Add a SpinWait step when waiting on a task. Fix 100% CPU bother.

After some time the call to SpinWait.SpinOnce will fall back to the more stalling Thread.Sleep (1) call.

mcs/class/corlib/System.Threading.Tasks/Internal/ThreadWorker.cs

@@ -212,6 +212,8 @@ bool WorkerMethod ()
 		public static void WorkerMethod (Func<bool> predicate, IProducerConsumerCollection<Task> sharedWorkQueue,
 		                                 ThreadWorker[] others)
 		{
+			SpinWait wait = new SpinWait ();
+
 			while (!predicate ()) {
 				Task value;
 
@@ -226,9 +228,8 @@ bool WorkerMethod ()
 				}
 
 				// First check to see if we comply to predicate
-				if (predicate ()) {
+				if (predicate ())
 					return;
-				}
 
 				// Try to complete other work by stealing since our desired tasks may be in other worker
 				ThreadWorker other;
@@ -245,10 +246,11 @@ bool WorkerMethod ()
 						}
 					}
 
-					if (predicate ()) {
+					if (predicate ())
 						return;
-					}
 				}
+
+				wait.SpinOnce ();
 			}
 		}