Re: Changing RT task to a different cgroup

[Rahul Bhattacharya <rahulimpdocs@xxxxxxxxx>]

Any idea or feedback for the person to contact?

Thanks

br

rb

On Wed, Sep 14, 2016 at 5:21 PM, Rahul Bhattacharya <rahulimpdocs@xxxxxxxxx> wrote:

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

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