Index: sys/kern/sched_ule.c
===================================================================
--- sys/kern/sched_ule.c
+++ sys/kern/sched_ule.c
@@ -45,6 +45,7 @@
 
 #include <sys/param.h>
 #include <sys/systm.h>
+#include <sys/counter.h>
 #include <sys/kdb.h>
 #include <sys/kernel.h>
 #include <sys/ktr.h>
@@ -298,6 +299,9 @@
 #define	TDQ_CPU(x)	(&tdq_cpu)
 #endif
 
+static COUNTER_U64_DEFINE_EARLY(ithread_demotions);
+static COUNTER_U64_DEFINE_EARLY(ithread_preemptions);
+
 #define	TDQ_LOCK_ASSERT(t, type)	mtx_assert(TDQ_LOCKPTR((t)), (type))
 #define	TDQ_LOCK(t)		mtx_lock_spin(TDQ_LOCKPTR((t)))
 #define	TDQ_LOCK_FLAGS(t, f)	mtx_lock_spin_flags(TDQ_LOCKPTR((t)), (f))
@@ -2336,6 +2340,15 @@
 		sched_interact_update(td);
 		sched_pctcpu_update(ts, 0);
 	}
+
+	/*
+	 * When resuming an idle ithread, restore its base ithread
+	 * priority.
+	 */
+	if (PRI_BASE(td->td_pri_class) == PRI_ITHD &&
+	    td->td_priority != td->td_base_ithread_pri)
+		sched_prio(td, td->td_base_ithread_pri);
+
 	/*
 	 * Reset the slice value since we slept and advanced the round-robin.
 	 */
@@ -2503,9 +2516,21 @@
 	thread_unlock(td);
 }
 
+/*
+ * Return time slice for a given thread.  For ithreads this is
+ * sched_slice.  For other threads it is tdq_slice(tdq).
+ */
+static inline int
+td_slice(struct thread *td, struct tdq *tdq)
+{
+	if (PRI_BASE(td->td_pri_class) == PRI_ITHD)
+		return (sched_slice);
+	return (tdq_slice(tdq));
+}
+
 /*
  * Handle a stathz tick.  This is really only relevant for timeshare
- * threads.
+ * and interrupt threads.
  */
 void
 sched_clock(struct thread *td, int cnt)
@@ -2562,9 +2587,22 @@
 	 * time slice (default is 100ms).
 	 */
 	ts->ts_slice += cnt;
-	if (ts->ts_slice >= tdq_slice(tdq)) {
+	if (ts->ts_slice >= td_slice(td, tdq)) {
 		ts->ts_slice = 0;
-		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) {
+			counter_u64_add(ithread_preemptions, 1);
+			td->td_owepreempt = 1;
+			if (td->td_base_pri + RQ_PPQ < PRI_MAX_ITHD) {
+				counter_u64_add(ithread_demotions, 1);
+				sched_prio(td, td->td_base_pri + RQ_PPQ);
+			}
+		} else
+			td->td_flags |= TDF_NEEDRESCHED | TDF_SLICEEND;
 	}
 }
 
@@ -3270,3 +3308,12 @@
 static int ccpu = 0;
 SYSCTL_INT(_kern, OID_AUTO, ccpu, CTLFLAG_RD, &ccpu, 0,
     "Decay factor used for updating %CPU in 4BSD scheduler");
+
+SYSCTL_NODE(_kern_sched, OID_AUTO, ithread, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
+    "Interrupt thread stats");
+SYSCTL_COUNTER_U64(_kern_sched_ithread, OID_AUTO, demotions,
+    CTLFLAG_RD, &ithread_demotions,
+    "Count of interrupt thread priority demotions");
+SYSCTL_COUNTER_U64(_kern_sched_ithread, OID_AUTO, preemptions,
+    CTLFLAG_RD, &ithread_preemptions,
+    "Count of interrupt thread preemptions due to time-sharing");