kernel - scheduler adjustments for large ncpus / 48-core monster

kernel - scheduler adjustments for large ncpus / 48-core monster

* Change the LWKT scheduler's token spinning algorithm.  It used to
  DELAY a short period of time and then simply retry, creating a lot
  of contention between cpus trying to acquire a token.

  Now the LWKT scheduler uses a FIFO index mechanic to resequence the
  contending cpus into 1uS retry slots using essentially just
  atomic_fetchadd_int(), so it is very cache friendly.  The spin-retry
  thus has a bounded cache management traffic load regardless of
  the number of cpus and contending cpus will not be tripping over
  each other.

  The new algorithm slightly regresses 4-cpu operation (~5% under heavy
  contention) but significantly improves 48-cpu operation.  It is also
  flexible enough for further work down the road.  The old algorithm
  simply did not scale very well.

  Add three sysctls:

  sysctl lwkt.spin_method=1

	0    Allow a user thread to be scheduled on a cpu while kernel
	     threads are contended on a token, using the IPI mechanic
	     to interrupt the user thread and reschedule on decontention.

	     This can potentially result in excessive IPI traffic.

	1    Allow a user thread to be scheduled on a cpu while kernel
	     threads are contended on a token, reschedule on the next clock
	     tick (100 Hz typically).  Decontention will NOT generate
	     any IPI traffic.  DEFAULT.

	2    Do not allow a user thread to be scheduled on a cpu while
	     kernel threads are contended.  Should not be used normally,
	     for debugging only.

  sysctl lwkt.spin_delay=1

	Slot time in microseconds, default 1uS.  Recommended values are
	1 or 2 but not longer.

  sysctl lwkt.spin_loops=10

	Number of times the LWKT scheduler loops on contended threads
	before giving up and allowing an idle-thread HLT.  In order to
	wake up from the HLT decontention will cause an IPI so you do
	not want to set this value too small and.  Values between
	10 and 100 are recommended.

* Redo the token decontention algorithm.  Use a new gd_reqflags flag,
  RQF_WAKEUP, coupled with RQF_AST_LWKT_RESCHED in the per-cpu globaldata
  structure to determine what cpus actually need to be IPId on token
  decontention (to wakeup their idle threads stuck in HLT).

  This requires that all gd_reqflags operations use locked atomic
  instructions rather than non-locked instructions.

* Decontention IPIs are a last-gasp effort if the LWKT scheduler has spun
  too many times.  Under normal conditions, even under heavy contention,
  actual IPIing should be minimal.