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

[Joshua West <jwest@xxxxxxxxxxxx>]

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
http://www.brandeis.edu


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