- Add a sched_preempt() routine to be called by md code after IPI_PREEMPT is

delivered.
 - Add a simple implementation to 4bsd.

sys/kern/sched_4bsd.c

@@ -1243,6 +1243,17 @@ sched_choose(void)
 	return (PCPU_GET(idlethread));
 }
 
+void
+sched_preempt(struct thread *td)
+{
+	thread_lock(td);
+	if (td->td_critnest > 1)
+		td->td_owepreempt = 1;
+	else
+		mi_switch(SW_INVOL | SW_PREEMPT, NULL);
+	thread_unlock(td);
+}
+
 void
 sched_userret(struct thread *td)
 {

sys/sys/sched.h

@@ -108,6 +108,7 @@ void	sched_unlend_user_prio(struct thread *td, u_char pri);
 void	sched_user_prio(struct thread *td, u_char prio);
 void	sched_userret(struct thread *td);
 void	sched_wakeup(struct thread *td);
+void	sched_preempt(struct thread *td);
 
 /*
  * Threads are moved on and off of run queues