linux-kernel - Re: [PATCH] [RFC] Potential fix for leapsecond caused futex related load spikes

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Date:	Sun, 01 Jul 2012 02:42:55 -0700
From:	John Stultz <johnstul@...ibm.com>
To:	John Stultz <johnstul@...ibm.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	stable@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] [RFC] Potential fix for leapsecond caused futex related
 load spikes

On 07/01/2012 02:36 AM, John Stultz wrote:
> As widely reported on the internet today, some Linux systems after
> the leapsecond was inserted are experiencing futex related load
> spikes (usually connected to MySQL, Firefox, Thunderbird, Java, etc).
>
> An apparent for this issue workaround 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 some extra logic to track when a leapsecond
> is added from update_wall_time() and calls clock_was_set()
> once the timekeeper.lock is released.
>
> I've been able to reproduce the load spike using Thunderbird
> when triggering a leap second and with this patch the issue
> did not crop up.

Also, attached is the test case I've been using to trigger leapseconds, 
in case anyone else is interested in trying to either test this fix or 
help reproduce the reported hard hangs.

To build:
gcc leaptest.c -o leaptest -lrt

thanks
-john




View attachment "leaptest.c" of type "text/x-csrc" (2077 bytes)