[RFC PATCH 1/3] powerpc: Allow platforms to force-enable CONFIG_SMP

[RFC PATCH 1/3] powerpc: Allow platforms to force-enable CONFIG_SMP

[Michael Ellerman]

Of the 64-bit Book3S platforms, only powermac supports booting on an
actual non-SMP system. The other platforms can be built with SMP
disabled, but it doesn't make a lot of sense given the CPUs they support
are all multicore or multithreaded.

So give platforms the option of forcing SMP=y.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/platforms/Kconfig.cputype | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
index 99b0ae8acb78..5c011e4baf0b 100644
--- a/arch/powerpc/platforms/Kconfig.cputype
+++ b/arch/powerpc/platforms/Kconfig.cputype
@@ -371,9 +371,15 @@ config PPC_PERF_CTRS
        help
          This enables the powerpc-specific perf_event back-end.
 
+config FORCE_SMP
+	# Allow platforms to force SMP=y by selecting this
+	bool
+	default n
+	select SMP
+
 config SMP
 	depends on PPC_BOOK3S || PPC_BOOK3E || FSL_BOOKE || PPC_47x
-	bool "Symmetric multi-processing support"
+	bool "Symmetric multi-processing support" if !FORCE_SMP
 	---help---
 	  This enables support for systems with more than one CPU. If you have
 	  a system with only one CPU, say N. If you have a system with more
-- 
2.7.4

[RFC PATCH 2/3] powerpc/powernv: Always enable SMP when building powernv

[Michael Ellerman]

The powernv platform supports Power7 and later CPUs, all of which are
multithreaded and multicore.

As such we never build a SMP=n kernel for those machines, other than
possibly for debugging or running in a simulator.

In the debugging case we can get a similar effect by booting with
nr_cpus=1, or there's always the option of building a custom kernel with
SMP hacked out.

For running in simulators the code size reduction from building without
SMP is not particularly important, what matters is the number of
instructions executed. A quick test shows that a SMP=y kernel takes ~6%
more instructions to boot to a shell. Booting with nr_cpus=1 recovers
about half that deficit.

On the flip side, keeping the SMP=n kernel building can be a pain at
times. And although we've mostly kept it building in recent years, no
one is regularly testing that the SMP=n kernel actually boots and works
well on these machines.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/platforms/powernv/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/platforms/powernv/Kconfig b/arch/powerpc/platforms/powernv/Kconfig
index 3a07e4dcf97c..bd8d41d3a1b3 100644
--- a/arch/powerpc/platforms/powernv/Kconfig
+++ b/arch/powerpc/platforms/powernv/Kconfig
@@ -19,6 +19,7 @@ config PPC_POWERNV
 	select CPU_FREQ_GOV_ONDEMAND
 	select CPU_FREQ_GOV_CONSERVATIVE
 	select PPC_DOORBELL
+	select FORCE_SMP
 	default y
 
 config OPAL_PRD
-- 
2.7.4

[RFC PATCH 3/3] powerpc/pseries: Always enable SMP when building pseries

[Michael Ellerman]

The pseries platform supports Power4 and later CPUs, all of which are
multithreaded and/or multicore.

In practice no one ever builds a SMP=n kernel for these machines. So as
we did for powernv, have the pseries platform imply SMP=y.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/platforms/pseries/Kconfig | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/platforms/pseries/Kconfig b/arch/powerpc/platforms/pseries/Kconfig
index 30ec04f1c67c..913c54e23eea 100644
--- a/arch/powerpc/platforms/pseries/Kconfig
+++ b/arch/powerpc/platforms/pseries/Kconfig
@@ -17,9 +17,10 @@ config PPC_PSERIES
 	select PPC_UDBG_16550
 	select PPC_NATIVE
 	select PPC_DOORBELL
-	select HOTPLUG_CPU if SMP
+	select HOTPLUG_CPU
 	select ARCH_RANDOM
 	select PPC_DOORBELL
+	select FORCE_SMP
 	default y
 
 config PPC_SPLPAR
-- 
2.7.4

Re: [RFC PATCH 1/3] powerpc: Allow platforms to force-enable CONFIG_SMP

[Michael Ellerman]

Michael Ellerman <mpe@ellerman.id.au> writes:

> Of the 64-bit Book3S platforms, only powermac supports booting on an
> actual non-SMP system. The other platforms can be built with SMP
> disabled, but it doesn't make a lot of sense given the CPUs they support
> are all multicore or multithreaded.
>
> So give platforms the option of forcing SMP=y.

For the record I'm planning to merge this.

cheers

> diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
> index 99b0ae8acb78..5c011e4baf0b 100644
> --- a/arch/powerpc/platforms/Kconfig.cputype
> +++ b/arch/powerpc/platforms/Kconfig.cputype
> @@ -371,9 +371,15 @@ config PPC_PERF_CTRS
>         help
>           This enables the powerpc-specific perf_event back-end.
>  
> +config FORCE_SMP
> +	# Allow platforms to force SMP=y by selecting this
> +	bool
> +	default n
> +	select SMP
> +
>  config SMP
>  	depends on PPC_BOOK3S || PPC_BOOK3E || FSL_BOOKE || PPC_47x
> -	bool "Symmetric multi-processing support"
> +	bool "Symmetric multi-processing support" if !FORCE_SMP
>  	---help---
>  	  This enables support for systems with more than one CPU. If you have
>  	  a system with only one CPU, say N. If you have a system with more
> -- 
> 2.7.4

Re: [RFC,1/3] powerpc: Allow platforms to force-enable CONFIG_SMP

[Michael Ellerman]

On Wed, 2017-04-05 at 02:44:49 UTC, Michael Ellerman wrote:
> Of the 64-bit Book3S platforms, only powermac supports booting on an
> actual non-SMP system. The other platforms can be built with SMP
> disabled, but it doesn't make a lot of sense given the CPUs they support
> are all multicore or multithreaded.
> 
> So give platforms the option of forcing SMP=y.
> 
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Series applied to powerpc next.

https://git.kernel.org/powerpc/c/ebbe9d7d3a2ca0d62f1a2c08c7e7a3

cheers