Index: sys/lwp.h
===================================================================
RCS file: /cvsroot/src/sys/sys/lwp.h,v
retrieving revision 1.179
diff -p -u -r1.179 lwp.h
--- sys/lwp.h	19 Apr 2018 21:19:07 -0000	1.179
+++ sys/lwp.h	28 Nov 2018 12:04:10 -0000
@@ -258,6 +258,7 @@ extern int		maxlwp __read_mostly;	/* max
 #define	LP_VFORKWAIT	0x00000200 /* Waiting at vfork() for a child */
 #define	LP_SINGLESTEP	0x00000400 /* Single step thread in ptrace(2) */
 #define	LP_TIMEINTR	0x00010000 /* Time this soft interrupt */
+#define	LP_PREEMPTING	0x00020000 /* mi_switch called involuntarily */
 #define	LP_RUNNING	0x20000000 /* Active on a CPU */
 #define	LP_BOUND	0x80000000 /* Bound to a CPU */
 
Index: kern/kern_synch.c
===================================================================
RCS file: /cvsroot/src/sys/kern/kern_synch.c,v
retrieving revision 1.318
diff -p -u -r1.318 kern_synch.c
--- kern/kern_synch.c	14 Aug 2018 01:06:01 -0000	1.318
+++ kern/kern_synch.c	28 Nov 2018 12:04:14 -0000
@@ -1,4 +1,4 @@
-/*	$NetBSD: kern_synch.c,v 1.318 2018/08/14 01:06:01 ozaki-r Exp $	*/
+/*	$NetBSD: kern_synch.c,v 1.319 2018/11/28 09:44:49 mlelstv Exp $	*/
 
 /*-
  * Copyright (c) 1999, 2000, 2004, 2006, 2007, 2008, 2009
@@ -69,7 +69,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_synch.c,v 1.318 2018/08/14 01:06:01 ozaki-r Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_synch.c,v 1.319 2018/11/28 09:44:49 mlelstv Exp $");
 
 #include "opt_kstack.h"
 #include "opt_dtrace.h"
@@ -286,12 +286,16 @@ preempt(void)
 {
 	struct lwp *l = curlwp;
 
+	/* check if the scheduler has another LWP to run */
+	if ((l->l_cpu->ci_schedstate.spc_flags & SPCF_SHOULDYIELD) == 0)
+		return;
+
 	KERNEL_UNLOCK_ALL(l, &l->l_biglocks);
 	lwp_lock(l);
 	KASSERT(lwp_locked(l, l->l_cpu->ci_schedstate.spc_lwplock));
 	KASSERT(l->l_stat == LSONPROC);
 	l->l_kpriority = false;
-	l->l_nivcsw++;
+	l->l_pflag |= LP_PREEMPTING;
 	(void)mi_switch(l);
 	KERNEL_LOCK(l->l_biglocks, l);
 }
@@ -649,6 +653,9 @@ mi_switch(lwp_t *l)
 		KASSERT(l->l_ctxswtch == 0);
 		l->l_ctxswtch = 1;
 		l->l_ncsw++;
+		if ((l->l_pflag & LP_PREEMPTING) != 0)
+			l->l_nivcsw++;
+		l->l_pflag &= ~LP_PREEMPTING;
 		KASSERT((l->l_pflag & LP_RUNNING) != 0);
 		l->l_pflag &= ~LP_RUNNING;
 
@@ -752,6 +759,7 @@ mi_switch(lwp_t *l)
 		/* Nothing to do - just unlock and return. */
 		pserialize_switchpoint();
 		mutex_spin_exit(spc->spc_mutex);
+		l->l_pflag &= ~LP_PREEMPTING;
 		lwp_unlock(l);
 		retval = 0;
 	}