os: reprogram timer solely if needed

If an alarm/timeout object became head of the timeout queue, then the
low level timer must be reprogrammed.

Issue #1106

os/include/os/alarm.h

@@ -131,6 +131,14 @@ namespace Genode {
 			 * \return          true if an alarm is scheduled
 			 */
 			bool next_deadline(Alarm::Time *deadline);
+
+			/**
+			 * Determine if given alarm object is current head element
+			 *
+			 * \param alarm  alarm object
+			 * \return true if alarm is head element of timeout queue
+			 */
+			bool head_timeout(const Alarm * alarm) { return _head == alarm; }
 	};
 }
 

os/src/drivers/timer/include/timer_session_component.h

@@ -193,7 +193,8 @@ namespace Timer {
 				} else schedule_absolute(alarm, now + timeout);
 
 				/* interrupt current 'wait_for_timeout' */
-				_platform_timer->schedule_timeout(0);
+				if (head_timeout(alarm))
+					_platform_timer->schedule_timeout(0);
 			}
 
 			unsigned long curr_time() const