[PATCH] x86/Kconfig: Allow NR_CPUS between 512 and 8192

[PATCH] x86/Kconfig: Allow NR_CPUS between 512 and 8192

[Saurabh Sengar]

Today there is no way one can choose any value between 512 to 8192
for NR_CPUS seamlessly. NR_CPUS is guarded by NR_CPUS_RANGE_END which
is further dependent on CPUMASK_OFFSTACK to allow NR_CPUs > 512.

For x86, CPUMASK_OFFSTACK can only be enabled either by selecting MAXSMP
or DEBUG_PER_CPU_MAPS. Both of these options has a cost to pay. MAXSMP
will increase the NR_CPUS to 8192 which will have impact on kernel image
size whereas DEBUG_PER_CPU_MAPS will have additional run time overheads.
Thus there is no good way to have NR_CPUS anything between 512 to 8192.

Fix this by selecting CPUMASK_OFFSTACK if NR_CPUS > 512 and
let NR_CPUS_RANGE_END set to 8192.

On a Hyper-V system where max number of CPUs are only 2048, this
patch saves around 1 MB of kernel image size, compare to MAXSMP.

Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
---

I want to mention that in ARM and other archs its very simple
to select any value for NR_CPUS. This is an attempt to have more
flexibilty in x86 arch as well to choose NR_CPUS.

Some of the earlier discussions reated to it which could be of interest:
https://lore.kernel.org/lkml/1708092603-14504-1-git-send-email-ssengar@linux.microsoft.com/
https://lore.kernel.org/lkml/794a1211-630b-3ee5-55a3-c06f10df1490@linux.com/

Another approach I can think of is to allow CPUMASK_OFFSTACK to be enabled
more freely like the below patch of Libo Chen, that will also solve the
problem I am addressing. But I feel this patch may have impact on other
archs as well and I am not sure if that is in best interest of all the archs.

https://lore.kernel.org/lkml/20220412231508.32629-2-libo.chen@oracle.com/

 arch/x86/Kconfig | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 07a0c8d4e9c7..458f3f250d7f 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -34,6 +34,7 @@ config X86_64
 	select SWIOTLB
 	select ARCH_HAS_ELFCORE_COMPAT
 	select ZONE_DMA32
+	select CPUMASK_OFFSTACK if NR_CPUS > 512
 
 config FORCE_DYNAMIC_FTRACE
 	def_bool y
@@ -1006,8 +1007,7 @@ config NR_CPUS_RANGE_END
 config NR_CPUS_RANGE_END
 	int
 	depends on X86_64
-	default 8192 if  SMP && CPUMASK_OFFSTACK
-	default  512 if  SMP && !CPUMASK_OFFSTACK
+	default 8192 if  SMP
 	default    1 if !SMP
 
 config NR_CPUS_DEFAULT
-- 
2.34.1

Re: [PATCH] x86/Kconfig: Allow NR_CPUS between 512 and 8192

[Saurabh Singh Sengar]

On Tue, Feb 20, 2024 at 01:50:13AM -0800, Saurabh Sengar wrote:
> Today there is no way one can choose any value between 512 to 8192
> for NR_CPUS seamlessly. NR_CPUS is guarded by NR_CPUS_RANGE_END which
> is further dependent on CPUMASK_OFFSTACK to allow NR_CPUs > 512.
> 
> For x86, CPUMASK_OFFSTACK can only be enabled either by selecting MAXSMP
> or DEBUG_PER_CPU_MAPS. Both of these options has a cost to pay. MAXSMP
> will increase the NR_CPUS to 8192 which will have impact on kernel image
> size whereas DEBUG_PER_CPU_MAPS will have additional run time overheads.
> Thus there is no good way to have NR_CPUS anything between 512 to 8192.
> 
> Fix this by selecting CPUMASK_OFFSTACK if NR_CPUS > 512 and
> let NR_CPUS_RANGE_END set to 8192.
> 
> On a Hyper-V system where max number of CPUs are only 2048, this
> patch saves around 1 MB of kernel image size, compare to MAXSMP.
> 
> Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
> ---
> 
> I want to mention that in ARM and other archs its very simple
> to select any value for NR_CPUS. This is an attempt to have more
> flexibilty in x86 arch as well to choose NR_CPUS.
> 
> Some of the earlier discussions reated to it which could be of interest:
> https://lore.kernel.org/lkml/1708092603-14504-1-git-send-email-ssengar@linux.microsoft.com/
> https://lore.kernel.org/lkml/794a1211-630b-3ee5-55a3-c06f10df1490@linux.com/
> 
> Another approach I can think of is to allow CPUMASK_OFFSTACK to be enabled
> more freely like the below patch of Libo Chen, that will also solve the
> problem I am addressing. But I feel this patch may have impact on other
> archs as well and I am not sure if that is in best interest of all the archs.
> 
> https://lore.kernel.org/lkml/20220412231508.32629-2-libo.chen@oracle.com/
> 
>  arch/x86/Kconfig | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 07a0c8d4e9c7..458f3f250d7f 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -34,6 +34,7 @@ config X86_64
>  	select SWIOTLB
>  	select ARCH_HAS_ELFCORE_COMPAT
>  	select ZONE_DMA32
> +	select CPUMASK_OFFSTACK if NR_CPUS > 512
>  
>  config FORCE_DYNAMIC_FTRACE
>  	def_bool y
> @@ -1006,8 +1007,7 @@ config NR_CPUS_RANGE_END
>  config NR_CPUS_RANGE_END
>  	int
>  	depends on X86_64
> -	default 8192 if  SMP && CPUMASK_OFFSTACK
> -	default  512 if  SMP && !CPUMASK_OFFSTACK
> +	default 8192 if  SMP
>  	default    1 if !SMP
>  
>  config NR_CPUS_DEFAULT
> 
> -- 
> 2.34.1
> 

x86 Maintainers,

Kind reminder to have your feedback on this patch.

- Saurabh

Re: [PATCH] x86/Kconfig: Allow NR_CPUS between 512 and 8192

[Saurabh Singh Sengar]

On Mon, Mar 04, 2024 at 08:13:23AM -0800, Saurabh Singh Sengar wrote:
> On Tue, Feb 20, 2024 at 01:50:13AM -0800, Saurabh Sengar wrote:
> > Today there is no way one can choose any value between 512 to 8192
> > for NR_CPUS seamlessly. NR_CPUS is guarded by NR_CPUS_RANGE_END which
> > is further dependent on CPUMASK_OFFSTACK to allow NR_CPUs > 512.
> > 
> > For x86, CPUMASK_OFFSTACK can only be enabled either by selecting MAXSMP
> > or DEBUG_PER_CPU_MAPS. Both of these options has a cost to pay. MAXSMP
> > will increase the NR_CPUS to 8192 which will have impact on kernel image
> > size whereas DEBUG_PER_CPU_MAPS will have additional run time overheads.
> > Thus there is no good way to have NR_CPUS anything between 512 to 8192.
> > 
> > Fix this by selecting CPUMASK_OFFSTACK if NR_CPUS > 512 and
> > let NR_CPUS_RANGE_END set to 8192.
> > 
> > On a Hyper-V system where max number of CPUs are only 2048, this
> > patch saves around 1 MB of kernel image size, compare to MAXSMP.
> > 
> > Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
> > ---
> > 
> > I want to mention that in ARM and other archs its very simple
> > to select any value for NR_CPUS. This is an attempt to have more
> > flexibilty in x86 arch as well to choose NR_CPUS.
> > 
> > Some of the earlier discussions reated to it which could be of interest:
> > https://lore.kernel.org/lkml/1708092603-14504-1-git-send-email-ssengar@linux.microsoft.com/
> > https://lore.kernel.org/lkml/794a1211-630b-3ee5-55a3-c06f10df1490@linux.com/
> > 
> > Another approach I can think of is to allow CPUMASK_OFFSTACK to be enabled
> > more freely like the below patch of Libo Chen, that will also solve the
> > problem I am addressing. But I feel this patch may have impact on other
> > archs as well and I am not sure if that is in best interest of all the archs.
> > 
> > https://lore.kernel.org/lkml/20220412231508.32629-2-libo.chen@oracle.com/
> > 
> >  arch/x86/Kconfig | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index 07a0c8d4e9c7..458f3f250d7f 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -34,6 +34,7 @@ config X86_64
> >  	select SWIOTLB
> >  	select ARCH_HAS_ELFCORE_COMPAT
> >  	select ZONE_DMA32
> > +	select CPUMASK_OFFSTACK if NR_CPUS > 512
> >  
> >  config FORCE_DYNAMIC_FTRACE
> >  	def_bool y
> > @@ -1006,8 +1007,7 @@ config NR_CPUS_RANGE_END
> >  config NR_CPUS_RANGE_END
> >  	int
> >  	depends on X86_64
> > -	default 8192 if  SMP && CPUMASK_OFFSTACK
> > -	default  512 if  SMP && !CPUMASK_OFFSTACK
> > +	default 8192 if  SMP
> >  	default    1 if !SMP
> >  
> >  config NR_CPUS_DEFAULT
> > 
> > -- 
> > 2.34.1
> > 
> 
> x86 Maintainers,
> 
> Kind reminder to have your feedback on this patch.
> 
> - Saurabh
> 
ping

Re: [PATCH] x86/Kconfig: Allow NR_CPUS between 512 and 8192

[Hardik Garg]

Hello Maintainers,
Please review the patch and share your feedback.

Some of the earlier discussions related to this patch:
https://lore.kernel.org/lkml/1708092603-14504-1-git-send-email-ssengar@linux.microsoft.com/
https://lore.kernel.org/lkml/794a1211-630b-3ee5-55a3-c06f10df1490@linux.com/



Thanks,
Hardik