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

From: Linas Vepstas <[email-masked]>
Date: Sat, 31 Mar 2012 15:15:00 -0500

Hi,

I didn't actually try to compile the patch below; it didn't look like
C code so I wasn't sure what compiler to run it through.  I guess maybe
its python?  However, I'm very sure that the patches are completely
correct, because I read them, and I also know Paul.  And I've heard of
Thomas Gleixner.

Thus, please add my ack --

Ack'ed by: Linas Vepstas <[email-masked]>

On Sun, Apr 01, 2012 at 12:33:21AM +0800, Paul E. McKenney was heard to remark:
> 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"
> 
> --
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