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 */