Based on discussions on the svn-src mailing list, rework r218195:

 - entirely eliminate some calls to uio_yeild() as being unnecessary,
   such as in a sysctl handler.

 - move should_yield() and maybe_yield() to kern_synch.c and move the
   prototypes from sys/uio.h to sys/proc.h

 - add a slightly more generic kern_yield() that can replace the
   functionality of uio_yield().

 - replace source uses of uio_yield() with the functional equivalent,
   or in some cases do not change the thread priority when switching.

 - fix a logic inversion bug in vlrureclaim(), pointed out by bde@.

 - instead of using the per-cpu last switched ticks, use a per thread
   variable for should_yield().  With PREEMPTION, the only reasonable
   use of this is to determine if a lock has been held a long time and
   relinquish it.  Without PREEMPTION, this is essentially the same as
   the per-cpu variable.

1 files changed, 33 insertions, 2 deletions

diff --git a/sys/kern/kern_synch.c b/sys/kern/kern_synch.c
index ddc2186..a3e920d 100644
--- a/sys/kern/kern_synch.c
+++ b/sys/kern/kern_synch.c
@@ -413,9 +413,10 @@ mi_switch(int flags, struct thread *newtd)
 	 */
 	if (kdb_active)
 		kdb_switch();
-	if (flags & SW_VOL)
+	if (flags & SW_VOL) {
 		td->td_ru.ru_nvcsw++;
-	else
+		td->td_swvoltick = ticks;
+	} else
 		td->td_ru.ru_nivcsw++;
 #ifdef SCHED_STATS
 	SCHED_STAT_INC(sched_switch_stats[flags & SW_TYPE_MASK]);
@@ -538,6 +539,36 @@ synch_setup(void *dummy)
 	loadav(NULL);
 }
 
+int
+should_yield(void)
+{
+
+	return (ticks - curthread->td_swvoltick >= hogticks);
+}
+
+void
+maybe_yield(void)
+{
+
+	if (should_yield())
+		kern_yield(curthread->td_user_pri);
+}
+
+void
+kern_yield(int prio)
+{
+	struct thread *td;
+
+	td = curthread;
+	DROP_GIANT();
+	thread_lock(td);
+	if (prio >= 0)
+		sched_prio(td, prio);
+	mi_switch(SW_VOL | SWT_RELINQUISH, NULL);
+	thread_unlock(td);
+	PICKUP_GIANT();
+}
+
 /*
  * General purpose yield system call.
  */