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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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