When I try to move a RT task to a different cgroup(shown below) I get a EINVAL error. However I can change the cgroup first and then change the sched policy. After looking into kernel source code(*I dont use CONFIG_RT_GROUP_SCHED config*) I was able to extract this piece of logic *chrt -p 777* pid 777's current scheduling policy: SCHED_OTHER pid 777's current scheduling priority: 0 *cat /proc/777/cgroup* 2:cpu,cpuacct:/system.slice 1:name=systemd:/system.slice/d.service *chrt -f -p 50 777* root@mgu-high:~# chrt -p 777 pid 777's current scheduling policy: SCHED_FIFO pid 777's current scheduling priority: 50 */bin/echo 777>/sys/fs/cgroup/cpu,cpuacct/interaction.slice/tasks**echo: write error: Invalid argument* After browsing kernel sources I was able to find out the reason of failure. static int cpu_cgroup_can_attach(struct cgroup_subsys_state *css, struct cgroup_taskset *tset) { struct task_struct *task; cgroup_taskset_for_each(task, tset) { #ifdef CONFIG_RT_GROUP_SCHED if (!sched_rt_can_attach(css_tg(css), task)) return -EINVAL; #else */* We don't support RT-tasks being in separate groups */ if (task->sched_class != &fair_sched_class) { return -EINVAL; }* #endif } return 0; } My question is why is this check required? Also, changing cgroups first and then the policy of FIFO works without issue(?) for the task. Thanks br rb