[PATCH 3/3] [RFC] timekeeping: Calculate freq adjustment directly

[John Stultz]

After working with Mirolsav's simulator and his initial patch,
I've grown more comfortable with his approach of calculating
the freq adjustment directly using a division, rather then
trying to aproximate it over a number of ticks.

Part of the rational here is that now the error adjustment
is very small (only a 0 or 1 unit adjustment to the multiplier)
it can take quite some time to reduce any accumulated error.

The approximation method strains this, since it takes
log(adjustment) number of updates to approximate, and we can
accumulate quite a bit of error in that period.

The downside with this approach is it requires doing a 64bit
divide and a few multiplies to properly make and apply the
calculation, which will occur about every second or whenever
the ntp_tick_length() value is adjusted.

The positive side of this approach is that we see very small
error accumulation.

I do have some concern that compared with the simulator, the
scale of the error without this patch in the real world will
be hard to ever observe, the extra overhead of the computation
(particularly on 32bit systems) won't actually provide a benefit.

Cc: Miroslav Lichvar <mlichvar@xxxxxxxxxx>
Cc: Richard Cochran <richardcochran@xxxxxxxxx>
Cc: Prarit Bhargava <prarit@xxxxxxxxxx>
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
---
 include/linux/timekeeper_internal.h |  2 ++
 kernel/time/timekeeping.c           | 67 +++++++++++++------------------------
 2 files changed, 25 insertions(+), 44 deletions(-)

diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
index 4fddf6c..03e5dff 100644
--- a/include/linux/timekeeper_internal.h
+++ b/include/linux/timekeeper_internal.h
@@ -48,6 +48,8 @@ struct timekeeper {
 	 * ntp shifted nano seconds. */
 	u32			ntp_error_shift;
 
+	/* Mult adjustment being applied to correct freq error */
+	u32			ntp_freq_mult;
 	/* Mult adjustment being applied to correct ntp_error */
 	u32			ntp_err_mult;
 
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 846603e..27c54a6 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -149,6 +149,7 @@ static void tk_setup_internals(struct timekeeper *tk, struct clocksource *clock)
 	 * to counteract clock drifting.
 	 */
 	tk->mult = clock->mult;
+	tk->ntp_freq_mult = clock->mult;
 	tk->ntp_err_mult = 0;
 }
 
@@ -1058,20 +1059,20 @@ device_initcall(timekeeping_init_ops);
  */
 static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk,
 							 s64 offset,
-							 bool negative,
-							 int adj_scale)
+							 int adj)
 {
 	s64 interval = tk->cycle_interval;
-	s32 mult_adj = 1;
 
-	if (negative) {
-		mult_adj = -mult_adj;
+	if (!adj)
+		return;
+
+	if (unlikely(abs(adj) > 1)) {
+		interval *= adj;
+		offset  *= adj;
+	} else if (adj < 0) {
 		interval = -interval;
 		offset  = -offset;
 	}
-	mult_adj <<= adj_scale;
-	interval <<= adj_scale;
-	offset <<= adj_scale;
 
 	/*
 	 * So the following can be confusing.
@@ -1079,7 +1080,7 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk,
 	 * To keep things simple, lets assume mult_adj == 1 for now.
 	 *
 	 * When mult_adj != 1, remember that the interval and offset values
-	 * have been appropriately scaled so the math is the same.
+	 * have been appropriately multiplied so the math is the same.
 	 *
 	 * The basic idea here is that we're increasing the multiplier
 	 * by one, this causes the xtime_interval to be incremented by
@@ -1122,7 +1123,7 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk,
 	 *
 	 * XXX - TODO: Doc ntp_error calculation.
 	 */
-	tk->mult += mult_adj;
+	tk->mult += adj;
 	tk->xtime_interval += interval;
 	tk->xtime_nsec -= offset;
 	tk->ntp_error -= (interval - offset) << tk->ntp_error_shift;
@@ -1135,36 +1136,13 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk,
 static __always_inline void timekeeping_freqadjust(struct timekeeper *tk,
 							s64 offset)
 {
-	s64 interval = tk->cycle_interval;
-	s64 xinterval = tk->xtime_interval;
-	s64 tick_error;
-	bool negative;
-	u32 adj;
-
-	/* Remove any current error adj from freq calculation */
-	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);
+	s64 tick_ns;
 
-	/* Don't worry about correcting it if its small */
-	if (likely((tick_error >= 0) && (tick_error <= interval)))
+	if (likely(tk->ntp_tick == ntp_tick_length()))
 		return;
-
-	/* preserve the direction of correction */
-	negative = (tick_error < 0);
-
-	/* Sort out the magnitude of the correction */
-	tick_error = abs(tick_error);
-	for (adj = 0; tick_error > interval; adj++)
-		tick_error >>= 1;
-
-	/* scale the corrections */
-	timekeeping_apply_adjustment(tk, offset, negative, adj);
+	tk->ntp_tick = ntp_tick_length();
+	tick_ns = (tk->ntp_tick >> tk->ntp_error_shift) - tk->xtime_remainder;
+	tk->ntp_freq_mult = div64_u64(tick_ns, tk->cycle_interval);
 }
 
 /*
@@ -1173,18 +1151,19 @@ static __always_inline void timekeeping_freqadjust(struct timekeeper *tk,
  */
 static void timekeeping_adjust(struct timekeeper *tk, s64 offset)
 {
+	u32 new_mult;
+
 	/* Correct for the current frequency error */
 	timekeeping_freqadjust(tk, offset);
 
 	/* Next make a small adjustment to fix any cumulative error */
-	if (!tk->ntp_err_mult && (tk->ntp_error > 0)) {
+	if (tk->ntp_error > 0)
 		tk->ntp_err_mult = 1;
-		timekeeping_apply_adjustment(tk, offset, 0, 0);
-	} else if (tk->ntp_err_mult && (tk->ntp_error <= 0)) {
-		/* Undo any existing error adjustment */
-		timekeeping_apply_adjustment(tk, offset, 1, 0);
+	else
 		tk->ntp_err_mult = 0;
-	}
+
+	new_mult = tk->ntp_freq_mult + tk->ntp_err_mult;
+	timekeeping_apply_adjustment(tk, offset, new_mult - tk->mult);
 
 	if (unlikely(tk->clock->maxadj &&
 		(tk->mult > tk->clock->mult + tk->clock->maxadj))) {
-- 
1.9.1

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