timekeeping: Account for monotonicity adjustment in ntp_error

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.

DWDavid Woodhouse committed 4 months ago