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.

sys/dev/sio/sio.c

@@ -1466,7 +1466,6 @@ sysctl_siots(SYSCTL_HANDLER_ARGS)
 		error = SYSCTL_OUT(req, buf, len);
 		if (error != 0)
 			return (error);
-		uio_yield();
 	}
 	return (0);
 }

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.
  */

sys/kern/kern_sysctl.c

@@ -1568,7 +1568,7 @@ userland_sysctl(struct thread *td, int *name, u_int namelen, void *old,
 		SYSCTL_XUNLOCK();
 		if (error != EAGAIN)
 			break;
-		uio_yield();
+		kern_yield(curthread->td_user_pri);
 	}
 
 	CURVNET_RESTORE();

sys/kern/subr_uio.c

@@ -352,33 +352,11 @@ ureadc(int c, struct uio *uio)
 	return (0);
 }
 
-int
-should_yield(void)
-{
-
-	return (ticks - PCPU_GET(switchticks) >= hogticks);
-}
-
-void
-maybe_yield(void)
-{
-
-	if (should_yield())
-		uio_yield();
-}
-
 void
 uio_yield(void)
 {
-	struct thread *td;
 
-	td = curthread;
-	DROP_GIANT();
-	thread_lock(td);
-	sched_prio(td, td->td_user_pri);
-	mi_switch(SW_INVOL | SWT_RELINQUISH, NULL);
-	thread_unlock(td);
-	PICKUP_GIANT();
+	kern_yield(curthread->td_user_pri);
 }
 
 int

sys/kern/vfs_bio.c

@@ -2234,7 +2234,7 @@ buf_daemon()
 		while (numdirtybuffers > lodirtybuffers) {
 			if (buf_do_flush(NULL) == 0)
 				break;
-			uio_yield();
+			kern_yield(-1);
 		}
 		lodirtybuffers = lodirtysave;
 

sys/kern/vfs_mount.c

@@ -1661,7 +1661,7 @@ __mnt_vnode_next(struct vnode **mvp, struct mount *mp)
 	KASSERT((*mvp)->v_mount == mp, ("marker vnode mount list mismatch"));
 	if (should_yield()) {
 		MNT_IUNLOCK(mp);
-		uio_yield();
+		kern_yield(-1);
 		MNT_ILOCK(mp);
 	}
 	vp = TAILQ_NEXT(*mvp, v_nmntvnodes);

sys/kern/vfs_subr.c

@@ -707,15 +707,15 @@ vlrureclaim(struct mount *mp)
 		vdropl(vp);
 		done++;
 next_iter_mntunlocked:
-		if (should_yield())
+		if (!should_yield())
 			goto relock_mnt;
 		goto yield;
 next_iter:
-		if (should_yield())
+		if (!should_yield())
 			continue;
 		MNT_IUNLOCK(mp);
 yield:
-		uio_yield();
+		kern_yield(-1);
 relock_mnt:
 		MNT_ILOCK(mp);
 	}
@@ -828,7 +828,7 @@ vnlru_proc(void)
 			vnlru_nowhere++;
 			tsleep(vnlruproc, PPAUSE, "vlrup", hz * 3);
 		} else
-			uio_yield();
+			kern_yield(-1);
 	}
 }
 

sys/kern/vfs_vnops.c

@@ -444,7 +444,7 @@ vn_rdwr(rw, vp, base, len, offset, segflg, ioflg, active_cred, file_cred,
  * Package up an I/O request on a vnode into a uio and do it.  The I/O
  * request is split up into smaller chunks and we try to avoid saturating
  * the buffer cache while potentially holding a vnode locked, so we 
- * check bwillwrite() before calling vn_rdwr().  We also call uio_yield()
+ * check bwillwrite() before calling vn_rdwr().  We also call kern_yield()
  * to give other processes a chance to lock the vnode (either other processes
  * core'ing the same binary, or unrelated processes scanning the directory).
  */
@@ -491,7 +491,7 @@ vn_rdwr_inchunks(rw, vp, base, len, offset, segflg, ioflg, active_cred,
 			break;
 		offset += chunk;
 		base = (char *)base + chunk;
-		uio_yield();
+		kern_yield(curthread->td_user_pri);
 	} while (len);
 	if (aresid)
 		*aresid = len + iaresid;

sys/pc98/cbus/sio.c

@@ -2250,7 +2250,6 @@ sysctl_siots(SYSCTL_HANDLER_ARGS)
 		error = SYSCTL_OUT(req, buf, len);
 		if (error != 0)
 			return (error);
-		uio_yield();
 	}
 	return (0);
 }

sys/sys/proc.h

@@ -242,6 +242,7 @@ struct thread {
 	u_int		td_estcpu;	/* (t) estimated cpu utilization */
 	int		td_slptick;	/* (t) Time at sleep. */
 	int		td_blktick;	/* (t) Time spent blocked. */
+	int		td_swvoltick;	/* (t) Time at last SW_VOL switch. */
 	struct rusage	td_ru;		/* (t) rusage information. */
 	struct rusage_ext td_rux;	/* (t) Internal rusage information. */
 	uint64_t	td_incruntime;	/* (t) Cpu ticks to transfer to proc. */
@@ -822,9 +823,11 @@ void	fork_exit(void (*)(void *, struct trapframe *), void *,
 	    struct trapframe *);
 void	fork_return(struct thread *, struct trapframe *);
 int	inferior(struct proc *p);
+void	kern_yield(int);
 void 	kick_proc0(void);
 int	leavepgrp(struct proc *p);
 int	maybe_preempt(struct thread *td);
+void	maybe_yield(void);
 void	mi_switch(int flags, struct thread *newtd);
 int	p_candebug(struct thread *td, struct proc *p);
 int	p_cansee(struct thread *td, struct proc *p);
@@ -847,6 +850,7 @@ void	sess_hold(struct session *);
 void	sess_release(struct session *);
 int	setrunnable(struct thread *);
 void	setsugid(struct proc *p);
+int	should_yield(void);
 int	sigonstack(size_t sp);
 void	sleepinit(void);
 void	stopevent(struct proc *, u_int, u_int);

sys/sys/uio.h

@@ -95,8 +95,6 @@ int	copyinstrfrom(const void * __restrict src, void * __restrict dst,
 	    size_t len, size_t * __restrict copied, int seg);
 int	copyinuio(struct iovec *iovp, u_int iovcnt, struct uio **uiop);
 void	uio_yield(void);
-void	maybe_yield(void);
-int	should_yield(void);
 int	uiomove(void *cp, int n, struct uio *uio);
 int	uiomove_frombuf(void *buf, int buflen, struct uio *uio);
 int	uiomove_fromphys(struct vm_page *ma[], vm_offset_t offset, int n,

sys/ufs/ffs/ffs_softdep.c

@@ -1380,7 +1380,7 @@ softdep_process_worklist(mp, full)
 		 */
 		if (should_yield()) {
 			FREE_LOCK(&lk);
-			uio_yield();
+			kern_yield(-1);
 			bwillwrite();
 			ACQUIRE_LOCK(&lk);
 		}