Re: [PATCH] thread-utils.c: detect online CPU count on OpenBSD / NetBSD

[Patrick Steinhardt <ps@pks.im>]

On Fri, May 09, 2025 at 02:13:13AM -0400, Brad Smith wrote:
> OpenBSD / NetBSD use HW_NCPUONLINE to detect the online CPU
> count. OpenBSD ships with SMT disabled on X86 systems so
> HW_NCPU would provide double the number of CPUs as opposed
> to the proper online count.
> 
> Signed-off-by: Brad Smith <brad@comstyle.com>
> ---
>  thread-utils.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/thread-utils.c b/thread-utils.c
> index 1f89ffab4c..374890e6b0 100644
> --- a/thread-utils.c
> +++ b/thread-utils.c
> @@ -46,11 +46,11 @@ int online_cpus(void)
>  	mib[0] = CTL_HW;
>  #  ifdef HW_AVAILCPU
>  	mib[1] = HW_AVAILCPU;
> -	len = sizeof(cpucount);
> -	if (!sysctl(mib, 2, &cpucount, &len, NULL, 0))
> -		return cpucount;
> -#  endif /* HW_AVAILCPU */
> +#  elif defined(HW_NCPUONLINE)
> +	mib[1] = HW_NCPUONLINE;
> +#  else
>  	mib[1] = HW_NCPU;
> +#  endif /* HW_AVAILCPU */
>  	len = sizeof(cpucount);
>  	if (!sysctl(mib, 2, &cpucount, &len, NULL, 0))
>  		return cpucount;

This change looks sensible to me and matches the documentation at [1].
Using the number of online CPUs instead of existing CPUs certainly
matches the expectation of what this function should reutrn.

Thanks!

Patrick

[1]: https://man.openbsd.org/sysctl.2#HW_NCPUONLINE~2