linux-kernel - [PATCH 4/4] time: Avoid making adjustments if we havne't accumulated anything

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Date:	Tue, 21 Aug 2012 20:30:49 -0400
From:	John Stultz <john.stultz@...aro.org>
To:	linux-kernel <linux-kernel@...r.kernel.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	Ingo Molnar <mingo@...nel.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, stable@...r.kernel.org
Subject: [PATCH 4/4] time: Avoid making adjustments if we havne't accumulated anything

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.

Cc: Ingo Molnar <mingo@...nel.org>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: stable@...r.kernel.org
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
 kernel/time/timekeeping.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index a5a9389..0c1485e 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1152,6 +1152,10 @@ static void update_wall_time(void)
 	offset = (clock->read(clock) - clock->cycle_last) & clock->mask;
 #endif
 
+	/* Check if there's really nothing to do */
+	if (offset < tk->cycle_interval)
+		goto out;
+
 	/*
 	 * 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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