remove code for dynamic offlining/onlining of CPUs on x86

The code has definitely been broken for SCHED_ULE, which is a default
scheduler.  It may have been broken for SCHED_4BSD in more subtle ways,
e.g. with manually configured CPU affinities and for interrupt devilery
purposes.
We still provide a way to disable individual CPUs or all hyperthreading
"twin" CPUs before SMP startup.  See the UPDATING entry for details.

Interaction between building CPU topology and disabling CPUs still
remains fuzzy: topology is first built using all availble CPUs and then
the disabled CPUs should be "subtracted" from it.  That doesn't work
well if the resulting topology becomes non-uniform.

This work is done in cooperation with Attilio Rao who in addition to
reviewing also provided parts of code.

PR:		kern/145385
Discussed with:	gcooper, ambrisko, mdf, sbruno
Reviewed by:	attilio
Tested by:	pho, pluknet
X-MFC after:	never

1 files changed, 17 insertions, 0 deletions

diff --git a/UPDATING b/UPDATING
index 6010a96..c8c3582 100644
--- a/UPDATING
+++ b/UPDATING
@@ -22,6 +22,23 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 9.x IS SLOW:
 	machines to maximize performance.  (To disable malloc debugging, run
 	ln -s aj /etc/malloc.conf.)
 
+20110608:
+	The following sysctls and tunables are retired on x86 platforms:
+		machdep.hlt_cpus
+		machdep.hlt_logical_cpus
+	The following sysctl is retired:
+		machdep.hyperthreading_allowed
+	The sysctls were supposed to provide a way to dynamically offline and
+	online selected CPUs on x86 platforms, but the implementation has not
+	been reliable especially with SCHED_ULE scheduler.
+	machdep.hyperthreading_allowed tunable is still available to ignore
+	hyperthreading CPUs at OS level.
+	Individual CPUs can be disabled using hint.lapic.X.disabled tunable,
+	where X is an APIC ID of a CPU.  Be advised, though, that disabling
+	CPUs in non-uniform fashion will result in non-uniform topology and
+	may lead to sub-optimal system performance with SCHED_ULE, which is
+	a default scheduler.
+
 20110607:
 	cpumask_t type is retired and cpuset_t is used in order to describe
 	a mask of CPUs.