Serge E. Hallyn wrote:
> Quoting Balbir Singh (balbir@linux.vnet.ibm.com):
>> On Fri, Apr 11, 2008 at 8:18 PM, Serge E. Hallyn <serue@us.ibm.com> wrote:
>>> Quoting Paul Menage (menage@google.com):
>>>  > This is a list of some of the sub-projects that I'm planning for
>>>  > Control Groups, or that I know others are planning on or working on.
>>>  > Any comments or suggestions are welcome.
>>>  >
>>>  >
>>>  > 1) Stateless subsystems
>>>  > -----
>>>  >
>>>  > This was motivated by the recent "freezer" subsystem proposal, which
>>>  > included a facility for sending signals to all members of a cgroup.
>>>  > This wasn't specifically freezer-related, and wasn't even something
>>>  > that needed particular per-cgroup state - its only state is that set
>>>  > of processes, which is already tracked by crgoups. So it could
>>>  > theoretically be mounted on multiple hierarchies at once, and wouldn't
>>>  > need an entry in the css_set array.
>>>  >
>>>  > This would require a few internal plumbing changes in cgroups, in particular:
>>>  >
>>>  > - hashing css_set objects based on their cgroups rather than their css pointers
>>>  > - allowing stateless subsystems to be in multiple hierarchies
>>>  > - changing the way hierarchy ids are calculated - simply ORing
>>>  > together the subsystem would no longer work since that could result in
>>>  > duplicates
>>>  >
>>>  > 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
>>>  doesn't sound like this would have any performance effect on the general
>>>  case, so it sounds good.
>>>
>>>  I'd stick with mount semantics.  Just
>>>         mount -t cgroup -o remount,devices,cpu none /devwh"
>>>  should handle all cases, no?
>>>
>>>
>>>
>>>  > 3) Subsystem dependencies
>>>  > -----
>>>  >
>>>  > This would be a fairly simple change, essentially allowing one
>>>  > subsystem to require that it only be mounted on a hierarchy when some
>>>  > other subsystem was also present. The implementation would probably be
>>>  > a callback that allows a subsystem to confirm whether it's prepared to
>>>  > be included in a proposed hierarchy containing a specified subsystem
>>>  > bitmask; it would be able to prevent the hierarchy from being created
>>>  > by giving an error return. An example of a use for this would be a
>>>  > swap subsystem that is mostly independent of the memory controller,
>>>  > but uses the page-ownership tracking of the memory controller to
>>>  > determine which cgroup to charge swap pages to. Hence it would require
>>>  > that it only be mounted on a hierarchy that also included a memory
>>>  > controller. The memory controller would make no such requirement by
>>>  > itself, so could be used on its own without the swap controller.
>>>  >
>>>  >
>>>  > 4) Subsystem Inheritance
>>>  > ------
>>>  >
>>>  > This is an idea that I've been kicking around for a while trying to
>>>  > figure out whether its usefulness is worth the in-kernel complexity,
>>>  > versus doing it in userspace. It comes from the idea that although
>>>  > cgroups supports multiple hierarchies so that different subsystems can
>>>  > see different task groupings, one of the more common uses of this is
>>>  > (I believe) to support a setup where say we have separate groups A, B
>>>  > and C for one resource X, but for resource Y we want a group
>>>  > consisting of A+B+C. E.g. we want individual CPU limits for A, B and
>>>  > C, but for disk I/O we want them all to share a common limit. This can
>>>  > be done from userspace by mounting two hierarchies, one for CPU and
>>>  > one for disk I/O, and creating appropriate groupings, but it could
>>>  > also be done in the kernel as follows:
>>>  >
>>>  > - each subsystem "foo" would have a "foo.inherit" file provided by
>>>  > (and handled by) cgroups in each group directory
>>>  >
>>>  > - setting the foo.inherit flag (i.e. writing 1 to it) would cause
>>>  > tasks in that cgroup to share the "foo" subsystem state with the
>>>  > parent cgroup
>>>  >
>>>  > - from the subsystem's point of view, it would only need to worry
>>>  > about its own foo_cgroup objects  and which task was associated with
>>>  > each object; the subsystem wouldn't need to care about which tasks
>>>  > were part of each cgroup, and which cgroups were sharing state; that
>>>  > would all be taken care of by the cgroup framework
>>>  >
>>>  > I've mentioned this a couple of times on the containers list as part
>>>  > of other random discussions; at one point Serge Hallyn expressed some
>>>  > interest but there's not been much noise about it either way. I
>>>  > figured I'd include it on this list anyway to see what people think of
>>>  > it.
>>>
>>>  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.
>>>
>> I thought of doing something like this in libcg (having a daemon and a
>> client socket interface), but dropped the idea later. When all
>> controllers support multi-levels well, the plan is to create a
>> sub-directory in the cgroup hierarchy and give subtree ownership to
>> the application administrator.
>>
>>>  So I'm basically being quiet until I see whether libcg will suffice.
>>>
>> If you do have any specific requirements, we can cater to them right
>> now. Please do let us know. The biggest challenge right now is getting
>> a stable API.
> 
> It sounds like what you're talking about should suffice - the container
> can only write to its own subdirectory, and the control files therein
> should not allow the container to escape the bounds set for it, only to
> partition it.
> 
> The only thing that worries me is how subtle it may turn out to be to
> properly set up a container this way.  I.e. you'll need to
> 	mount --bind /etc/cgroups/mycontainer /vps/container1/etc/cgroups
> before the container is off and running and be able to then prevent
> the cgroup from mounting the host's /etc any other way.
> 
> As in so many other cases it shouldn't be too difficult with selinux,
> otherwise I suppose one thing you could do is to put the host's
> /etc/cgroup (or really the host's /) on partitionN, mount
> /etc/cgroup/container from another partitionM, and use the device
> whitelist (eventually, device namespaces) to allow the container to
> mount partitionM but not partitionN.
> 
> So that's the one place where kernel support might be kind of seductive,
> but I suspect it would just lead to either an unsafe, an inflexible, or
> just a hokey "solution".  So let's stick with libcg for now.  A daemon
> can always be written on top of it if people want, and if at some point
> we see a real need for kernel support we can talk about it then.
> 

Sounds fair to me. We intend to provide the basis for building a good daemon if
ever required. You see left overs in libcg.h (that I need to clean up).

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