This patch limits the cpu span of each node's scheduler domain to prevent
  balancing across too many cpus.  The cpus included in a node's domain are
  determined by the SD_NODES_PER_DOMAIN define and the arch specific
  sched_domain_node_span routine if ARCH_HAS_SCHED_DOMAIN is defined.  If
  ARCH_HAS_SCHED_DOMAIN is not defined, behavior is unchanged--all possible
  cpus will be included in each node's scheduling domain.  Currently, only
  ia64 provides an arch specific sched_domain_node_span routine.

From: Jesse Barnes <jbarnes@engr.sgi.com>

  This patch adds some more NUMA specific logic to the creation of scheduler
  domains.  Domains spanning all CPUs in a large system are too large to
  schedule across efficiently, leading to livelocks and inordinate amounts of
  time being spent in scheduler routines.  With this patch applied, the node
  scheduling domains for NUMA platforms will only contain a specified number
  of nearby CPUs, based on the value of SD_NODES_PER_DOMAIN.  It also allows
  arches to override SD_NODE_INIT, which sets the domain scheduling parameters
  for each node's domain.  This is necessary especially for large systems.

  Possible future directions:

  o multilevel node hierarchy (e.g.  node domains could contain 4 nodes
    worth of CPUs, supernode domains could contain 32 nodes worth, etc.  each
    with their own SD_NODE_INIT values)

  o more tweaking of SD_NODE_INIT values for good load balancing vs. 
    overhead tradeoffs

From: mita akinobu <amgta@yacht.ocn.ne.jp>

  Compile fix
Signed-off-by: Jesse Barnes <jbarnes@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>