Greetings,
I just hunted down a time related bug which caused the Linux internal
xtime to drift away from the precise hardware clock provided by the
TOD clock found in the s390 architecture.
After a long search I came along this lovely piece of code in
kernel/time/timekeeping.c:
#ifdef CONFIG_GENERIC_TIME_VSYSCALL_OLD
static inline void old_vsyscall_fixup(struct timekeeper *tk)
s64 remainder;
/*
* Store only full nanoseconds into xtime_nsec after rounding
* it up and add the remainder to the error difference.
* XXX - This is necessary to avoid small 1ns inconsistnecies caused
* by truncating the remainder in vsyscalls. However, it causes
* additional work to be done in timekeeping_adjust(). Once
* the vsyscall implementations are converted to use xtime_nsec
* (shifted nanoseconds), and CONFIG_GENERIC_TIME_VSYSCALL_OLD
* users are removed, this can be killed.
*/
remainder = tk->xtime_nsec & ((1ULL << tk->shift) - 1);
tk->xtime_nsec -= remainder;
tk->xtime_nsec += 1ULL << tk->shift;
tk->ntp_error += remainder << tk->ntp_error_shift;
}
#else
#define old_vsyscall_fixup(tk)
#endif
The highly precise result of our TOD clock source ends up in
tk->xtime_sec / tk->xtime_nsec where old_vsyscall_fixup just rounds
it up to the next nano-second (booo). To add insult to injury an
incorrect delta gets added to ntp_error, xtime has been forwarded by
((1ULL << tk->shift) - (tk->xtime_nsec & ((1ULL << tk->shift) - 1)))
and not set back by (tk->xtime_nsec & ((1ULL << tk->shift) - 1)).
xtime is too fast by one nano-second per tick. To verify that this
is indeed the problem I removed the line that adds the nano-second
to xtime_nsec and voila the clocks are in sync.
A possible patch to fix this would be:
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1347,6 +1347,7 @@ static inline void old_vsyscall_fixup(struct timekeeper *t
k)
tk->xtime_nsec -= remainder;
tk->xtime_nsec += 1ULL << tk->shift;
tk->ntp_error += remainder << tk->ntp_error_shift;
+ tk->ntp_error -= (1ULL << tk->shift) << tk->ntp_error_shift;
}
#else
But that has the downside that it creates a negative ntp_error that
can only be corrected with an adjustment of tk->mult which takes a
long time.
The fix I am going to use is to convert s390 to GENERIC_TIME_VSYSCALL,
you might want to think about doing that for powerpc and ia64 as well.
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
