Date Sun, 1 Apr 2012 00:33:21 +0800 From "Paul E. McKenney" <> Subject [PATCH RFC] Simplify the Linux kernel by reducing its state space 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 <paulmck@linux.vnet.ibm.com>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

---



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"





