linux-kernel - Re: [PATCH] clocksource: Avoid selecting mult values that might overflow when adjusted

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Date:	Thu, 3 Nov 2011 22:10:00 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	John Stultz <john.stultz@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Yong Zhang <yong.zhang0@...il.com>,
	David Daney <ddaney.cavm@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] clocksource: Avoid selecting mult values that might
 overflow when adjusted


* John Stultz <john.stultz@...aro.org> wrote:

> For some frequqencies, the clocks_calc_mult_shift() function will
> unfortunately select mult values very close to 0xffffffff.  This
> has the potential to overflow when NTP adjusts the clock, adding
> to the mult value.
> 
> This patch adds a clocksource.maxadj value, which provides
> an approximation of an 11% adjustment(NTP limits adjustments to
> 500ppm and the tick adjustment is limited to 10%), which could
> be made to the clocksource.mult value. This is then used to both
> check that the current mult value won't overflow/underflow, as
> well as warning us if the timekeeping_adjust() code pushes over
> that 11% boundary.
> 
> CC: Yong Zhang <yong.zhang0@...il.com>
> CC: David Daney <ddaney.cavm@...il.com>
> CC: Thomas Gleixner <tglx@...utronix.de>
> Reported-by: Chen Jie <chenj@...ote.com>
> Reported-by: zhangfx <zhangfx@...ote.com>
> Signed-off-by: John Stultz <john.stultz@...aro.org>
> ---
>  include/linux/clocksource.h |    3 +-
>  kernel/time/clocksource.c   |   53 ++++++++++++++++++++++++++++++++++--------
>  kernel/time/timekeeping.c   |    3 ++
>  3 files changed, 48 insertions(+), 11 deletions(-)

This patch (included in tip:timers/urgent) causes the following boot 
warning x86:

[    0.000000] Fast TSC calibration using PIT
[    0.000000] ------------[ cut here ]------------
[    0.000000] WARNING: at kernel/time/timekeeping.c:855 do_timer+0x47f/0x4c0()
[    0.000000] Hardware name: System Product Name
[    0.000000] Adjusting more then 11%
[    0.000000] Modules linked in:
[    0.000000] Pid: 0, comm: swapper Not tainted 3.1.0-tip+ #161792
[    0.000000] Call Trace:
[    0.000000]  <IRQ>  [<ffffffff81042d0a>] warn_slowpath_common+0x7a/0xb0
[    0.000000]  [<ffffffff81042de1>] warn_slowpath_fmt+0x41/0x50
[    0.000000]  [<ffffffff8106e78f>] do_timer+0x47f/0x4c0
[    0.000000]  [<ffffffff81073953>] tick_periodic+0x63/0x80
[    0.000000]  [<ffffffff810739f1>] tick_handle_periodic+0x21/0x70
[    0.000000]  [<ffffffff810046d8>] timer_interrupt+0x18/0x20
[    0.000000]  [<ffffffff8109ff9e>] handle_irq_event_percpu+0x5e/0x220
[    0.000000]  [<ffffffff810a019b>] handle_irq_event+0x3b/0x60
[    0.000000]  [<ffffffff810a295c>] handle_level_irq+0x6c/0xd0
[    0.000000]  [<ffffffff81003f34>] handle_irq+0x44/0xa0

Full bootlog and config attached.

i've excluded it from tip:master for now.

Thanks,

	Ingo

View attachment "boot.log" of type "text/plain" (117907 bytes)

View attachment "config" of type "text/plain" (76376 bytes)