[Bug libc/28991] sysconf(_SC_NPROCESSORS_CONF) should read /sys/devices/system/cpu/possible

[adhemerval.zanella at linaro dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28991

Adhemerval Zanella <adhemerval.zanella at linaro dot org> changed:

           What    |Removed                     |Added
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                 CC|                            |adhemerval.zanella at linaro dot o
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--- Comment #1 from Adhemerval Zanella <adhemerval.zanella at linaro dot org> ---
I am not sure if we can actually /sys/devices/system/cpu/possible, since for
some architecture it reports possible hotplug cpus that might not actually be
present in the system.

For instance, on my system with a Ryzen 5900x (12c, 24t) I see:

$ sudo dmesg
[...]
[    0.065499] smpboot: Allowing 32 CPUs, 8 hotplug CPUs
[...]
$ cat /sys/devices/system/cpu/possible 
0-31
$ cat /sys/devices/system/cpu/online 
0-23
$ cat /sys/devices/system/cpu/offline 
24-31

And thus using 'possible' will wrongly report 32 cpus on a system with the
maximum of 24. 

The Linux source give us some hints why it is doing it:

arch/x86/kernel/smpboot.c
1513  * Three ways to find out the number of additional hotplug CPUs:
1514  * - If the BIOS specified disabled CPUs in ACPI/mptables use that.
1515  * - The user can overwrite it with possible_cpus=NUM
1516  * - Otherwise don't reserve additional CPUs.
1517  * We do this because additional CPUs waste a lot of memory.

So I am guessing that BIOS in my case it reporting 32 because it supports the
Ryzen 5950x, even thought he system does not actually support a large number of
cpus.

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