Exception starting xend ((13, ‘Permission denied’))

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After a yum upgrade of one of our CentOS 5 Xen server, xend would not start properly. The logs contained the following error messages below.
xend-debug.log:

Xend started at Wed Aug 26 18:15:57 2009.
sysctl operation failed -- need to rebuild the user-space tool set?
Exception starting xend: (13, 'Permission denied')

xend.log

[2009-08-26 18:15:57 3310] ERROR (SrvDaemon:347) Exception starting xend ((13, 'Permission denied'))Traceback (most recent call last): File "/usr/lib/python2.4/site-packages/xen/xend/server/SrvDaemon.py", line 339, in run servers = SrvServer.create() File "/usr/lib/python2.4/site-packages/xen/xend/server/SrvServer.py", line 251, in create root.putChild('xend', SrvRoot()) File "/usr/lib/python2.4/site-packages/xen/xend/server/SrvRoot.py", line 40, in __init__ self.get(name) File "/usr/lib/python2.4/site-packages/xen/web/SrvDir.py", line 82, in get val = val.getobj() File "/usr/lib/python2.4/site-packages/xen/web/SrvDir.py", line 52, in getobj self.obj = klassobj() File "/usr/lib/python2.4/site-packages/xen/xend/server/SrvNode.py", line 30, in __init__ self.xn = XendNode.instance()
File "/usr/lib/python2.4/site-packages/xen/xend/XendNode.py", line 752, in instance
inst = XendNode()
File "/usr/lib/python2.4/site-packages/xen/xend/XendNode.py", line 87, in __init__
self.other_config["xen_pagesize"] = self.xeninfo_dict()["xen_pagesize"]
File "/usr/lib/python2.4/site-packages/xen/xend/XendNode.py", line 741, in xeninfo_dict
return dict(self.xeninfo())
File "/usr/lib/python2.4/site-packages/xen/xend/XendNode.py", line 685, in xeninfo
info['xen_scheduler'] = self.xenschedinfo()
File "/usr/lib/python2.4/site-packages/xen/xend/XendNode.py", line 675, in xenschedinfo
sched_id = self.xc.sched_id_get()
Error: (13, 'Permission denied')

After some investigation this was quite easy to solve. The yum upgrade updated the kernel and modified the grub.conf. So after the reboot, the new xen kernel booted. However, this kernel did not match the xen tools installed. This is easily fixed by changing the grub.conf to boot the correct xen kernel. See the examples below for the exact change.

The grub.conf after the yum update that caused the problem:

title CentOS (2.6.18-128.7.1.el5xen)
root (hd0,0)
kernel /xen.gz-2.6.18-128.7.1.el5
module /vmlinuz-2.6.18-128.7.1.el5xen ro root=/dev/VolGroup00/LogVol00
module /initrd-2.6.18-128.7.1.el5xen.img

The changed grub.conf after the yum updated that fixed the problem:

title CentOS (2.6.18-128.7.1.el5xen)
root (hd0,0)
kernel /xen.gz-3.3.1
module /vmlinuz-2.6.18-128.7.1.el5xen ro root=/dev/VolGroup00/LogVol00
module /initrd-2.6.18-128.7.1.el5xen.img

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