Recommit "tc_windup: separate timecounter.tc_freq_adj from timehands.th_adjustment"
Reverted with backout of tickless timeouts.
Original commit message:
We currently mix timecounter.tc_freq_adj and timehands.th_adjtimedelta
in ntp_update_second() to produce timehands.th_adjustment, our net skew.
But if you set a low enough adjfreq(2) adjustment you can freeze time.
This prevents ntp_update_second() from running again. So even if you
then set a sane adjfreq(2) you cannot unfreeze time without rebooting.
If we just reread timecounter.tc_freq_adj every time we recompute
timehands.th_scale we avoid this trap. visa@ notes that this is
more costly than what we currently do but that the cost itself is
Intuitively, timecounter.tc_freq_adj is a constant skew and should be
handled separately from timehands.th_adjtimedelta, an adjustment that
we chip away at very slowly.
tedu@ notes that this problem is sort-of an argument for imposing range
limits on adjfreq(2) inputs. He's right, but I think we should still
separate the counter adjustment from the adjtime(2) adjustment, with
or without range limits.