[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>
diff --git a/kernel/timer.c b/kernel/timer.c
index 890a56937..5bb6b79 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -770,7 +770,7 @@
* 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 @@
* 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 @@
/* 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 @@
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 @@
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 @@
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 @@
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 @@
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);
-
- /* correct the clock when NTP error is too big */
- remainder_snsecs += make_ntp_adj(clock, offset, &error);
-
- if (remainder_snsecs >= snsecs_per_sec) {
- remainder_snsecs -= snsecs_per_sec;
- xtime.tv_sec++;
- second_overflow();
- }
+ clock->error += current_tick_length();
+ clock->error -= clock->xtime_interval << (TICK_LENGTH_SHIFT - clock->shift);
}
+
+ /* correct the clock when NTP error is too big */
+ clocksource_adjust(clock, offset);
+
/* 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);
}
}
