path: root/kernel
diff options
authorMartin Schwidefsky <schwidefsky@de.ibm.com>2013-11-22 11:44:51 -0800
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2013-12-11 22:36:27 -0800
commit78f8d9b5647283bdea224d9bb7fb99f8f37a7614 (patch)
tree994efe10fdd9d07e0d103c1b333507bd7999d8f1 /kernel
parent0d08c381362aca8ef20c7fe1fa4c1ce36728208b (diff)
time: Fix 1ns/tick drift w/ GENERIC_TIME_VSYSCALL_OLD
commit 4be77398ac9d948773116b6be4a3c91b3d6ea18c upstream. Since commit 1e75fa8be9f (time: Condense timekeeper.xtime into xtime_sec - merged in v3.6), there has been an problem with the error accounting in the timekeeping code, such that when truncating to nanoseconds, we round up to the next nsec, but the balancing adjustment to the ntp_error value was dropped. This causes 1ns per tick drift forward of the clock. In 3.7, this logic was isolated to only GENERIC_TIME_VSYSCALL_OLD architectures (s390, ia64, powerpc). The fix is simply to balance the accounting and to subtract the added nanosecond from ntp_error. This allows the internal long-term clock steering to keep the clock accurate. While this fix removes the regression added in 1e75fa8be9f, the ideal solution is to move away from GENERIC_TIME_VSYSCALL_OLD and use the new VSYSCALL method, which avoids entirely the nanosecond granular rounding, and the resulting short-term clock adjustment oscillation needed to keep long term accurate time. [ jstultz: Many thanks to Martin for his efforts identifying this subtle bug, and providing the fix. ] Originally-from: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Tony Luck <tony.luck@intel.com> Cc: Paul Mackerras <paulus@samba.org> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Paul Turner <pjt@google.com> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Richard Cochran <richardcochran@gmail.com> Cc: Prarit Bhargava <prarit@redhat.com> Cc: Fenghua Yu <fenghua.yu@intel.com> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/1385149491-20307-1-git-send-email-john.stultz@linaro.org Signed-off-by: John Stultz <john.stultz@linaro.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'kernel')
1 files changed, 1 insertions, 1 deletions
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index fcc261c5df2..1c5b0fcd83b 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1328,7 +1328,7 @@ static inline void old_vsyscall_fixup(struct timekeeper *tk)
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;
#define old_vsyscall_fixup(tk)