diff --git a/sys/kern/sched_4bsd.c b/sys/kern/sched_4bsd.c
--- a/sys/kern/sched_4bsd.c
+++ b/sys/kern/sched_4bsd.c
@@ -703,6 +703,10 @@
 	return (imax(1, (sched_slice * hz + realstathz / 2) / realstathz));
 }
 
+SCHED_STAT_DEFINE(ithread_demotions, "Interrupt thread priority demotions");
+SCHED_STAT_DEFINE(ithread_preemptions,
+    "Interrupt thread preemptions due to time-sharing");
+
 /*
  * We adjust the priority of the current process.  The priority of a
  * process gets worse as it accumulates CPU time.  The cpu usage
@@ -739,7 +743,20 @@
 	 */
 	if (!TD_IS_IDLETHREAD(td) && --ts->ts_slice <= 0) {
 		ts->ts_slice = sched_slice;
-		td->td_flags |= TDF_NEEDRESCHED | TDF_SLICEEND;
+
+		/*
+		 * If an ithread uses a full quantum, demote its
+		 * priority and preempt it.
+		 */
+		if (PRI_BASE(td->td_pri_class) == PRI_ITHD) {
+			SCHED_STAT_INC(ithread_preemptions);
+			td->td_owepreempt = 1;
+			if (td->td_base_pri + RQ_PPQ < PRI_MAX_ITHD) {
+				SCHED_STAT_INC(ithread_demotions);
+				sched_prio(td, td->td_base_pri + RQ_PPQ);
+			}
+		} else
+			td->td_flags |= TDF_NEEDRESCHED | TDF_SLICEEND;
 	}
 
 	stat = DPCPU_PTR(idlestat);
@@ -1134,6 +1151,15 @@
 	td->td_slptick = 0;
 	ts->ts_slptime = 0;
 	ts->ts_slice = sched_slice;
+
+	/*
+	 * When resuming an idle ithread, restore its base ithread
+	 * priority.
+	 */
+	if (PRI_BASE(td->td_pri_class) == PRI_ITHD &&
+	    td->td_base_pri != td->td_base_ithread_pri)
+		sched_prio(td, td->td_base_ithread_pri);
+
 	sched_add(td, srqflags);
 }