Re: INFO: rcu_preempt detected stalls on CPUs/tasks: { 1} (detected by 0, t=10002 jiffies)

[Avi Kivity <avi@redhat.com>]

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)?


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