Add hw.ncpuonline to count the number of online CPUs.

The introduction of hw.smt means that logical CPUs can be disabled
after boot and prior to suspend/resume.  If hw.smt=0 (the default),
there needs to be a way to count the number of hardware threads
available on the system at any given time.

So, import HW_NCPUONLINE/hw.ncpuonline from NetBSD and document it.
hw.ncpu becomes equal to the number of CPUs given to sched_init_cpu()
during boot, while hw.ncpuonline is equal to the number of CPUs available
to the scheduler in the cpuset "sched_all_cpus". Set_SC_NPROCESSORS_ONLN
equal to this new sysctl and keep _SC_NPROCESSORS_CONF equal to hw.ncpu.

This is preferable to adding a new sysctl to count the number of
configured CPUs and keeping hw.ncpu equal to the number of online
CPUs because such a change would break software in the ecosystem
that relies on HW_NCPU/hw.ncpu to measure CPU usage and the like.
Such software in base includes top(1), systat(1), and snmpd(8),
and perhaps others.

We don't need additional locking to count the cardinality of a cpuset
in this case because the only interfaces that can modify said cardinality
are sysctl(2) and ioctl(2), both of which are under the KERNEL_LOCK.

Software using HW_NCPU/hw.ncpu to determine optimal parallism will need
to be updated to use HW_NCPUONLINE/hw.ncpuonline.  Until then, such software
may perform suboptimally.  However, most changes will be similar to the
change included here for libcxx's std::thread:hardware_concurrency():
using HW_NCPUONLINE in lieu of HW_NCPU should be sufficient for determining
optimal parallelism for most software if the change to _SC_NPROCESSORS_ONLN
is insufficient.

Prompted by deraadt. Discussed at length with kettenis, deraadt, and sthen.
Lots of patch tweaks from kettenis.

ok kettenis, "proceed" deraadt

1 files changed, 6 insertions, 3 deletions

diff --git a/lib/libc/sys/sysctl.2 b/lib/libc/sys/sysctl.2
index 6a1de7b0231..b600074fea1 100644
--- a/lib/libc/sys/sysctl.2
+++ b/lib/libc/sys/sysctl.2
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: sysctl.2,v 1.9 2018/06/21 20:28:36 jmc Exp $
+.\"	$OpenBSD: sysctl.2,v 1.10 2018/07/12 01:23:38 cheloha Exp $
 .\"
 .\" Copyright (c) 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -27,7 +27,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd $Mdocdate: June 21 2018 $
+.Dd $Mdocdate: July 12 2018 $
 .Dt SYSCTL 2
 .Os
 .Sh NAME
@@ -286,6 +286,7 @@ privileges may change the value.
 .It Dv HW_MODEL Ta "string" Ta "no"
 .It Dv HW_NCPU Ta "integer" Ta "no"
 .It Dv HW_NCPUFOUND Ta "integer" Ta "no"
+.It Dv HW_NCPUONLINE Ta "integer" Ta "no"
 .It Dv HW_PAGESIZE Ta "integer" Ta "no"
 .It Dv HW_PERFPOLICY Ta "string" Ta "yes"
 .It Dv HW_PHYSMEM Ta "integer" Ta "no"
@@ -328,9 +329,11 @@ The machine class.
 .It Dv HW_MODEL Pq Va hw.model
 The machine model.
 .It Dv HW_NCPU Pq Va hw.ncpu
-The number of CPUs being used.
+The number of CPUs configured.
 .It Dv HW_NCPUFOUND Pq Va hw.ncpufound
 The number of CPUs found.
+.It Dv HW_NCPUONLINE Pq Va hw.ncpuonline
+The number of CPUs online.
 .It Dv HW_PAGESIZE Pq Va hw.pagesize
 The software page size.
 .It Dv HW_PERFPOLICY Pq Va hw.perfpolicy