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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
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patrick
robert
cheloha