sched,numa: add cond_resched to task_numa_work

Normally task_numa_work scans over a fairly small amount of memory,
but it is possible to run into a large unpopulated part of virtual
memory, with no pages mapped. In that case, task_numa_work can run
for a while, and it may make sense to reschedule as required.

Cc: akpm@linux-foundation.org
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Rik van Riel <riel@redhat.com>
Reported-by: Xing Gang <gang.xing@hp.com>
Tested-by: Chegu Vinod <chegu_vinod@hp.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1392761566-24834-2-git-send-email-riel@redhat.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/sched/fair.c

@@ -1757,6 +1757,8 @@ void task_numa_work(struct callback_head *work)
 			start = end;
 			if (pages <= 0)
 				goto out;
+
+			cond_resched();
 		} while (end != vma->vm_end);
 	}