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Message-ID: <20120331201500.GA27640@linas.org>
Date: Sat, 31 Mar 2012 15:15:00 -0500
From: Linas Vepstas <linasvepstas@...il.com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org,
uclinux-dist-devel@...ckfin.uclinux.org,
linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
linux-m32r@...linux-m32r.org, linux-m32r-ja@...linux-m32r.org,
linux-mips@...ux-mips.org, linux-am33-list@...hat.com,
linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
sparclinux@...r.kernel.org, tglx@...utronix.de,
linux@....linux.org.uk, dhowells@...hat.com, jejb@...isc-linux.org,
linux390@...ibm.com, x86@...nel.org, cmetcalf@...era.com
Subject: Re: [PATCH RFC] Simplify the Linux kernel by reducing its state space
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 <linasvepstas@...il.com>
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 <paulmck@...ux.vnet.ibm.com>
> Reviewed-by: Thomas Gleixner <tglx@...utronix.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"
>
> --
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