[RFC] Default child of a cgroup

[Balbir Singh]

Srivatsa Vaddagiri wrote:
> Hi,
> 	As we were implementing multiple-hierarchy support for CPU
> controller, we hit some oddities in its implementation, partly related
> to current cgroups implementation. Peter and I have been debating on the 
> exact solution and I thought of bringing that discussion to lkml.
> 
> Consider the cgroup filesystem structure for managing cpu resource.
> 
> 	# mount -t cgroup -ocpu,cpuacct none /cgroup
> 	# mkdir /cgroup/A
> 	# mkdir /cgroup/B
> 	# mkdir /cgroup/A/a1
> 
> will result in:
> 
> 	/cgroup
> 	   |------<tasks>
> 	   |------<cpuacct.usage>
>  	   |------<cpu.shares>
> 	   |
> 	   |----[A]
> 	   |     |----<tasks>
> 	   |     |----<cpuacct.usage>
> 	   |     |----<cpu.shares>
> 	   |     |
> 	   |     |---[a1]
> 	   |           |----<tasks>
> 	   |   	       |----<cpuacct.usage>
> 	   |           |----<cpu.shares>
> 	   |           |
> 	   |
> 	   |----[B]
> 	   |     |----<tasks>
> 	   |     |----<cpuacct.usage>
> 	   |     |----<cpu.shares>
> 	   |     
> 
> 
> Here are some questions that arise in this picture:
> 
> 1. What is the relationship of the task-group in A/tasks with the
>    task-group in A/a1/tasks? In otherwords do they form siblings
>    of the same parent A?
> 

I consider them to be the same relationship between directories and files.
A/tasks are siblings of A/a1 and A/other children, *but* the entities of
interest are A and A/a1.

> 2. Somewhat related to the above question, how much resource should the 
>    task-group A/a1/tasks get in relation to A/tasks? Is it 1/2 of parent
>    A's share or 1/(1 + N) of parent A's share (where N = number of tasks
>    in A/tasks)?
> 

I propose that it gets 1/2 of the bandwidth, here is why

1. Assume that a task in A/tasks forks 1000 children, what happens to the
bandwidth of A/a1's tasks then? We have no control over how many tasks can be
created on A/tasks as a consequence of moving one task to A/tasks. Doing it the
other way would mean, that A/a1/tasks will get 1/1001 of the bandwidth (sounds
very unfair and prone to Denial of Service/Fairness)


> 3. What should A/cpuacct.usage reflect? CPU usage of A/tasks? Or CPU usage
>    of all siblings put together? It can reflect only one, in which case
>    user has to manually derive the other component of the statistics.
> 

It should reflect the accumulated usage of A's children and the tasks in A.

> It seems to me that tasks in A/tasks form what can be called the
> "default" child group of A, in which case:
> 
> 4. Modifications to A/cpu.shares should affect the parent or its default
>    child group (A/tasks)?
> 
> To avoid these ambiguities, it may be good if cgroup create this
> "default child group" automatically whenever a cgroup is created?
> Something like below (not the absence of tasks file in some directories
> now):
> 

I think the concept makes sense, but creating a default child is going to be
confusing, as it is not really a child of A.

> 
> 	/cgroup
> 	   |
> 	   |------<cpuacct.usage>
>  	   |------<cpu.shares>
> 	   |
>   	   |---[def_child]
> 	   |     |----<tasks>
> 	   |     |----<cpuacct.usage>
> 	   |     |----<cpu.shares>
> 	   |     |
> 	   |
> 	   |----[A]
> 	   |     |
> 	   |     |----<cpuacct.usage>
> 	   |     |----<cpu.shares>
> 	   |     |
> 	   |     |---[def_child]
> 	   |     |     |----<tasks>
> 	   |   	 |     |----<cpuacct.usage>
> 	   |     |     |----<cpu.shares>
> 	   |     |     |
> 	   |     | 
> 	   |     |---[a1]
> 	   |           |
> 	   |   	       |----<cpuacct.usage>
> 	   |           |----<cpu.shares>
> 	   |           |
> 	   | 	       |---[def_child]
> 	   |	       |       |---<tasks>
> 	   |	       |       |---<cpuacct.usage>
> 	   | 	       |       |---<cpu.shares>
> 	   |	       |       |
> 	   |
> 	   |----[B]
> 	   |     |
> 	   |     |----<cpuacct.usage>
> 	   |     |----<cpu.shares>
> 	   |     | 
> 	   |     |---[def_child]
> 	   |     |     |----<tasks>
> 	   |   	 |     |----<cpuacct.usage>
> 	   |     |     |----<cpu.shares>
> 	   |     |     |
> 
> Note that user cannot create subdirectories under def_child with this
> scheme! I am also not sure what impact this will have on other resources
> like cpusets ..
> 

Which means we'll need special logic in the cgroup filesystem to handle
def_child. Not a very good idea.

> Thoughts?
> 
> 


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL