[Xen-devel] Re: PROBLEM: 3.0-rc kernels unbootable since -rc3

["Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>]

On Tue, Jul 12, 2011 at 10:12:28AM -0400, Konrad Rzeszutek Wilk wrote:
> > >   [<c042d0f5>] task_waking_fair+0x14  <--
> > 
> > Hmmm...  This is a 32-bit system, isn't it?
> 
> Yes. I ran this little loop:
> 
> #!/bin/bash
> 
> ID=`xl list | grep Fedora | awk '  { print $2}'`
> 
> rm -f cpu*.log
> while (true) do
>       xl pause $ID
>        /usr/lib64/xen/bin/xenctx -s 
> /mnt/tmp/FC15-32/System.map-3.0.0-rc6-julie-tested-dirty -a $ID 0 >> cpu0.log
>        /usr/lib64/xen/bin/xenctx -s 
> /mnt/tmp/FC15-32/System.map-3.0.0-rc6-julie-tested-dirty -a $ID 1 >> cpu1.log
>        /usr/lib64/xen/bin/xenctx -s 
> /mnt/tmp/FC15-32/System.map-3.0.0-rc6-julie-tested-dirty -a $ID 2 >> cpu2.log
>        /usr/lib64/xen/bin/xenctx -s 
> /mnt/tmp/FC15-32/System.map-3.0.0-rc6-julie-tested-dirty -a $ID 3 >> cpu3.log
>       xl unpause $ID
> done
> 
> To get an idea what the CPU is doing before it hits the task_waking_fair
> and there isn't anything daming. Here are the logs:
> 
> http://darnok.org/xen/cpu1.log

OK, a fair amount of variety, then lots and lots of task_waking_fair(),
so I still feel good about asking you for the following.

> > Could you please add a check to the loop in task_waking_fair() and
> > do a printk() if the loop does (say) more than 1000 passes without
> > exiting?
> 
> Of course. Let me queue that up.

Hmmm...  Given that this is persisting for many many seconds, it might
be better to check for at least 10,000,000 passes.  In contrast, 1000
passes might elapse just waiting for a cache miss to complete.

Other possible causes include:

o       A mismatch between Xen's and RCU's ideas of how CONFIG_NO_HZ
        works.  If Xen thinks that the CPU is in CONFIG_NO_HZ's
        dyntick-idle mode, but RCU thinks otherwise, the grace period
        might stall.

o       Problems due to portions of the code attempting to use
        RCU read-side critical sections while in dyntick-idle mode.
        Frederic Weisbecker has located some of these, (though not yet
        in Xen) and he has some diagnositics which may be found at:

        git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git

        on branch eqscheck.2011.07.08a.

        You need to enable CONFIG_PROVE_RCU for these diagnostics to
        be executed.

o       As always, there might be bugs in RCU.  ;-)

But the loop in task_waking_fair() looks like the most prominent smoking
gun at the moment.

                                                        Thanx, Paul

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