timekeeping: Use cached ntp_tick_length when accumulating error

By caching the ntp_tick_length() when we correct the frequency error,
and then using that cached value to accumulate error, we avoid large
initial errors when the tick length is changed.

This makes convergence happen much faster in the simulator, since the
initial error doesn't have to be slowly whittled away.

This initially seems like an accounting error, but Miroslav pointed out
that ntp_tick_length() can change mid-tick, so when we apply it in the
error accumulation, we are applying any recent change to the entire tick.

This approach chooses to apply changes in the ntp_tick_length() only to
the next tick, which allows us to calculate the freq correction before
using the new tick length, which avoids accummulating error.

Credit to Miroslav for pointing this out and providing the original patch
this functionality has been pulled out from, along with the rational.

Cc: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Reported-by: Miroslav Lichvar <mlichvar@redhat.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>

include/linux/timekeeper_internal.h

@@ -90,6 +90,15 @@ struct timekeeper {
 	u64			xtime_interval;
 	s64			xtime_remainder;
 	u32			raw_interval;
+	/* The ntp_tick_length() value currently being used.
+	 * This cached copy ensures we consistently apply the tick
+	 * length for an entire tick, as ntp_tick_length may change
+	 * mid-tick, and we don't want to apply that new value to
+	 * the tick in progress.
+	 */
+	u64			ntp_tick;
+	/* Difference between accumulated time and NTP time in ntp
+	 * shifted nano seconds. */
 	s64			ntp_error;
 	u32			ntp_error_shift;
 	u32			ntp_err_mult;

kernel/time/timekeeping.c

@@ -171,6 +171,7 @@ static void tk_setup_internals(struct timekeeper *tk, struct clocksource *clock)
 
 	tk->ntp_error = 0;
 	tk->ntp_error_shift = NTP_SCALE_SHIFT - clock->shift;
+	tk->ntp_tick = ntpinterval << tk->ntp_error_shift;
 
 	/*
 	 * The timekeeper keeps its own mult values for the currently
@@ -1352,6 +1353,8 @@ static __always_inline void timekeeping_freqadjust(struct timekeeper *tk,
 	if (tk->ntp_err_mult)
 		xinterval -= tk->cycle_interval;
 
+	tk->ntp_tick = ntp_tick_length();
+
 	/* Calculate current error per tick */
 	tick_error = ntp_tick_length() >> tk->ntp_error_shift;
 	tick_error -= (xinterval + tk->xtime_remainder);
@@ -1497,7 +1500,7 @@ static cycle_t logarithmic_accumulation(struct timekeeper *tk, cycle_t offset,
 	tk->raw_time.tv_nsec = raw_nsecs;
 
 	/* Accumulate error between NTP and clock interval */
-	tk->ntp_error += ntp_tick_length() << shift;
+	tk->ntp_error += tk->ntp_tick << shift;
 	tk->ntp_error -= (tk->xtime_interval + tk->xtime_remainder) <<
 						(tk->ntp_error_shift + shift);