Ross S. W. Walker wrote:

Brett Worth wrote:

...

Hello All.

I've just started looking into Xen and have a test environment in place. I'm seeing an annoying problem that I thought worthy of a post.

Config:

I have 2 x HP DL585 servers each with 4 Dual core Opterons (non-vmx) and 16GB RAM configured as Xen servers. These run CentOS 5.1 with the latest updates applied. These system both attach to an iSCSI target which is an HP DL385 running ietd and serving SAN based storage.

I have a test VM running CentOS 5.1 also updated.

Problem:

If I run the VM on a single server everything is OK. If I do a migrate of the VM to the other server I start getting random "BUG: soft lockup detected on CPU#?" messages on the VM console. The messages seem to happen with IO but not every time. A reboot of the VM on the new server will stop these messages.

I've also left the VM running overnight a couple of times and when I do I find that any external sessions (ssh) are hung in the morning but the console session is not. New ssh sessions can be started and seem to work.

After much googling it looks like the kernel messages can occur if dom0 is very busy but mine is not.

Any suggestions?

The soft lockup is technically not a BUG.

You will see these errors if an IRQ takes more then 10 seconds to respond.

In your case I would take a look at your iSCSI setup and the time it takes to migrate the VM from one node to another along with SCSI reserve/release setup on the iSCSI target.

I also have been using the Xen 3.2 RPMs off xen.org to CentOS 5.1 which good results, the VM migration may run smoother and quicker in Xen 3.2, but in doing so you take Xen off the reservation, if your OK with that it may fix your issues.

After seeing this same issue on my Xen 3.2 install, but with NO migration or iSCSI happening I decided it is probably NOT iSCSI's fault, so I decided to research it a little more and this is what I found:

http://docs.xensource.com/XenServer/4.0.1/guest/ch04s08.html#rhel5_limitatio...

XenSource does provide a repo of CentOS 5 kernels that have been patched to fix this though:

http://updates.xensource.com/XenServer/4.0.1/centos5x/

But these seem to be woefully out of date.

I wonder if a kind soul would add the fix to the centosplus kernel with XenSource's patch so those rogue Xen users could benefit from this fix until upstream decides to include it.

I suppose the centosplus patch would need to be flagged interm in case it needs removed when upstream has their own fix.

-Ross

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