Changing RT task to a different cgroup

[Juan Luis De Sousa-Valadas Castano]

I would try the cgroups mailing list:

http://vger.kernel.org/vger-lists.html#cgroups

Juan.

On Tue, Sep 20, 2016 at 2:39 PM, Rahul Bhattacharya
<rahulimpdocs at gmail.com> wrote:
> Any idea or feedback for the person to contact?
> Thanks
>
> br
> rb
>
> On Wed, Sep 14, 2016 at 5:21 PM, Rahul Bhattacharya <rahulimpdocs at gmail.com>
> 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 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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-- 
Juan-Luis de Sousa-Valadas Castaño
Iberia Infrastructure Consultant - Red Hat