Index: sys/kern/sched_ule.c
===================================================================
--- sys/kern/sched_ule.c
+++ sys/kern/sched_ule.c
@@ -298,6 +298,9 @@
 #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 +2339,15 @@
 		sched_interact_update(td);
 		sched_pctcpu_update(ts, 0);
 	}
+
+	/*
+	 * When resuming an idle thread, 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.
 	 */
@@ -2505,7 +2517,7 @@
 
 /*
  * Handle a stathz tick.  This is really only relevant for timeshare
- * threads.
+ * and interrupt threads.
  */
 void
 sched_clock(struct thread *td, int cnt)
@@ -2557,6 +2569,34 @@
 		sched_priority(td);
 	}
 
+	if (PRI_BASE(td->td_pri_class) == PRI_ITHD) {
+		/*
+		 * If an ithread uses a full quantum, demote its
+		 * priority and preempt it if there is another ithread
+		 * of the same or higher priority to run.
+		 *
+		 * The check for preemption is performed before
+		 * demoting the priority so that the current thread
+		 * isn't preempted to run an ithread in the priority
+		 * band the running thread is being demoted to while
+		 * still ensuring that preemptions occur once the
+		 * ithread is demoted to the lowest ithread priority.
+		 */
+		ts->ts_slice += cnt;
+		if (ts->ts_slice >= sched_slice) {
+			ts->ts_slice = 0;
+			if (td->td_priority >= tdq->tdq_lowpri) {
+				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);
+			}
+		}
+		return;
+	}
+
 	/*
 	 * Force a context switch if the current thread has used up a full
 	 * time slice (default is 100ms).
@@ -3270,3 +3310,12 @@
 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");