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>
master commit: 054b6dfb61eab00d86ddd5d0ac508f5302da0d52
master date: 2014-05-28 10:07:50 +0200

diff --git a/xen/common/timer.c b/xen/common/timer.c
index 39de52dc359022d1d9f34473a69f47dd704d330c..5a7e0f630ccaedce5c83764999a27b05ea30c783 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);