[PATCH] fix and optimize clock source update

This fixes the clock source updates in update_wall_time() to correctly
track the time coming in via current_tick_length().  Optimize the fast
paths to be as short as possible to keep the overhead low.

Signed-off-by: Roman Zippel <zippel@linux-m68k.org>
Acked-by: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 109 insertions, 42 deletions

diff --git a/kernel/timer.c b/kernel/timer.c
index 890a56937cfa..5bb6b7976eec 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -770,7 +770,7 @@ static void update_ntp_one_tick(void)
  * specified number of bits to the right of the binary point.
  * This function has no side-effects.
  */
-u64 current_tick_length(long shift)
+u64 current_tick_length(void)
 {
         long delta_nsec;
         u64 ret;
@@ -779,14 +779,8 @@ u64 current_tick_length(long shift)
          *    ie: nanosecond value shifted by (SHIFT_SCALE - 10)
          */
         delta_nsec = tick_nsec + adjtime_adjustment() * 1000;
-        ret = ((u64) delta_nsec << (SHIFT_SCALE - 10)) + time_adj;
-
-        /* convert from (SHIFT_SCALE - 10) to specified shift scale: */
-        shift = shift - (SHIFT_SCALE - 10);
-        if (shift < 0)
-                ret >>= -shift;
-        else
-                ret <<= shift;
+        ret = (u64)delta_nsec << TICK_LENGTH_SHIFT;
+        ret += (s64)time_adj << (TICK_LENGTH_SHIFT - (SHIFT_SCALE - 10));
 
         return ret;
 }
@@ -794,7 +788,6 @@ u64 current_tick_length(long shift)
 /* XXX - all of this timekeeping code should be later moved to time.c */
 #include <linux/clocksource.h>
 static struct clocksource *clock; /* pointer to current clocksource */
-static cycle_t last_clock_cycle;  /* cycle value at last update_wall_time */
 
 #ifdef CONFIG_GENERIC_TIME
 /**
@@ -813,7 +806,7 @@ static inline s64 __get_nsec_offset(void)
         cycle_now = clocksource_read(clock);
 
         /* calculate the delta since the last update_wall_time: */
-        cycle_delta = (cycle_now - last_clock_cycle) & clock->mask;
+        cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
 
         /* convert to nanoseconds: */
         ns_offset = cyc2ns(clock, cycle_delta);
@@ -927,7 +920,7 @@ static int change_clocksource(void)
                 timespec_add_ns(&xtime, nsec);
 
                 clock = new;
-                last_clock_cycle = now;
+                clock->cycle_last = now;
                 printk(KERN_INFO "Time: %s clocksource has been installed.\n",
                                         clock->name);
                 return 1;
@@ -968,7 +961,7 @@ void __init timekeeping_init(void)
         write_seqlock_irqsave(&xtime_lock, flags);
         clock = clocksource_get_next();
         clocksource_calculate_interval(clock, tick_nsec);
-        last_clock_cycle = clocksource_read(clock);
+        clock->cycle_last = clocksource_read(clock);
         ntp_clear();
         write_sequnlock_irqrestore(&xtime_lock, flags);
 }
@@ -988,7 +981,7 @@ static int timekeeping_resume(struct sys_device *dev)
 
         write_seqlock_irqsave(&xtime_lock, flags);
         /* restart the last cycle value */
-        last_clock_cycle = clocksource_read(clock);
+        clock->cycle_last = clocksource_read(clock);
         write_sequnlock_irqrestore(&xtime_lock, flags);
         return 0;
 }
@@ -1015,60 +1008,134 @@ static int __init timekeeping_init_device(void)
 device_initcall(timekeeping_init_device);
 
 /*
+ * If the error is already larger, we look ahead another tick,
+ * to compensate for late or lost adjustments.
+ */
+static __always_inline int clocksource_bigadjust(int sign, s64 error, s64 *interval, s64 *offset)
+{
+        int adj;
+
+        /*
+         * As soon as the machine is synchronized to the external time
+         * source this should be the common case.
+         */
+        error >>= 2;
+        if (likely(sign > 0 ? error <= *interval : error >= *interval))
+                return sign;
+
+        /*
+         * 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.
+         */
+        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++;
+        }
+
+        /*
+         * 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++;
+
+        *interval <<= adj;
+        *offset <<= adj;
+        return sign << adj;
+}
+
+/*
+ * Adjust the multiplier to reduce the error value,
+ * this is optimized for the most common adjustments of -1,0,1,
+ * for other values we can do a bit more work.
+ */
+static void clocksource_adjust(struct clocksource *clock, s64 offset)
+{
+        s64 error, interval = clock->cycle_interval;
+        int adj;
+
+        error = clock->error >> (TICK_LENGTH_SHIFT - clock->shift - 1);
+        if (error > interval) {
+                adj = clocksource_bigadjust(1, error, &interval, &offset);
+        } else if (error < -interval) {
+                interval = -interval;
+                offset = -offset;
+                adj = clocksource_bigadjust(-1, error, &interval, &offset);
+        } else
+                return;
+
+        clock->mult += adj;
+        clock->xtime_interval += interval;
+        clock->xtime_nsec -= offset;
+        clock->error -= (interval - offset) << (TICK_LENGTH_SHIFT - clock->shift);
+}
+
+/*
  * update_wall_time - Uses the current clocksource to increment the wall time
  *
  * Called from the timer interrupt, must hold a write on xtime_lock.
  */
 static void update_wall_time(void)
 {
-        static s64 remainder_snsecs, error;
-        s64 snsecs_per_sec;
-        cycle_t now, offset;
+        cycle_t offset;
 
-        snsecs_per_sec = (s64)NSEC_PER_SEC << clock->shift;
-        remainder_snsecs += (s64)xtime.tv_nsec << clock->shift;
+        clock->xtime_nsec += (s64)xtime.tv_nsec << clock->shift;
 
-        now = clocksource_read(clock);
-        offset = (now - last_clock_cycle)&clock->mask;
+#ifdef CONFIG_GENERIC_TIME
+        offset = (clocksource_read(clock) - clock->cycle_last) & clock->mask;
+#else
+        offset = clock->cycle_interval;
+#endif
 
         /* normally this loop will run just once, however in the
          * case of lost or late ticks, it will accumulate correctly.
          */
-        while (offset > clock->interval_cycles) {
-                /* get the ntp interval in clock shifted nanoseconds */
-                s64 ntp_snsecs  = current_tick_length(clock->shift);
-
+        while (offset >= clock->cycle_interval) {
                 /* accumulate one interval */
-                remainder_snsecs += clock->interval_snsecs;
-                last_clock_cycle += clock->interval_cycles;
-                offset -= clock->interval_cycles;
+                clock->xtime_nsec += clock->xtime_interval;
+                clock->cycle_last += clock->cycle_interval;
+                offset -= clock->cycle_interval;
+
+                if (clock->xtime_nsec >= (u64)NSEC_PER_SEC << clock->shift) {
+                        clock->xtime_nsec -= (u64)NSEC_PER_SEC << clock->shift;
+                        xtime.tv_sec++;
+                        second_overflow();
+                }
 
                 /* interpolator bits */
-                time_interpolator_update(clock->interval_snsecs
+                time_interpolator_update(clock->xtime_interval
                                                 >> clock->shift);
                 /* increment the NTP state machine */
                 update_ntp_one_tick();
 
                 /* accumulate error between NTP and clock interval */
-                error += (ntp_snsecs - (s64)clock->interval_snsecs);
+                clock->error += current_tick_length();
+                clock->error -= clock->xtime_interval << (TICK_LENGTH_SHIFT - clock->shift);
+        }
 
-                /* correct the clock when NTP error is too big */
-                remainder_snsecs += make_ntp_adj(clock, offset, &error);
+        /* correct the clock when NTP error is too big */
+        clocksource_adjust(clock, offset);
 
-                if (remainder_snsecs >= snsecs_per_sec) {
-                        remainder_snsecs -= snsecs_per_sec;
-                        xtime.tv_sec++;
-                        second_overflow();
-                }
-        }
         /* store full nanoseconds into xtime */
-        xtime.tv_nsec = remainder_snsecs >> clock->shift;
-        remainder_snsecs -= (s64)xtime.tv_nsec << clock->shift;
+        xtime.tv_nsec = 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 */
         if (change_clocksource()) {
-                error = 0;
-                remainder_snsecs = 0;
+                clock->error = 0;
+                clock->xtime_nsec = 0;
                 clocksource_calculate_interval(clock, tick_nsec);
         }
 }