Index: sys/kern/sched_4bsd.c
===================================================================
--- sys/kern/sched_4bsd.c
+++ sys/kern/sched_4bsd.c
@@ -42,6 +42,7 @@
 
 #include <sys/param.h>
 #include <sys/systm.h>
+#include <sys/counter.h>
 #include <sys/cpuset.h>
 #include <sys/kernel.h>
 #include <sys/ktr.h>
@@ -262,6 +263,19 @@
 	   "allow threads to share a quantum");
 #endif
 
+SYSCTL_NODE(_kern_sched, OID_AUTO, ithread, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
+    "Interrupt thread stats");
+
+static COUNTER_U64_DEFINE_EARLY(ithread_demotions);
+SYSCTL_COUNTER_U64(_kern_sched_ithread, OID_AUTO, demotions,
+    CTLFLAG_RD, &ithread_demotions,
+    "Count of interrupt thread priority demotions");
+
+static COUNTER_U64_DEFINE_EARLY(ithread_preemptions);
+SYSCTL_COUNTER_U64(_kern_sched_ithread, OID_AUTO, preemptions,
+    CTLFLAG_RD, &ithread_preemptions,
+    "Count of interrupt thread preemptions due to time-sharing");
+
 SDT_PROVIDER_DEFINE(sched);
 
 SDT_PROBE_DEFINE3(sched, , , change__pri, "struct thread *", 
@@ -739,7 +753,20 @@
 	 */
 	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) {
+			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;
 	}
 
 	stat = DPCPU_PTR(idlestat);
@@ -1134,6 +1161,15 @@
 	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);
 }