timers: set the deadline more accurately

Program the timer to the deadline of the closest timer if it is further
than 50us ahead, otherwise set it 50us ahead.  This way a single event
fires on time rather than 50us late (as it would have previously) while
still preventing too many timer wakeups in the case of having many
timers scheduled close together.

(where 50us is the timer_slop)

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>

diff --git a/xen/common/timer.c b/xen/common/timer.c
index 1895a78ba344081dd5f78e11ed6c1342878db817..f36aebcf4333d816cc9cb962f2642d2f8af6236b 100644 (file)
--- a/xen/common/timer.c
+++ b/xen/common/timer.c
@@ -492,8 +492,9 @@ static void timer_softirq_action(void)
         deadline = heap[1]->expires;
     if ( (ts->list != NULL) && (ts->list->expires < deadline) )
         deadline = ts->list->expires;
+    now = NOW();
     this_cpu(timer_deadline) =
-        (deadline == STIME_MAX) ? 0 : deadline + timer_slop;
+        (deadline == STIME_MAX) ? 0 : MAX(deadline, now + timer_slop);
 
     if ( !reprogram_timer(this_cpu(timer_deadline)) )
         raise_softirq(TIMER_SOFTIRQ);