1 files changed, 47 insertions, 38 deletions

diff --git a/kernel/timer.c b/kernel/timer.c
index 396a3c024c2c..2a87430a58d4 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -891,6 +891,7 @@ int do_settimeofday(struct timespec *tv)
 	set_normalized_timespec(&xtime, sec, nsec);
 	set_normalized_timespec(&wall_to_monotonic, wtm_sec, wtm_nsec);
 
+	clock->error = 0;
 	ntp_clear();
 
 	write_sequnlock_irqrestore(&xtime_lock, flags);
@@ -1008,52 +1009,52 @@ static int __init timekeeping_init_device(void)
 device_initcall(timekeeping_init_device);
 
 /*
- * If the error is already larger, we look ahead another tick,
+ * If the error is already larger, we look ahead even further
  * to compensate for late or lost adjustments.
  */
-static __always_inline int clocksource_bigadjust(int sign, s64 error, s64 *interval, s64 *offset)
+static __always_inline int clocksource_bigadjust(s64 error, s64 *interval, s64 *offset)
 {
-	int adj;
+	s64 tick_error, i;
+	u32 look_ahead, adj;
+	s32 error2, mult;
 
 	/*
-	 * As soon as the machine is synchronized to the external time
-	 * source this should be the common case.
+	 * Use the current error value to determine how much to look ahead.
+	 * The larger the error the slower we adjust for it to avoid problems
+	 * with losing too many ticks, otherwise we would overadjust and
+	 * produce an even larger error.  The smaller the adjustment the
+	 * faster we try to adjust for it, as lost ticks can do less harm
+	 * here.  This is tuned so that an error of about 1 msec is adusted
+	 * within about 1 sec (or 2^20 nsec in 2^SHIFT_HZ ticks).
 	 */
-	error >>= 2;
-	if (likely(sign > 0 ? error <= *interval : error >= *interval))
-		return sign;
+	error2 = clock->error >> (TICK_LENGTH_SHIFT + 22 - 2 * SHIFT_HZ);
+	error2 = abs(error2);
+	for (look_ahead = 0; error2 > 0; look_ahead++)
+		error2 >>= 2;
 
 	/*
-	 * An extra look ahead dampens the effect of the current error,
-	 * which can grow quite large with continously late updates, as
-	 * it would dominate the adjustment value and can lead to
-	 * oscillation.
+	 * Now calculate the error in (1 << look_ahead) ticks, but first
+	 * remove the single look ahead already included in the error.
 	 */
-	error += current_tick_length() >> (TICK_LENGTH_SHIFT - clock->shift + 1);
-	error -= clock->xtime_interval >> 1;
-
-	adj = 0;
-	while (1) {
-		error >>= 1;
-		if (sign > 0 ? error <= *interval : error >= *interval)
-			break;
-		adj++;
+	tick_error = current_tick_length() >> (TICK_LENGTH_SHIFT - clock->shift + 1);
+	tick_error -= clock->xtime_interval >> 1;
+	error = ((error - tick_error) >> look_ahead) + tick_error;
+
+	/* Finally calculate the adjustment shift value.  */
+	i = *interval;
+	mult = 1;
+	if (error < 0) {
+		error = -error;
+		*interval = -*interval;
+		*offset = -*offset;
+		mult = -1;
 	}
-
-	/*
-	 * Add the current adjustments to the error and take the offset
-	 * into account, the latter can cause the error to be hardly
-	 * reduced at the next tick. Check the error again if there's
-	 * room for another adjustment, thus further reducing the error
-	 * which otherwise had to be corrected at the next update.
-	 */
-	error = (error << 1) - *interval + *offset;
-	if (sign > 0 ? error > *interval : error < *interval)
-		adj++;
+	for (adj = 0; error > i; adj++)
+		error >>= 1;
 
 	*interval <<= adj;
 	*offset <<= adj;
-	return sign << adj;
+	return mult << adj;
 }
 
 /*
@@ -1068,11 +1069,19 @@ static void clocksource_adjust(struct clocksource *clock, s64 offset)
 
 	error = clock->error >> (TICK_LENGTH_SHIFT - clock->shift - 1);
 	if (error > interval) {
-		adj = clocksource_bigadjust(1, error, &interval, &offset);
+		error >>= 2;
+		if (likely(error <= interval))
+			adj = 1;
+		else
+			adj = clocksource_bigadjust(error, &interval, &offset);
 	} else if (error < -interval) {
-		interval = -interval;
-		offset = -offset;
-		adj = clocksource_bigadjust(-1, error, &interval, &offset);
+		error >>= 2;
+		if (likely(error >= -interval)) {
+			adj = -1;
+			interval = -interval;
+			offset = -offset;
+		} else
+			adj = clocksource_bigadjust(error, &interval, &offset);
 	} else
 		return;
 
@@ -1129,7 +1138,7 @@ static void update_wall_time(void)
 	clocksource_adjust(clock, offset);
 
 	/* store full nanoseconds into xtime */
-	xtime.tv_nsec = clock->xtime_nsec >> clock->shift;
+	xtime.tv_nsec = (s64)clock->xtime_nsec >> clock->shift;
 	clock->xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;
 
 	/* check to see if there is a new clocksource to use */