Determining the number of CPU cores and hyperthreads from userspace

Hello,

In the interest of fetching this information from Ansible for the GCC
compile farm [1], I would like to determine the number of cores
vs. hyper-threads on a system.

With OpenBSD 6.9, this is what I get with "sysctl hw" on a dual 6-core Xeon
system:

hw.model=Intel(R) Xeon(R) CPU E5-2640 0 @ 2.50GHz
hw.ncpu=24
hw.ncpufound=24
hw.smt=0
hw.ncpuonline=12

Because SMT is disabled, I can guess that there are 12 cores (ncpuonline)
and 24 threads (ncpu), which is good.

However, if SMT is enabled, this "guess" no longer works:

hw.ncpu=24
hw.ncpufound=24
hw.smt=1
hw.ncpuonline=24

Is there another method that always works?

I could parse the output of dmesg, it would work on this system:

# dmesg | grep smt
cpu0: smt 0, core 0, package 0
...
cpu12: smt 1, core 0, package 0
...

However, parsing dmesg was removed from Ansible in 2016 because it was not
considered reliable enough:

https://github.com/ansible/ansible/commit/c17dad0def2fa86733c07610189e94486e056203

In addition, this method would only work on amd64/i386 (according to the
comment added in this commit).

Thanks,
Baptiste

[1] https://cfarm.tetaneutral.net