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Message-Id: <1708422613-15714-1-git-send-email-ssengar@linux.microsoft.com> Date: Tue, 20 Feb 2024 01:50:13 -0800 From: Saurabh Sengar <ssengar@...ux.microsoft.com> To: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org Cc: ssengar@...rosoft.com, libo.chen@...cle.com, mhklinux@...look.com Subject: [PATCH] x86/Kconfig: Allow NR_CPUS between 512 and 8192 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@...ux.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
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