Re: [RFC] Default child of a cgroup

Serge E. Hallyn
2008-01-31T17:44:40Z

Quoting Srivatsa Vaddagiri (vatsa@linux.vnet.ibm.com):
> 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?
>
> 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)?
>
> 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 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 didn't think it was actually ambiguous. /A/cpu.shares will specify
what all tasks under /A and its children (just /A/a1/tass in this
example) get to share, while /A/a1/cpu.share specifies what tasks under
/A/a1/tasks get. Tasks which are in /A/tasks get whatever is left over,
that is /A/cpu.share - /A/a1/cpu.shares. /A/cpuacct.usage reflects all
usage by tasks under /A and its children.

>
>
> /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 ..
>
> Thoughts?
>
>
> --
> Regards,
> vatsa
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