kernel by reducing its state space

From: [email-masked] (Paul E. McKenney)
Date: Sun, 1 Apr 2012 00:33:21 +0800

Although there have been numerous complaints about the complexity of
parallel programming (especially over the past 5-10 years), the plain
truth is that the incremental complexity of parallel programming over
that of sequential programming is not as large as is commonly believed.
Despite that you might have heard, the mind-numbing complexity of modern
computer systems is not due so much to there being multiple CPUs, but
rather to there being any CPUs at all.  In short, for the ultimate in
computer-system simplicity, the optimal choice is NR_CPUS=0.

This commit therefore limits kernel builds to zero CPUs.  This change
has the beneficial side effect of rendering all kernel bugs harmless.
Furthermore, this commit enables additional beneficial changes, for
example, the removal of those parts of the kernel that are not needed
when there are zero CPUs.

Signed-off-by: Paul E. McKenney <[email-masked]>
Reviewed-by: Thomas Gleixner <[email-masked]>
---

 alpha/Kconfig                     |   11 ++++++-----
 arm/Kconfig                       |    6 +++---
 blackfin/Kconfig                  |    3 ++-
 hexagon/Kconfig                   |    9 +++++----
 ia64/Kconfig                      |    9 +++++----
 m32r/Kconfig                      |   10 ++++++----
 mips/Kconfig                      |   21 +++++++++++----------
 mn10300/Kconfig                   |    3 ++-
 parisc/Kconfig                    |    6 +++---
 powerpc/platforms/Kconfig.cputype |    8 ++++----
 s390/Kconfig                      |   12 +++++++-----
 sh/Kconfig                        |   11 ++++++-----
 sparc/Kconfig                     |    8 ++++----
 tile/Kconfig                      |    9 +++++----
 x86/Kconfig                       |   16 +++++++++-------
 15 files changed, 78 insertions(+), 64 deletions(-)

diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index 56a4df9..1766b4a 100644
--- a/arch/alpha/Kconfig
+++ b/arch/alpha/Kconfig
@@ -541,14 +541,15 @@ config HAVE_DEC_LOCK
 	default y
 
 config NR_CPUS
-	int "Maximum number of CPUs (2-32)"
-	range 2 32
+	int "Maximum number of CPUs (0-0)"
+	range 0 0
 	depends on SMP
-	default "32" if ALPHA_GENERIC || ALPHA_MARVEL
-	default "4" if !ALPHA_GENERIC && !ALPHA_MARVEL
+	default "0" if ALPHA_GENERIC || ALPHA_MARVEL
+	default "0" if !ALPHA_GENERIC && !ALPHA_MARVEL
 	help
 	  MARVEL support can handle a maximum of 32 CPUs, all the others
-          with working support have a maximum of 4 CPUs.
+          with working support have a maximum of 4 CPUs.  But why take
+	  chances?  Just stick with zero CPUs.
 
 config ARCH_DISCONTIGMEM_ENABLE
 	bool "Discontiguous Memory Support (EXPERIMENTAL)"
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index a48aecc..1f07a3a 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1551,10 +1551,10 @@ config PAGE_OFFSET
 	default 0xC0000000
 
 config NR_CPUS
-	int "Maximum number of CPUs (2-32)"
-	range 2 32
+	int "Maximum number of CPUs (0-0)"
+	range 0 0
 	depends on SMP
-	default "4"
+	default "0"
 
 config HOTPLUG_CPU
 	bool "Support for hot-pluggable CPUs (EXPERIMENTAL)"
diff --git a/arch/blackfin/Kconfig b/arch/blackfin/Kconfig
index abe5a9e..6a78549 100644
--- a/arch/blackfin/Kconfig
+++ b/arch/blackfin/Kconfig
@@ -241,7 +241,8 @@ config SMP
 config NR_CPUS
 	int
 	depends on SMP
-	default 2 if BF561
+	range 0 0
+	default 0 if BF561
 
 config HOTPLUG_CPU
 	bool "Support for hot-pluggable CPUs"
diff --git a/arch/hexagon/Kconfig b/arch/hexagon/Kconfig
index 9059e39..daab009 100644
--- a/arch/hexagon/Kconfig
+++ b/arch/hexagon/Kconfig
@@ -158,13 +158,14 @@ config SMP
 
 config NR_CPUS
 	int "Maximum number of CPUs" if SMP
-	range 2 6 if SMP
-	default "1" if !SMP
-	default "6" if SMP
+	range 0 0 if SMP
+	default "0" if !SMP
+	default "0" if SMP
 	---help---
 	  This allows you to specify the maximum number of CPUs which this
 	  kernel will support.  The maximum supported value is 6 and the
-	  minimum value which makes sense is 2.
+	  minimum value which makes sense is 2.  But a limit of zero is
+	  so much safer!
 
 	  This is purely to save memory - each supported CPU adds
 	  approximately eight kilobytes to the kernel image.
diff --git a/arch/ia64/Kconfig b/arch/ia64/Kconfig
index bd72669..fea0e6d 100644
--- a/arch/ia64/Kconfig
+++ b/arch/ia64/Kconfig
@@ -373,16 +373,17 @@ config SMP
 	  If you don't know what to do here, say N.
 
 config NR_CPUS
-	int "Maximum number of CPUs (2-4096)"
-	range 2 4096
+	int "Maximum number of CPUs (0-0)"
+	range 0 0
 	depends on SMP
-	default "4096"
+	default "0"
 	help
 	  You should set this to the number of CPUs in your system, but
 	  keep in mind that a kernel compiled for, e.g., 2 CPUs will boot but
 	  only use 2 CPUs on a >2 CPU system.  Setting this to a value larger
 	  than 64 will cause the use of a CPU mask array, causing a small
-	  performance hit.
+	  performance hit.  And setting it larger than zero risks all
+	  manner of software bugs, so we just play it safe.
 
 config HOTPLUG_CPU
 	bool "Support for hot-pluggable CPUs (EXPERIMENTAL)"
diff --git a/arch/m32r/Kconfig b/arch/m32r/Kconfig
index ef80a65..68b9e88 100644
--- a/arch/m32r/Kconfig
+++ b/arch/m32r/Kconfig
@@ -300,14 +300,16 @@ config CHIP_M32700_TS1
 	default n
 
 config NR_CPUS
-	int "Maximum number of CPUs (2-32)"
-	range 2 32
+	int "Maximum number of CPUs (0-0)"
+	range 0 0
 	depends on SMP
-	default "2"
+	default "0"
 	help
 	  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.
+	  minimum value which makes sense is 2.  Zero may not make sense,
+	  but given that there is much in this world that does not make
+	  sense, zero it is!
 
 	  This is purely to save memory - each supported CPU adds
 	  approximately eight kilobytes to the kernel image.
diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 5ab6e89..3d7d06c 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -2192,16 +2192,16 @@ config NR_CPUS_DEFAULT_64
 	bool
 
 config NR_CPUS
-	int "Maximum number of CPUs (2-64)"
-	range 1 64 if NR_CPUS_DEFAULT_1
+	int "Maximum number of CPUs (0-0)"
+	range 0 0 if NR_CPUS_DEFAULT_1
 	depends on SMP
-	default "1" if NR_CPUS_DEFAULT_1
-	default "2" if NR_CPUS_DEFAULT_2
-	default "4" if NR_CPUS_DEFAULT_4
-	default "8" if NR_CPUS_DEFAULT_8
-	default "16" if NR_CPUS_DEFAULT_16
-	default "32" if NR_CPUS_DEFAULT_32
-	default "64" if NR_CPUS_DEFAULT_64
+	default "0" if NR_CPUS_DEFAULT_1
+	default "0" if NR_CPUS_DEFAULT_2
+	default "0" if NR_CPUS_DEFAULT_4
+	default "0" if NR_CPUS_DEFAULT_8
+	default "0" if NR_CPUS_DEFAULT_16
+	default "0" if NR_CPUS_DEFAULT_32
+	default "0" if NR_CPUS_DEFAULT_64
 	help
 	  This allows you to specify the maximum number of CPUs which this
 	  kernel will support.  The maximum supported value is 32 for 32-bit
@@ -2212,7 +2212,8 @@ config NR_CPUS
 	  This is purely to save memory - each supported CPU adds
 	  approximately eight kilobytes to the kernel image.  For best
 	  performance should round up your number of processors to the next
-	  power of two.
+	  power of two.  And just think how much more memory we will
+	  save by setting the limit to zero!
 
 source "kernel/time/Kconfig"
 
diff --git a/arch/mn10300/Kconfig b/arch/mn10300/Kconfig
index 8f1c40d..85fc112 100644
--- a/arch/mn10300/Kconfig
+++ b/arch/mn10300/Kconfig
@@ -201,7 +201,8 @@ config SMP
 config NR_CPUS
 	int
 	depends on SMP
-	default "2"
+	range 0 0
+	default "0"
 
 source "kernel/Kconfig.preempt"
 
diff --git a/arch/parisc/Kconfig b/arch/parisc/Kconfig
index 242a1b7..358eaf8 100644
--- a/arch/parisc/Kconfig
+++ b/arch/parisc/Kconfig
@@ -254,10 +254,10 @@ config HPUX
 	depends on !64BIT
 
 config NR_CPUS
-	int "Maximum number of CPUs (2-32)"
-	range 2 32
+	int "Maximum number of CPUs (0-0)"
+	range 0 0
 	depends on SMP
-	default "32"
+	default "0"
 
 endmenu
 
diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
index 425db18..5e607e0 100644
--- a/arch/powerpc/platforms/Kconfig.cputype
+++ b/arch/powerpc/platforms/Kconfig.cputype
@@ -356,11 +356,11 @@ config SMP
 	  If you don't know what to do here, say N.
 
 config NR_CPUS
-	int "Maximum number of CPUs (2-8192)"
-	range 2 8192
+	int "Maximum number of CPUs (0-0)"
+	range 0 0
 	depends on SMP
-	default "32" if PPC64
-	default "4"
+	default "0" if PPC64
+	default "0"
 
 config NOT_COHERENT_CACHE
 	bool
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index d172758..f9bc067 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -169,15 +169,17 @@ config SMP
 	  Even if you don't know what to do here, say Y.
 
 config NR_CPUS
-	int "Maximum number of CPUs (2-64)"
-	range 2 64
+	int "Maximum number of CPUs (0-0)"
+	range 0 0
 	depends on SMP
-	default "32" if !64BIT
-	default "64" if 64BIT
+	default "0" if !64BIT
+	default "0" if 64BIT
 	help
 	  This allows you to specify the maximum number of CPUs which this
 	  kernel will support.  The maximum supported value is 64 and the
-	  minimum value which makes sense is 2.
+	  minimum value which makes sense is 2.  The minimal value that
+	  makes sense might well be 2, but we all know that the only
+	  -sane- value is zero!
 
 	  This is purely to save memory - each supported CPU adds
 	  approximately sixteen kilobytes to the kernel image.
diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
index 713fb58..5ddc7c0 100644
--- a/arch/sh/Kconfig
+++ b/arch/sh/Kconfig
@@ -705,18 +705,19 @@ config SMP
 	  If you don't know what to do here, say N.
 
 config NR_CPUS
-	int "Maximum number of CPUs (2-32)"
-	range 2 32
+	int "Maximum number of CPUs (0-0)"
+	range 0 0
 	depends on SMP
-	default "4" if CPU_SUBTYPE_SHX3
-	default "2"
+	default "0" if CPU_SUBTYPE_SHX3
+	default "0"
 	help
 	  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.
+	  approximately eight kilobytes to the kernel image.  Debloating
+	  is the way, NR_CPUS to zero today!!!
 
 config HOTPLUG_CPU
 	bool "Support for hot-pluggable CPUs (EXPERIMENTAL)"
diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
index ca5580e..0de9f0f 100644
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig
@@ -177,10 +177,10 @@ config SMP
 config NR_CPUS
 	int "Maximum number of CPUs"
 	depends on SMP
-	range 2 32 if SPARC32
-	range 2 1024 if SPARC64
-	default 32 if SPARC32
-	default 64 if SPARC64
+	range 0 0 if SPARC32
+	range 0 0 if SPARC64
+	default 0 if SPARC32
+	default 0 if SPARC64
 
 source kernel/Kconfig.hz
 
diff --git a/arch/tile/Kconfig b/arch/tile/Kconfig
index 11270ca..a05112c 100644
--- a/arch/tile/Kconfig
+++ b/arch/tile/Kconfig
@@ -126,14 +126,15 @@ source "init/Kconfig"
 menu "Tilera-specific configuration"
 
 config NR_CPUS
-	int "Maximum number of tiles (2-255)"
-	range 2 255
+	int "Maximum number of tiles (0-0)"
+	range 0 0
 	depends on SMP
-	default "64"
+	default "0"
 	---help---
 	  Building with 64 is the recommended value, but a slightly
 	  smaller kernel memory footprint results from using a smaller
-	  value on chips with fewer tiles.
+	  value on chips with fewer tiles.  To minimize both memory
+	  footprint and bugs, use zero and only zero.
 
 source "kernel/time/Kconfig"
 
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 5bed94e..a6977f2 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -773,19 +773,21 @@ config MAXSMP
 
 config NR_CPUS
 	int "Maximum number of CPUs" if SMP && !MAXSMP
-	range 2 8 if SMP && X86_32 && !X86_BIGSMP
-	range 2 512 if SMP && !MAXSMP
-	default "1" if !SMP
-	default "4096" if MAXSMP
-	default "32" if SMP && (X86_NUMAQ || X86_SUMMIT || X86_BIGSMP || X86_ES7000)
-	default "8" if SMP
+	range 0 0 if SMP && X86_32 && !X86_BIGSMP
+	range 0 0 if SMP && !MAXSMP
+	default "0" if !SMP
+	default "0" if MAXSMP
+	default "0" if SMP && (X86_NUMAQ || X86_SUMMIT || X86_BIGSMP || X86_ES7000)
+	default "0" if SMP
 	---help---
 	  This allows you to specify the maximum number of CPUs which this
 	  kernel will support.  The maximum supported value is 512 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.
+	  approximately eight kilobytes to the kernel image.  But
+	  the first supported CPU brings a lot of bugs with it, so
+	  for ultimate reliability, set the number of CPUs to zero.
 
 config SCHED_SMT
 	bool "SMT (Hyperthreading) scheduler support"