Changing RT task to a different cgroup

[Rahul Bhattacharya]

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 at 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
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