Index: head/sys/kern/kern_timeout.c
===================================================================
--- head/sys/kern/kern_timeout.c
+++ head/sys/kern/kern_timeout.c
@@ -65,6 +65,7 @@
 
 #ifdef DDB
 #include <ddb/ddb.h>
+#include <ddb/db_sym.h>
 #include <machine/_inttypes.h>
 #endif
 
@@ -143,12 +144,14 @@
 struct cc_exec {
 	struct callout		*cc_curr;
 	void			(*cc_drain)(void *);
+	void			*cc_last_func;
+	void			*cc_last_arg;
 #ifdef SMP
 	void			(*ce_migration_func)(void *);
 	void			*ce_migration_arg;
-	int			ce_migration_cpu;
 	sbintime_t		ce_migration_time;
 	sbintime_t		ce_migration_prec;
+	int			ce_migration_cpu;
 #endif
 	bool			cc_cancel;
 	bool			cc_waiting;
@@ -177,6 +180,8 @@
 #define	callout_migrating(c)	((c)->c_iflags & CALLOUT_DFRMIGRATION)
 
 #define	cc_exec_curr(cc, dir)		cc->cc_exec_entity[dir].cc_curr
+#define	cc_exec_last_func(cc, dir)	cc->cc_exec_entity[dir].cc_last_func
+#define	cc_exec_last_arg(cc, dir)	cc->cc_exec_entity[dir].cc_last_arg
 #define	cc_exec_drain(cc, dir)		cc->cc_exec_entity[dir].cc_drain
 #define	cc_exec_next(cc)		cc->cc_next
 #define	cc_exec_cancel(cc, dir)		cc->cc_exec_entity[dir].cc_cancel
@@ -686,6 +691,8 @@
 		c->c_iflags &= ~CALLOUT_PENDING;
 	
 	cc_exec_curr(cc, direct) = c;
+	cc_exec_last_func(cc, direct) = c_func;
+	cc_exec_last_arg(cc, direct) = c_arg;
 	cc_exec_cancel(cc, direct) = false;
 	cc_exec_drain(cc, direct) = NULL;
 	CC_UNLOCK(cc);
@@ -1669,5 +1676,43 @@
 	}
 
 	_show_callout((struct callout *)addr);
+}
+
+static void
+_show_last_callout(int cpu, int direct, const char *dirstr)
+{
+	struct callout_cpu *cc;
+	void *func, *arg;
+
+	cc = CC_CPU(cpu);
+	func = cc_exec_last_func(cc, direct);
+	arg = cc_exec_last_arg(cc, direct);
+	db_printf("cpu %d last%s callout function: %p ", cpu, dirstr, func);
+	db_printsym((db_expr_t)func, DB_STGY_ANY);
+	db_printf("\ncpu %d last%s callout argument: %p\n", cpu, dirstr, arg);
+}
+
+DB_SHOW_COMMAND(callout_last, db_show_callout_last)
+{
+	int cpu, last;
+
+	if (have_addr) {
+		if (addr < 0 || addr > mp_maxid || CPU_ABSENT(addr)) {
+			db_printf("no such cpu: %d\n", (int)addr);
+			return;
+		}
+		cpu = last = addr;
+	} else {
+		cpu = 0;
+		last = mp_maxid;
+	}
+
+	while (cpu <= last) {
+		if (!CPU_ABSENT(cpu)) {
+			_show_last_callout(cpu, 0, "");
+			_show_last_callout(cpu, 1, " direct");
+		}
+		cpu++;
+	}
 }
 #endif /* DDB */
Index: head/tools/test/callout_free/Makefile
===================================================================
--- head/tools/test/callout_free/Makefile
+++ head/tools/test/callout_free/Makefile
@@ -0,0 +1,6 @@
+# $FreeBSD$
+
+KMOD=	callout_free
+SRCS=	callout_free.c
+
+.include <bsd.kmod.mk>
Index: head/tools/test/callout_free/callout_free.c
===================================================================
--- head/tools/test/callout_free/callout_free.c
+++ head/tools/test/callout_free/callout_free.c
@@ -0,0 +1,87 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2019 Eric van Gyzen
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+/*
+ * Free a pending callout.  This was useful for testing the
+ * "show callout_last" ddb command.
+ */
+
+#include <sys/param.h>
+#include <sys/conf.h>
+#include <sys/kernel.h>
+#include <sys/lock.h>
+#include <sys/module.h>
+#include <sys/mutex.h>
+#include <sys/systm.h>
+
+static struct callout callout_free;
+static struct mtx callout_free_mutex;
+static int callout_free_arg;
+
+static void
+callout_free_func(void *arg)
+{
+	printf("squirrel!\n");
+	mtx_destroy(&callout_free_mutex);
+	memset(&callout_free, 'C', sizeof(callout_free));
+}
+
+static int
+callout_free_load(module_t mod, int cmd, void *arg)
+{
+	int error;
+
+	switch (cmd) {
+	case MOD_LOAD:
+		mtx_init(&callout_free_mutex, "callout_free", NULL, MTX_DEF);
+		/*
+		 * Do not pass CALLOUT_RETURNUNLOCKED so the callout
+		 * subsystem will unlock the "destroyed" mutex.
+		 */
+		callout_init_mtx(&callout_free, &callout_free_mutex, 0);
+		printf("callout_free_func = %p\n", callout_free_func);
+		printf("callout_free_arg = %p\n", &callout_free_arg);
+		callout_reset(&callout_free, hz/10, callout_free_func,
+		    &callout_free_arg);
+		error = 0;
+		break;
+
+	case MOD_UNLOAD:
+		error = 0;
+		break;
+
+	default:
+		error = EOPNOTSUPP;
+		break;
+	}
+
+	return (error);
+}
+
+DEV_MODULE(callout_free, callout_free_load, NULL);