[PATCH v7 2/7] timekeeping: Account for monotonicity adjustment in ntp_error

[David Woodhouse <dwmw2@infradead.org>]

From: David Woodhouse <dwmw@amazon.co.uk>

timekeeping_apply_adjustment() modifies xtime_nsec to ensure monotonicity
when mult changes:

    xtime_nsec -= offset

This ensures that the time reported to userspace does not jump when the
multiplier is adjusted from one tick to the next. However, the ntp_error
accumulator which tracks the difference between intended and actual
clock position was not being updated to reflect this additional
discrepancy.

An earlier attempt at this compensation existed as:

    ntp_error -= (interval - offset) << ntp_error_shift

but was removed in commit c2cda2a5bda9 ("timekeeping/ntp: Don't align
NTP frequency adjustments to ticks") because it was a major source of
NTP error. That's because (interval - offset) was wrong: the subtraction
of "interval" prematurely accounted for the changed xtime_interval of
the next tick, which would be correctly accounted in the next
accumulation anyway — a double subtraction.

What is actually needed is just the "offset" part: ntp_error must be
told that xtime_nsec moved by "offset" without a corresponding change
in the intended position. For the normal ±1 mult dithering this is
negligible (the adjustments cancel over time), but for larger mult
changes — such as when an external reference clock sets a new
frequency — the one-time uncompensated offset is significant.

Fix by adjusting ntp_error by the correct amount:

    ntp_error += offset << ntp_error_shift

This keeps ntp_error consistent with the actual xtime_nsec position
after the adjustment, and ensures the discrepancy is correctly smoothed
away over time and the clock returns to where it should have been.

Fixes: c2cda2a5bda9 ("timekeeping/ntp: Don't align NTP frequency adjustments to ticks")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: Kiro:claude-opus-4.6-1m
Acked-by: John Stultz <jstultz@google.com>
---
 kernel/time/timekeeping.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 0d5b67f609bb..d847bba0481b 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -2389,6 +2389,11 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk,
 	 *	xtime_nsec_2 = xtime_nsec_1 - offset
 	 * Which simplifies to:
 	 *	xtime_nsec -= offset
+	 *
+	 * When subtracting offset from xtime_nsec, the same amount
+	 * (in appropriate units) has to be added to ntp_error, in
+	 * order to correctly track the delta between the time
+	 * reported in xtime_nsec, and the intended time.
 	 */
 	if ((mult_adj > 0) && (tk->tkr_mono.mult + mult_adj < mult_adj)) {
 		/* NTP adjustment caused clocksource mult overflow */
@@ -2399,6 +2404,7 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk,
 	tk->tkr_mono.mult += mult_adj;
 	tk->xtime_interval += interval;
 	tk->tkr_mono.xtime_nsec -= offset;
+	tk->ntp_error += offset << tk->ntp_error_shift;
 }
 
 /*
-- 
2.54.0