[libvirt] libvirt qemu cgroup problem

[faicker mo]

I want to use the cgroup with libvirt+qemu. The VM runs well.
But the libvirt report error, here is the libvirt debug info:

2012-07-19 09:40:10.726+0000: 32341: debug : virCgroupNew:603 : New group / 
2012-07-19 09:40:10.726+0000: 32341: info : qemudStartup:597 : Unable to create cgroup for driver: No such device or address 
2012-07-19 09:40:10.726+0000: 32341: debug : virLockManagerNopInit:35 : version=1000000 configFile=(null) flags=0 
2012-07-19 09:40:10.726+0000: 32341: info : qemudLoadDriverConfig:332 : Configured cgroup controller 'cpu' 
2012-07-19 09:40:10.726+0000: 32341: info : qemudLoadDriverConfig:332 : Configured cgroup controller 'cpuacct' 
2012-07-19 09:40:10.726+0000: 32341: info : qemudLoadDriverConfig:332 : Configured cgroup controller 'cpuset' 
2012-07-19 09:40:10.726+0000: 32341: info : qemudLoadDriverConfig:332 : Configured cgroup controller 'memory' 
2012-07-19 09:40:10.726+0000: 32341: info : qemudLoadDriverConfig:332 : Configured cgroup controller 'devices' 
2012-07-19 09:40:10.726+0000: 32341: info : qemudLoadDriverConfig:332 : Configured cgroup controller 'blkio'

I can't find the /sys/fs/cgroup dir or any other dir?

virsh blkiotune test
error: Unable to get blkio parameters
error: Requested operation is not valid: blkio cgroup isn't mounted
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thank you.