[PATCH 2/2] [PRECISE] (pre-upstream) Fix leapsecond triggered hrtimer/futex load spike issue

[Brad Figg]

From: John Stultz <john.stultz at linaro.org>

BugLink: http://bugs.launchpad.net/bugs/1020285

Backport for 3.2.21

As widely reported on the internet, some Linux systems after
the leapsecond was inserted are experiencing futex related load
spikes (usually connected to MySQL, Firefox, Thunderbird, Java, etc).

An apparent  workaround for this issue is running:
$ date -s "`date`"

Credit: http://www.sheeri.com/content/mysql-and-leap-second-high-cpu-and-fix

I believe this issue is due to the leapsecond being added without
calling clock_was_set() to notify the hrtimer subsystem of the
change. (Although I've not yet chased all the way down to the
hrtimer code to validate exactly what's going on there).

The workaround functions as it forces a clock_was_set()
call from settimeofday().

This fix adds the required clock_was_set() calls to where
we adjust for leapseconds.

NOTE: This fix *depends* on the previous fix, which allows
clock_was_set to be called from atomic context. Do not try
to apply just this patch.

CC: Prarit Bhargava <prarit at redhat.com>
CC: stable at vger.kernel.org
CC: Thomas Gleixner <tglx at linutronix.de>
Reported-by: Jan Engelhardt <jengelh at inai.de>
Signed-off-by: John Stultz <johnstul at us.ibm.com>
Signed-off-by: Brad Figg <brad.figg at canonical.com>
---
 kernel/time/timekeeping.c |    1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 2378413..6d88c1f 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -176,6 +176,7 @@ void timekeeping_leap_insert(int leapsecond)
 	wall_to_monotonic.tv_sec -= leapsecond;
 	update_vsyscall(&xtime, &wall_to_monotonic, timekeeper.clock,
 			timekeeper.mult);
+	clock_was_set();
 }
 
 /**
-- 
1.7.9.5