From: Robert Love <rml@tech9.net>

- Let real-time tasks dip further into the reserves than usual in
  __alloc_pages().  There are a lot of ways to special case this.  This
  patch just cuts z->pages_low in half, before doing the incremental min
  thing, for real-time tasks.  I do not do anything in the low memory slow
  path.  We can be a _lot_ more aggressive if we want.  Right now, we just
  give real-time tasks a little help.

- Never ever call balance_dirty_pages() on a real-time task.  Where and
  how exactly we handle this is up for debate.  We could, for example,
  special case real-time tasks inside balance_dirty_pages().  This would
  allow us to perform some of the work (say, waking up pdflush) but not
  other work (say, the active throttling).  As it stands now, we do the
  per-processor accounting in balance_dirty_pages_ratelimited() but we
  never call balance_dirty_pages().  Lots of approaches work.  What we want
  to do is never engage the real-time task in forced writeback.