[RFC] Default child of a cgroup

[Peter Zijlstra]

On Thu, 2008-01-31 at 23:39 +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)

And I oppose this, it means not all siblings are treated equal. Also, I
miss the story of the 'hidden' group here. The biggest objection is this
hidden group with no direct controls.

My proposal is to make it a hard constraint, either a group has task
children or a group has group children, but not mixed. That keeps the
interface explicit and doesn't hide the tricks we play.

> > 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.

A's children includes tasks in this context. See where the confusion is?

> > 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.

Quite so. I really hate this hidden group.

> > 
> > 	/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.

agreed.