linux-kernel - [PATCH 2/3] 3.4.y: time: Avoid making adjustments if we haven't accumulated anything

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Message-Id: <1347390259-54838-3-git-send-email-john.stultz@linaro.org>
Date:	Tue, 11 Sep 2012 15:04:18 -0400
From:	John Stultz <john.stultz@...aro.org>
To:	stable@...r.kernel.org
Cc:	John Stultz <john.stultz@...aro.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: [PATCH 2/3] 3.4.y: time: Avoid making adjustments if we haven't accumulated anything

This is a -stable backport of bf2ac312195155511a0f79325515cbb61929898a

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@...aro.org>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: stable@...r.kernel.org
Link: http://lkml.kernel.org/r/1345595449-34965-5-git-send-email-john.stultz@linaro.org
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Linux Kernel <linux-kernel@...r.kernel.org>
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
 kernel/time/timekeeping.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index dd31d0e..104d549 100644
--- 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,
-- 
1.7.9.5

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