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

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Date:	Fri, 4 Nov 2011 16:20:38 +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:

> On Thu, 2011-11-03 at 22:10 +0100, Ingo Molnar wrote:
> > * 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.
> 
> Sounds good. Thanks for the heads up.  Do you have a dmesg for this 
> system as well, so I can narrow down what I should try to reproduce 
> this on?

The full bootlog that i attached to my report is a dmesg - do you 
need any other info beyond that?

Thanks,

	Ingo
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