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Description: When disable 'io' controller, the error message is code-level when try to set values to blk related params Versions: libvirt-5.5.0-2.module+el8.1.0+3773+7dd501bf.x86_64 How reproducible: 100% Pls note: This is a regression but not a serious issue. And happens with cgroup v1 and v2. Steps: 1. Disable blkio controller with qemu.conf $ cat /etc/libvirt/qemu.conf | grep "^cgroup" cgroup_controllers = [ "cpu", "devices", "memory", "cpuset", "cpuacct" ] $ systemctl restart libvirtd 2. Try to use 'blkiotune' cmd to set values to some params $ virsh blkiotune avocado-vt-vm1 --device-read-iops-sec /dev/sda,330 --device-write-iops-sec /dev/sda,293 error: Unable to change blkio parameters error: internal error: failed to get cgroup backend for 'setBlkioDeviceWriteIops' $ virsh blkiotune avocado-vt-vm1 --device-weights /dev/sda,100 error: Unable to change blkio parameters error: internal error: failed to get cgroup backend for 'setBlkioDeviceWeight' $ virsh blkiotune avocado-vt-vm1 --weight 100 error: Unable to change blkio parameters error: internal error: failed to get cgroup backend for 'setBlkioWeight' <==== we can see these "internal error" is code level, not suitable for end-user Additional info: 1. When just try to getting the values, the error message is clear virsh blkiotune avocado-vt-vm1 error: Unable to get blkio parameters error: Requested operation is not valid: blkio cgroup isn't mounted 2. other controllers don't have this problem. 3. With rhel7.7 libvirt-4.5.0-23.el7.x86_64, the error message is clear # virsh blkiotune avocado-vt-vm1 --weight 100 error: Unable to change blkio parameters error: internal error: Controller 'blkio' is not mounted 3 Expected result: Should keep the error message as before.