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SubjectRe: [PATCH] x86/Kconfig: Further simplify the NR_CPUS config

* Ingo Molnar <mingo@kernel.org> wrote:

> =========================>
> Subject: x86/Kconfig: Further simplify the NR_CPUS config
> From: Ingo Molnar <mingo@kernel.org>
> Date: Sat Feb 10 11:51:57 CET 2018
>
> Clean up various aspects of the x86 CONFIG_NR_CPUS configuration switches:
>
> - Rename the three CONFIG_NR_CPUS related variables to create a common
> namespace for them:
>
> RANGE_BEGIN_CPUS => NR_CPUS_RANGE_BEGIN
> RANGE_END_CPUS => NR_CPUS_RANGE_END
> DEF_CONFIG_CPUS => NR_CPUS_DEFAULT
>
> - Align them vertically, such as:
>
> config NR_CPUS_RANGE_END
> int
> depends on X86_64
> default 8192 if SMP && ( MAXSMP || CPUMASK_OFFSTACK)
> default 512 if SMP && (!MAXSMP && !CPUMASK_OFFSTACK)
> default 1 if !SMP
>
> - Update help text, add more comments.
>
> Test results:
>
> # i386 allnoconfig:
> CONFIG_NR_CPUS_RANGE_BEGIN=1
> CONFIG_NR_CPUS_RANGE_END=1
> CONFIG_NR_CPUS_DEFAULT=1
> CONFIG_NR_CPUS=1
>
> # i386 defconfig:
> CONFIG_NR_CPUS_RANGE_BEGIN=2
> CONFIG_NR_CPUS_RANGE_END=8
> CONFIG_NR_CPUS_DEFAULT=8
> CONFIG_NR_CPUS=8
>
> # i386 allyesconfig:
> CONFIG_NR_CPUS_RANGE_BEGIN=2
> CONFIG_NR_CPUS_RANGE_END=64
> CONFIG_NR_CPUS_DEFAULT=32
> CONFIG_NR_CPUS=32
>
> # x86_64 allnoconfig:
> CONFIG_NR_CPUS_RANGE_BEGIN=1
> CONFIG_NR_CPUS_RANGE_END=1
> CONFIG_NR_CPUS_DEFAULT=1
> CONFIG_NR_CPUS=1
>
> # x86_64 defconfig:
> CONFIG_NR_CPUS_RANGE_BEGIN=2
> CONFIG_NR_CPUS_RANGE_END=512
> CONFIG_NR_CPUS_DEFAULT=64
> CONFIG_NR_CPUS=64
>
> # x86_64 allyesconfig:
> CONFIG_NR_CPUS_RANGE_BEGIN=8192
> CONFIG_NR_CPUS_RANGE_END=8192
> CONFIG_NR_CPUS_DEFAULT=8192
> CONFIG_NR_CPUS=8192
>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
> arch/x86/Kconfig | 66 +++++++++++++++++++++++++++++++++----------------------
> 1 file changed, 40 insertions(+), 26 deletions(-)

Here is a full combo patch of your patch and mine, for easier review:

---
arch/x86/Kconfig | 75 ++++++++++++++++++++++++++++++++++++++++++-------------
1 file changed, 58 insertions(+), 17 deletions(-)

Index: tip/arch/x86/Kconfig
===================================================================
--- tip.orig/arch/x86/Kconfig
+++ tip/arch/x86/Kconfig
@@ -423,12 +423,6 @@ config X86_MPPARSE
For old smp systems that do not have proper acpi support. Newer systems
(esp with 64bit cpus) with acpi support, MADT and DSDT will override it

-config X86_BIGSMP
- bool "Support for big SMP systems with more than 8 CPUs"
- depends on X86_32 && SMP
- ---help---
- This option is needed for the systems that have more than 8 CPUs
-
config GOLDFISH
def_bool y
depends on X86_GOLDFISH
@@ -460,6 +454,12 @@ config INTEL_RDT
Say N if unsure.

if X86_32
+config X86_BIGSMP
+ bool "Support for big SMP systems with more than 8 CPUs"
+ depends on SMP
+ ---help---
+ This option is needed for the systems that have more than 8 CPUs
+
config X86_EXTENDED_PLATFORM
bool "Support for extended (non-PC) x86 platforms"
default y
@@ -949,25 +949,66 @@ config MAXSMP
Enable maximum number of CPUS and NUMA Nodes for this architecture.
If unsure, say N.

+#
+# The maximum number of CPUs supported:
+#
+# The main config value is NR_CPUS, which defaults to NR_CPUS_DEFAULT,
+# and which can be configured interactively in the
+# [NR_CPUS_RANGE_BEGIN ... NR_CPUS_RANGE_END] range.
+#
+# The ranges are different on 32-bit and 64-bit kernels, depending on
+# hardware capabilities and scalability features of the kernel.
+#
+# ( If MAXSMP is enabled we just use the highest possible value and disable
+# interactive configuration. )
+#
+
+config NR_CPUS_RANGE_BEGIN
+ int
+ default NR_CPUS_RANGE_END if MAXSMP
+ default 1 if !SMP
+ default 2
+
+config NR_CPUS_RANGE_END
+ int
+ depends on X86_32
+ default 64 if SMP && X86_BIGSMP
+ default 8 if SMP && !X86_BIGSMP
+ default 1 if !SMP
+
+config NR_CPUS_RANGE_END
+ int
+ depends on X86_64
+ default 8192 if SMP && ( MAXSMP || CPUMASK_OFFSTACK)
+ default 512 if SMP && (!MAXSMP && !CPUMASK_OFFSTACK)
+ default 1 if !SMP
+
+config NR_CPUS_DEFAULT
+ int
+ depends on X86_32
+ default 32 if X86_BIGSMP
+ default 8 if SMP
+ default 1 if !SMP
+
+config NR_CPUS_DEFAULT
+ int
+ depends on X86_64
+ default 8192 if MAXSMP
+ default 64 if SMP
+ default 1 if !SMP
+
config NR_CPUS
int "Maximum number of CPUs" if SMP && !MAXSMP
- range 2 8 if SMP && X86_32 && !X86_BIGSMP
- range 2 64 if SMP && X86_32 && X86_BIGSMP
- range 2 512 if SMP && !MAXSMP && !CPUMASK_OFFSTACK && X86_64
- range 2 8192 if SMP && !MAXSMP && CPUMASK_OFFSTACK && X86_64
- default "1" if !SMP
- default "8192" if MAXSMP
- default "32" if SMP && X86_BIGSMP
- default "8" if SMP && X86_32
- default "64" if SMP
+ range NR_CPUS_RANGE_BEGIN NR_CPUS_RANGE_END
+ default NR_CPUS_DEFAULT
---help---
This allows you to specify the maximum number of CPUs which this
kernel will support. If CPUMASK_OFFSTACK is enabled, the maximum
supported value is 8192, otherwise the maximum value is 512. 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.
+ This is purely to save memory: each supported CPU adds about 8KB
+ to the kernel image.

config SCHED_SMT
bool "SMT (Hyperthreading) scheduler support"
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