[RFC PATCH v2 2/8] timekeeping: Account for clawback adjustment in ntp_error

[David Woodhouse]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

timekeeping_apply_adjustment() modifies xtime_nsec to maintain vDSO
monotonicity when mult changes:

    xtime_nsec -= offset

This ensures that the time reported to userspace doesn't jump when the
multiplier is adjusted. However, ntp_error — which tracks the difference
between intended and actual clock position — was not updated to reflect
this change.

After a mult change, xtime_nsec has moved but ntp_error still reflects
the old position. For the normal ±1 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 (~38ns for a 700-count mult change).

Fix by adjusting ntp_error by the same amount:

    ntp_error += offset << ntp_error_shift

This keeps ntp_error consistent with the actual xtime_nsec position
after the clawback.

Fixes: 1b1b3e2a3671 ("timekeeping: Rework frequency adjustments to work better w/ nohz")
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 kernel/time/timekeeping.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 3da7167ceb0d..050123fc179b 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -2317,6 +2317,7 @@ 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;
 }
 
 /*
-- 
2.51.0