Index: sys/sys/callout.h
===================================================================
--- sys/sys/callout.h	(revision 242516)
+++ sys/sys/callout.h	(working copy)
@@ -51,29 +51,23 @@
 #define	CALLOUT_DIRECT 		0x0100 /* allow exec from hw int context */
 
 #define	C_DIRECT_EXEC		0x0001 /* direct execution of callout */
-#define	C_P1S			0x0002 /* fields related to precision */ 
-#define	C_P500MS		0x0006 	
-#define	C_P250MS		0x000a 	
-#define	C_P125MS		0x000e
-#define	C_P64MS			0x0012
-#define	C_P32MS			0x0016
-#define	C_P16MS			0x001a
-#define	C_P8MS			0x001e
-#define	C_P4MS			0x0022
-#define	C_P2MS			0x0026
-#define	C_P1MS			0x002a
-#define	C_P500US		0x002e
-#define	C_P250US		0x0032
-#define	C_P125US		0x0036
-#define	C_P64US			0x003a
-#define	C_P32US			0x003e
-#define	C_P16US			0x0042
-#define	C_P8US			0x0046
-#define	C_P4US			0x004a
-#define	C_P2US			0x004e
-#define	PRECISION_BITS		7	
-#define	PRECISION_RANGE		((1 << PRECISION_BITS) - 1)	
+#define C_PRECISION		0x0002 /* precision specified */
+#define	C_PREC(x)		(((x) << 2) | C_PRECISION)
 
+/*
+ * Common values used for precision. In some cases, e.g. 1ms, the precision
+ * value is approximated to the nearest power-of-two, as long as we deal with
+ * that timescale.
+ */
+#define C_P1S			(C_PREC(0))	
+#define	C_P500MS		(C_PREC(1))
+#define C_P1MS			(C_PREC(10))
+
+u_int precision_flags, pthreshold;
+
+#define TIMESEL(x, bt)	\
+   (((x) < (pthreshold)) ? binuptime(&bt) : getbinuptime(&bt))
+
 struct callout_handle {
 	struct callout *callout;
 };
Index: sys/kern/kern_timeout.c
===================================================================
--- sys/kern/kern_timeout.c	(revision 242516)
+++ sys/kern/kern_timeout.c	(working copy)
@@ -94,6 +94,11 @@ SYSCTL_INT(_debug, OID_AUTO, to_avg_mpcalls_dir, C
     0, "Average number of MP direct callouts made per callout_process call. "
     "Units = 1/1000");
 #endif
+SYSCTL_INT(_kern, OID_AUTO, pthreshold, CTLFLAG_RW, &pthreshold, 0, 
+    "Precision threshold for callout time-interval measurements");
+SYSCTL_INT(_kern, OID_AUTO, precision_flags, CTLFLAG_RW, &precision_flags, 0,
+    "Precision tolerance for callouts");
+
 /*
  * TODO:
  *	allocate more timeout table slots when table overflows.
@@ -170,7 +175,6 @@ struct callout_cpu cc_cpu;
 #define	CC_LOCK(cc)	mtx_lock_spin(&(cc)->cc_lock)
 #define	CC_UNLOCK(cc)	mtx_unlock_spin(&(cc)->cc_lock)
 #define	CC_LOCK_ASSERT(cc)	mtx_assert(&(cc)->cc_lock, MA_OWNED)
-#define	C_PRECISION	0x2
 
 #define FREQ2BT(freq, bt)                                               \
 {                                                                       \
@@ -369,6 +373,21 @@ start_softclock(void *dummy)
 
 SYSINIT(start_softclock, SI_SUB_SOFTINTR, SI_ORDER_FIRST, start_softclock, NULL);
 
+static void
+callout_init_precision(void) 
+{
+	u_int pow2;
+
+	pthreshold = 20 * imin(1000000000 / hz, 1000000);  
+	pow2 = (pthreshold & (pthreshold - 1)) == 0;
+	precision_flags = fls(pthreshold);
+	if (pow2)
+		precision_flags += 1;
+}
+
+SYSINIT(callout_init_precision, SI_SUB_CLOCKS, SI_ORDER_ANY,
+    callout_init_precision, NULL); 
+
 static inline int
 callout_hash(struct bintime *bt)
 {
@@ -567,7 +586,7 @@ callout_cc_add(struct callout *c, struct callout_c
 	c->c_time = to_bintime;
 	bintime_clear(&c->c_precision);
 	if (flags & C_PRECISION) {
-		r_shift = ((flags >> 2) & PRECISION_RANGE);
+		r_shift = flags >> 2;
 		r_val = (r_shift != 0) ? (uint64_t)1 << (64 - r_shift) : 0;
 		/*
 		 * Round as far as precision specified is coarse (up to 8ms).