[Devel] Re: [RFC] Control Groups Roadmap ideas

[Paul Menage]

On Fri, Apr 11, 2008 at 7:48 AM, Serge E. Hallyn <serue at us.ibm.com> wrote:
>  > 2) More flexible binding/unbinding/rebinding
>  > -----
>  >
>  > Currently you can only add/remove subsystems to a hierarchy when it
>  > has just a single (root) cgroup. This is a bit inflexible, so I'm
>  > planning to support:
>  >
>  > - adding a subsystem to an existing hierarchy by automatically
>  > creating a subsys state object for the new subsystem for each existing
>  > cgroup in the hierarchy and doing the appropriate
>  > can_attach()/attach_tasks() callbacks for all tasks in the system
>  >
>  > - removing a subsystem from an existing hierarchy by moving all tasks
>  > to that subsystem's root cgroup and destroying the child subsystem
>  > state objects
>  >
>  > - merging two existing hierarchies that have identical cgroup trees
>  >
>  > - (maybe) splitting one hierarchy into two separate hierarchies
>  >
>  > Whether all these operations should be forced through the mount()
>  > system call, or whether they should be done via operations on cgroup
>  > control files, is something I've not figured out yet.
>
>  I'm tempted to ask what the use case is for this (I assume you have one,
>  you don't generally introduce features for no good reason), but it

Back during the early versions of control groups, Paul Jackson
proposed a bind/unbind API that would let you affect the subsystems on
an active hierarchy, and it was always a goal of mine to implement
that - current inflexibility is something that I've never been that
keen on, but it was OK for the first big release and could be extended
later.

One of the potential scenarios was that you might want to have a very
early boot script set up cpusets and node isolation for a set of
system daemons, and then bind other subsystems on to the same
hierarchy later in the boot process.

>  I'd stick with mount semantics.  Just
>         mount -t cgroup -o remount,devices,cpu none /devwh"
>  should handle all cases, no?

Yes, probably - particularly if we restrict it to adding/removing
subsystems from an existing tree, rather than splitting and merging
multiple hierarchies.

>
>  I guess I'm hoping that if libcg goes well then a userspace daemon can
>  do all we need.  Of course the use case I envision is having a container
>  which is locked to some amount of ram, wherein the container admin wants
>  to lock some daemon to a subset of that ram.  If the host admin lets the
>  container admin edit a config file (or talk to a daemon through some
>  sock designated for the container) that will only create a child of the
>  container's cgroup, that's probably great.

That's a different issue, and one that I left out of the roadmap
email. We can have a virtualization subsystem that controls what
subset of a given hierarchy you can see - if the virtualization
subsystem is bound to a given hierarchy, and a cgroup is marked as
virtualized, then a mount of that hierarchy by a process in the
virtualized cgroup will see that cgroup as the root of the hierarchy.
It would be a bit like doing a bind mount of a subtree of the main
hierarchy, but automatically enforced by the kernel.

>  > 8) per-mm owner field
>  > ----
>  >
>  > To remove the need for per-subsystem counted references from the mm.
>  > Being developed by Balbir Singh
>
>  I'm slooowly trying to whip together a swapfile namespace - not a
>  cgroup - which ties a swapfns to a list of swapfiles (where each
>  swapfile belongs to only one swapfns).

This would be to allow virtual servers to mount their own swapfiles?
Presumably there'd still be a use for a swap cgroup for job systems
that want to isolate swap usage without virtualization or requiring
jobs to mount their own swapfiles?

Paul
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