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
negligible.

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.

ok visa@

1 files changed, 4 insertions, 4 deletions

diff --git a/sys/kern/kern_tc.c b/sys/kern/kern_tc.c
index a9b853b0a14..96ee6dd2c55 100644
--- a/sys/kern/kern_tc.c
+++ b/sys/kern/kern_tc.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: kern_tc.c,v 1.53 2019/12/02 21:47:54 cheloha Exp $ */
+/*	$OpenBSD: kern_tc.c,v 1.54 2019/12/12 17:52:25 cheloha Exp $ */
 
 /*
  * Copyright (c) 2000 Poul-Henning Kamp <phk@FreeBSD.org>
@@ -594,7 +594,8 @@ tc_windup(struct bintime *new_boottime, struct bintime *new_offset,
 	 *
 	 */
 	scale = (u_int64_t)1 << 63;
-	scale += (th->th_adjustment / 1024) * 2199;
+	scale += \
+	    ((th->th_adjustment + th->th_counter->tc_freq_adj) / 1024) * 2199;
 	scale /= th->th_counter->tc_frequency;
 	th->th_scale = scale * 2;
 
@@ -751,7 +752,7 @@ sysctl_tc(int *name, u_int namelen, void *oldp, size_t *oldlenp,
 }
 
 /*
- * Skew the timehands according to any adjfreq(2)/adjtime(2) adjustments.
+ * Skew the timehands according to any adjtime(2) adjustment.
  */
 void
 ntp_update_second(struct timehands *th)
@@ -766,7 +767,6 @@ ntp_update_second(struct timehands *th)
 		adj = MAX(-5000, th->th_adjtimedelta);
 	th->th_adjtimedelta -= adj;
 	th->th_adjustment = (adj * 1000) << 32;
-	th->th_adjustment += th->th_counter->tc_freq_adj;
 }
 
 void