[ 187/218] time: Avoid making adjustments if we havent accumulated anything

[Greg Kroah-Hartman]

3.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: John Stultz <john.stultz@xxxxxxxxxx>

commit bf2ac312195155511a0f79325515cbb61929898a upstream.

If update_wall_time() is called and the current offset isn't large
enough to accumulate, avoid re-calling timekeeping_adjust which may
change the clock freq and can cause 1ns inconsistencies with
CLOCK_REALTIME_COARSE/CLOCK_MONOTONIC_COARSE.

Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Prarit Bhargava <prarit@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/1345595449-34965-5-git-send-email-john.stultz@xxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/time/timekeeping.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1057,9 +1057,12 @@ static void update_wall_time(void)
 #else
 	offset = (clock->read(clock) - clock->cycle_last) & clock->mask;
 #endif
+	/* Check if there's really nothing to do */
+	if (offset < timekeeper.cycle_interval)
+		goto out;
+
 	timekeeper.xtime_nsec = (s64)timekeeper.xtime.tv_nsec <<
 						timekeeper.shift;
-
 	/*
 	 * With NO_HZ we may have to accumulate many cycle_intervals
 	 * (think "ticks") worth of time at once. To do this efficiently,


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