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Date:	Sun, 30 Sep 2012 13:32:46 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Fengguang Wu <fengguang.wu@...el.com>
CC:	paulmck@...ux.vnet.ibm.com, Josh Boyer <jwboyer@...hat.com>,
	Christian Hoffmann <email@...istianhoffmann.info>,
	LKML <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, johnstul@...ibm.com,
	tglx@...utronix.de
Subject: Re: INFO: rcu_preempt detected stalls on CPUs/tasks: { 1} (detected
 by 0, t=10002 jiffies)

On 09/30/2012 01:23 PM, Fengguang Wu wrote:
> On Sun, Sep 30, 2012 at 01:10:55PM +0200, Avi Kivity wrote:
>> On 09/28/2012 05:35 AM, Paul E. McKenney wrote:
>> > On Thu, Sep 27, 2012 at 12:40:44PM +0800, Fengguang Wu wrote:
>> >> On Wed, Sep 26, 2012 at 09:28:50PM -0700, Paul E. McKenney wrote:
>> >> > On Thu, Sep 27, 2012 at 10:54:00AM +0800, Fengguang Wu wrote:
>> >> > > On Wed, Sep 26, 2012 at 09:45:43AM -0700, Paul E. McKenney wrote:
>> >> > > > On Wed, Sep 26, 2012 at 04:15:01PM +0800, Fengguang Wu wrote:
>> > 
>> > [ . . . ]
>> > 
>> >> > > > But could you also please send your .config file and a description of
>> >> > > 
>> >> > > .config attached.
>> >> > > 
>> >> > > > the workload you are running?
>> >> > > 
>> >> > > It's basically the below commands. The exact initrd is not relevant in
>> >> > > this case because it's a boot time warning before user space is
>> >> > > started. The stalls roughly happen 1 time on every 10 boots.
>> >> > 
>> >> > Yow!!!
>> >> > 
>> >> > You have severe cross-CPU time-synchronization problems.  See for
>> >> > example the first dmesg, with the relevant part extracted right here.
>> >> > One CPU believes that it is about 37 seconds past boot, and the other
>> >> > CPU beleives that it is about 137 seconds past boot.  Given that large
>> >> > of a time difference, an RCU CPU stall warning is expected behavior.
>> >> 
>> >> Good spot! Yeah I noticed that huge timestamp gap, however didn't take
>> >> it seriously enough..
>> >> 
>> >> > Get your two CPUs in agreement about what time it is, and I bet that
>> >> > the CPU stall warnings will go away.
>> >> 
>> >> Possibly KVM related? Because the warnings show up in many test boxes
>> >> running KVM and so is not likely some hardware specific issue.
>> > 
>> > I vaguely recall seeing something recently.  But let's ask the KVM and
>> > timekeeping guys.
>> 
>> >From the logs it looks like hpet (why not kvmclock?) is used for the
>> clock, it should not generate such drifts since it is a global clock.
>> Can you verify current_clocksource on a boot that actually failed (in
>> case the clocksource is switched during runtime)?
> 
> I've checked out the dmesg that's cited by Paul, attached. Yes it
> contains lines
> 
> [    4.970051] Switching to clocksource hpet
> 
> and then
> 
> [    7.250353] Switching to clocksource tsc
> 
> And there is no kvm-clock lines. Oh well for this particular kernel:
> 

Ah, tsc will certainly break on kvm if the hardware doesn't provide a
constant tsc source.  I'm surprised the guest kernel didn't detect it
and switch back to hpet though.


-- 
error compiling committee.c: too many arguments to function
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