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[PRESEND] timekeeping: Cap adjustments so they don't exceede the maxadj value

Message ID 1449169351-1901-1-git-send-email-john.stultz@linaro.org
State New
Headers show

Commit Message

John Stultz Dec. 3, 2015, 7:02 p.m. UTC
Thus its been occasionally noted that users have seen
confusing warnings like:

    Adjusting tsc more than 11% (5941981 vs 7759439)

We try to limit the maximum total adjustment to 11% (10% tick
adjustment + 0.5% frequency adjustment). But this is done by
bounding the requested adjustment values, and the internal
steering that is done by tracking the error from what was
requested and what was applied, does not have any such limits.

This is usually not problematic, but in some cases has a risk
that an adjustment could cause the clocksource mult value to
overflow, so its an indication things are outside of what is
expected.

It ends up most of the reports of this 11% warning are on systems
using chrony, which utilizes the adjtimex() ADJ_TICK interface
(which allows a +-10% adjustment). The original ratonal for
ADJ_TICK unlcear to me but my assumption it was originally added
to allow broken systems to get a big constant correction at boot
(see adjtimex userspace package for an example) which would allow
the system to work w/ ntpd's 0.5% adjustment limit.

Chrony uses ADJ_TICK to make very aggressive short term corrections
(usually right at startup). Which push us close enough to the max
bound that a few late ticks can cause the internal steering to push
past the max adjust value (tripping the warning).

Thus this patch adds some extra logic to enforce the max adjustment
cap in the internal steering.

Signed-off-by: John Stultz <john.stultz@linaro.org>

---
 kernel/time/timekeeping.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

-- 
1.9.1
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Patch

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index d563c19..4e38902 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1594,7 +1594,9 @@  static __always_inline void timekeeping_freqadjust(struct timekeeper *tk,
 	s64 xinterval = tk->xtime_interval;
 	s64 tick_error;
 	bool negative;
-	u32 adj;
+	u32 adj_scale;
+	s32 cur_adj;
+	u32 max;
 
 	/* Remove any current error adj from freq calculation */
 	if (tk->ntp_err_mult)
@@ -1613,13 +1615,24 @@  static __always_inline void timekeeping_freqadjust(struct timekeeper *tk,
 	/* preserve the direction of correction */
 	negative = (tick_error < 0);
 
-	/* Sort out the magnitude of the correction */
+	/*
+	 * Sort out the magnitude of the correction, but
+	 * avoid making so large a correction that we go
+	 * over the max adjustment.
+	 */
+	adj_scale = 0;
+	max = tk->tkr_mono.clock->maxadj;
+	cur_adj = tk->tkr_mono.mult - tk->tkr_mono.clock->mult;
 	tick_error = abs(tick_error);
-	for (adj = 0; tick_error > interval; adj++)
+	while((tick_error > interval) &&
+			(abs(cur_adj - (1 << (adj_scale + 1)) < max)) &&
+			(abs(cur_adj + (1 << (adj_scale + 1)) < max))) {
+		adj_scale++;
 		tick_error >>= 1;
+	}
 
 	/* scale the corrections */
-	timekeeping_apply_adjustment(tk, offset, negative, adj);
+	timekeeping_apply_adjustment(tk, offset, negative, adj_scale);
 }
 
 /*