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[Xen-users] RHEL6 domU migrate issues w/ higher to lower frequency CPU's

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Subject: [Xen-users] RHEL6 domU migrate issues w/ higher to lower frequency CPU's
From: Joshua West <jwest@xxxxxxxxxxxx>
Date: Wed, 15 Dec 2010 18:03:07 -0500
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Hey folks,

I've encountered a rather interesting/frustrating issue with RHEL6 domU's and live migration.

I have no problems booting a RHEL6 domU using its stock/native kernel on Xen 3.4.1 or Xen 3.4.3. But in terms of live migration, there seems to be a problem when moving from a higher (in terms of CPU MHz) to lower (MHz) system -- even if the higher of the two is a much older CPU model.

For example, I can reproduce the bug under Xen 3.4.3 with the following:

* Migrating from X5450 @ 3.00GHz to X5355 @ 2.66GHz fails, but the opposite (increasing in CPU frequency) succeeds. * Migrating from Xeon(TM) CPU 2.80GHz to E5310 @ 1.60GHz fails, but the opposite (increasing in CPU frequency) succeeds.

BTW, when I say "fails", what I really mean is the migration succeeds but the domU is no longer responsive. I can attach to the console via 'xm console' but nothing is displayed, although occasionally a new line is printed as i bang my hands on the Enter key. Occasionally ping works and occasionally I can establish a connection to the domU's port 22 and see the OpenSSH banner, but thats as far as I get. Its not like the domU is runaway with 100% cpu. It sits with state "-b----" (xm list).

I have tested cpuid masking but this doesn't help. Its an issue with going from a higher CPU frequency system to a lower CPU frequency system.

This is using the stock RHEL6 kernel 'vmlinuz-2.6.32-71.7.1.el6.x86_64'.

Anybody have suggestions on the cause or a workaround? Experience this issue too?

I've heard through the grapevine that this bug is also confirmed with RHEL6 domU's on XCP 1.0.

Thanks for any help you can provide!

Joshua West
Senior Systems Engineer
Brandeis University

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