Re: [RFC] Default child of a cgroup

[Dhaval Giani]

On Thu, Jan 31, 2008 at 11:39:12PM +0530, Balbir Singh wrote:
> 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.
> 

I've been taking the root group as an example, and extending it. The
root group does not reflect the usage of all the tasks in it. (IIRC,
can't seem to find the stats file there now, please correct me if I am
wrong)

> > 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.
> 

For all practical purposes, it is the same as the init_task_group which
is at the parent level.

> > 
> > 	/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.
> 

Not really. That issue would come into play if every task group was
assigned a control group. The task group is not exposed to the outside
world. (That's why its a hidden task group)

Thanks,
-- 
regards,
Dhaval
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