Sunday, February 20, 2022

[SOLVED] NR_CPUS define translated to 8192 during kernel compilation

Issue

I'm trying to compile 4.14 Linux kernel for Ubuntu 18.04, with some minor changes I've made in the code (not relevant to the discussion).

From some reason the NR_CPUS define translates to number 8192 for me, which is a way too big number, and it messes up with my code. (I have only 4 CPU cores on my Kabylake machine). Why is it so big? Did I miss something in the configs before I tried to compile the kernel? How do I change the definition correctly?


Solution

In your kernel tree, check arch/x86/Kconfig to see which kernel options you've set from menuconfig may be affecting Kabylake NR_CPUS. According to my 4.14.2 version of this Kconfig:

config NR_CPUS
        int "Maximum number of CPUs" if SMP && !MAXSMP
        range 2 8 if SMP && X86_32 && !X86_BIGSMP
        range 2 512 if SMP && !MAXSMP && !CPUMASK_OFFSTACK
        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
        ---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.

So setting MAXSMP (appears in menuconfig session as "Enable Maximum number of SMP Processors and NUMA Nodes [N/y]") could be getting you to 8192. Note that with NR_CPUS you are specifying the maximum number of CPUs to be used with this kernel; it's not expected to match your current CPU count exactly. That should be done on the fly.



Answered By - Frank Hoeflich
Answer Checked By - Willingham (WPSolving Volunteer)
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