Message ID | 1708422613-15714-1-git-send-email-ssengar@linux.microsoft.com |
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State | New |
Headers |
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Series |
x86/Kconfig: Allow NR_CPUS between 512 and 8192
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Commit Message
Saurabh Singh Sengar
Feb. 20, 2024, 9:50 a.m. UTC
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(-)
Comments
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
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