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timekeeping: Account for monotonicity adjustment in ntp_error

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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 c2cda2a5bd ("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: c2cda2a5bd ("timekeeping/ntp: Don't align NTP frequency adjustments to ticks")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Assisted-by: Kiro:claude-opus-4.6-1m
Acked-by: John Stultz <jstultz@google.com>
Link: https://patch.msgid.link/20260621220051.1030462-3-dwmw2@infradead.org
This commit is contained in:
David Woodhouse
2026-07-08 00:17:26 +02:00
committed by Thomas Gleixner
parent 79b8bd857b
commit b7befd6d91
1 changed files with 6 additions and 0 deletions
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kernel/time/timekeeping.c
+6
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@@ -2404,6 +2404,11 @@ 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 */
@@ -2414,6 +2419,7 @@ 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;
}
/*