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Subject[PATCH 0/7] Containers (V8): Generic Process Containers
--

This is an update to my multi-hierarchy multi-subsystem generic
process containers patch. Changes since V7 (12th Feb) include:

- Removed the config-time choice of the number of supported
hierarchies - this is now completely dynamic; new hierarchies are
allocated on demand, and freed when no longer in use.

- Subsystems are now registered at compile-time in
linux/container_subsys.h. This allows for faster access to subsystem
state since the id is a compile-time constant, so there's only a
single extra pointer dereference compared to having a pointer directly
in the task_struct. It also avoids wasting space with unused subsystem
pointers.

- Removed the container pointers from container_group - this results
in a structure very similar to Srivatsa Vaddagiri's rcfs
approach. (RCFS uses the nsproxy object rather than the
container_group object; merging container_group and nsproxy would be
pretty straightforward if desired).

- Removed callback_mutex from container subsystem to be purely back in
the cpuset subsystem. Renamed manage_mutex to container_mutex.

- Condensed post_attach_task() into attach_task() now that
callback_mutex is purely within cpuset.c

- Simplified the container_subsys_state reference counting - stricter
rules on liveness make adding reference counts cheaper.

Still TODO:

- decide whether "Containers" is an acceptable name for the system
given its usage by some other development groups, or whether something
else (ProcessSets? ResourceGroups?) would be better

- decide whether merging container_group and nsproxy is desirable

- add a hash-table based lookup for container_group objects.

- use seq_file properly in container tasks files (and also in
cpuset_attach_task) to avoid having to allocate a big array for all
the container's task pointers.

- add back support for the "release agent" functionality

- lots more testing

- define standards for container file names

Generic Process Containers
--------------------------
There have recently been various proposals floating around for
resource management/accounting and other task grouping subsystems in
the kernel, including ResGroups, User BeanCounters, NSProxy
containers, and others. These all need the basic abstraction of being
able to group together multiple processes in an aggregate, in order to
track/limit the resources permitted to those processes, or control
other behaviour of the processes, and all implement this grouping in
different ways.
Already existing in the kernel is the cpuset subsystem; this has a
process grouping mechanism that is mature, tested, and well documented
(particularly with regards to synchronization rules).

This patchset extracts the process grouping code from cpusets into a
generic container system, and makes the cpusets code a client of
the container system.

It also provides several example clients of the container system,
including ResGroups, BeanCounters and namespace proxy.

The change is implemented in three implementation patches, plus four example
subsystems that aren't necessarily intended to be merged as part of
this patch set, but demonstrate the applicability of the framework.

1) extract the process grouping code from cpusets into a standalone system

2) remove the process grouping code from cpusets and hook into the
container system

3) convert the container system to present a generic multi-hierarchy
API, and make cpusets a client of that API

4) example of a simple CPU accounting container subsystem. Useful as a
boilerplate for people implementing their own subsystems.

5) example of implementing ResGroups and its numtasks controller over
generic containers

6) example of implementing BeanCounters and its numfiles counter over
generic containers

7) example of integrating the namespace isolation code (sys_unshare()
or various clone flags) with generic containers, allowing virtual
servers to take advantage of other resource control efforts.

The intention is that the various resource management and
virtualization efforts can also become container clients, with the
result that:

- the userspace APIs are (somewhat) normalised

- it's easier to test out e.g. the ResGroups CPU controller in
conjunction with the BeanCounters memory controller, or use either of
them as the resource-control portion of a virtual server system.

- the additional kernel footprint of any of the competing resource
management systems is substantially reduced, since it doesn't need
to provide process grouping/containment, hence improving their
chances of getting into the kernel

Signed-off-by: Paul Menage <menage@google.com>
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