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 rational for
ADJ_TICK unclear 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.
Note: This has the potential to slow corrections when the ADJ_TICK
value is furthest away from the default value. So it would be good to
get some testing from folks using chrony, to make sure we don't
cause any troubles there.
Cc: Miroslav Lichvar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Richard Cochran <[email protected]>
Cc: Prarit Bhargava <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Reported-by: Andy Lutomirski <[email protected]>
Signed-off-by: John Stultz <[email protected]>
---
v2: Catch single unit adjustment that was being made
repeatedly to push us past the limit, as pointed
out by Miroslav.
v3: Fix off-by-one case that could result in warning
still triggering, pointed out by Miroslav.
kernel/time/timekeeping.c | 31 +++++++++++++++++++++++++++----
1 file changed, 27 insertions(+), 4 deletions(-)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index d9249da..a17cdc3 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1591,9 +1591,12 @@ static __always_inline void timekeeping_freqadjust(struct timekeeper *tk,
{
s64 interval = tk->cycle_interval;
s64 xinterval = tk->xtime_interval;
+ u32 base = tk->tkr_mono.clock->mult;
+ u32 max = tk->tkr_mono.clock->maxadj;
+ u32 cur_adj = tk->tkr_mono.mult;
s64 tick_error;
bool negative;
- u32 adj;
+ u32 adj_scale;
/* Remove any current error adj from freq calculation */
if (tk->ntp_err_mult)
@@ -1612,13 +1615,33 @@ 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 */
+ /* If any adjustment would pass the max, just return */
+ if (negative && (cur_adj - 1) <= (base - max))
+ return;
+ if (!negative && (cur_adj + 1) >= (base + max))
+ return;
+ /*
+ * Sort out the magnitude of the correction, but
+ * avoid making so large a correction that we go
+ * over the max adjustment.
+ */
+ adj_scale = 0;
tick_error = abs(tick_error);
- for (adj = 0; tick_error > interval; adj++)
+ while (tick_error > interval) {
+ u32 adj = 1 << (adj_scale + 1);
+
+ /* Check if adjustment gets us within 1 unit from the max */
+ if (negative && (cur_adj - adj) <= (base - max))
+ break;
+ if (!negative && (cur_adj + adj) >= (base + max))
+ break;
+
+ adj_scale++;
tick_error >>= 1;
+ }
/* scale the corrections */
- timekeeping_apply_adjustment(tk, offset, negative, adj);
+ timekeeping_apply_adjustment(tk, offset, negative, adj_scale);
}
/*
--
1.9.1
On Tue, Dec 15, 2015 at 09:48:06PM -0800, John Stultz wrote:
> Thus its been occasionally noted that users have seen
> confusing warnings like:
>
> Adjusting tsc more than 11% (5941981 vs 7759439)
> v2: Catch single unit adjustment that was being made
> repeatedly to push us past the limit, as pointed
> out by Miroslav.
> v3: Fix off-by-one case that could result in warning
> still triggering, pointed out by Miroslav.
Looks good to me. There are no warnings in the simulator and chronyd
still seems to be working as expected.
Tested-by: Miroslav Lichvar <[email protected]>
--
Miroslav Lichvar