Section 2.9.2, reads:

Therefore, it is advisable to move all processes from the root group to a specific subgroup when using the blkio and cpu configuration options which divide an available resource based on a weight or a share (for example, cpu.shares or blkio.weight). To move all tasks from the root group into a specific subgroup, you can use the following command:

rootgroup]# for i in `cat tasks`; do echo $i > red/tasks; done


The for loop above suffer from two issues:
1) It copies the tasks from root group to subgroup rather than moving them.
2) It overrides any tasks in the subgroup.

To avoid these, I suggest the code would become:

rootgroup]# for i in `cat tasks`; do echo $i >> red/tasks; done
rootgroup]# echo '' > tasks

That's all.