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    Subject[PATCH][plugsched 0/28] Pluggable cpu scheduler framework
    With the recent interest in varying the cpu schedulers in linux, this
    set of patches provides a modular framework for adding multiple
    boot-time selectable cpu schedulers. William Lee Irwin III came up with
    the original design and I based my patchset on that.

    This code was designed to touch the least number of files, be completely
    arch-independant, and allow extra schedulers to be coded in by only
    touching Kconfig, scheduler.c and scheduler.h. It should incur no
    overhead when run and will allow you to compile in only the scheduler(s)
    you desire. This allows, for example, embedded hardware to have a tiny
    new scheduler that takes up minimal code space.

    This works by taking all functions that will be common to all scheduler
    designs and moving them from kernel/sched.c into kernel/scheduler.c.

    Then it adds the scheduler driver struct into scheduler.h which is a set
    of pointers to functions that will all have per-scheduler versions.
    include/linux/scheduler.h has the definitions for the scheduler driver

    kernel/sched.c remains as the default cpu scheduler in the same place to
    minimise the patch size and portability of the patch set.

    All variables of the task_struct that could be unique to a different
    scheduler are now in a private struct held within a union in
    task_struct. rt_priority and static_priority are kept global for
    userspace interface and for the possibility of adding run-time switching
    later on.

    The main disadvantage of this design is that there will (initially) be a
    lot of code duplication by different scheduler designs in their own
    private space. This will mean that if a new scheduler uses the same smp
    balancing algorithm then it will need to be modified to keep in sync
    with changes to the default scheduler. If, for example, you modified
    just the dynamic priority component of the current scheduler and left
    the runqueue and task_struct the same, you could make it depend on the
    default scheduler and point most functions to that one.

    However, the same disadvantage can be a major advantage. The fact that
    so much of the scheduler is privatised means that wildly different
    designs can be plugged in without any reference to the number of
    runqueues, frame schedulers could be plugged in, shared runqueues (eg on
    numa designs), and we could even keep new balancing in a "developing"
    arm of the scheduler that can be booted by testers and so on.

    What is left to do is add a per-scheduler entry into /sys which can be
    used to modify the unique controls each scheduler has, and write up some
    documentation for this and staircase.

    Anyway the patches will follow shortly, and then (not surprisingly) a
    port of the staircase scheduler to plug into this framework which can
    also be used as an example for others wishing to code up or port their

    While I have tried to build this on as many configurations as possible,
    I am sure breakage will creep in given the type of modification so I
    apologise in advance.

    Patches for those who want to download them separately here:

    Here is a diffstat of the patches rolled up.

    fs/proc/array.c | 2
    fs/proc/proc_misc.c | 14
    include/linux/init_task.h | 5
    include/linux/sched.h | 39 -
    include/linux/scheduler.h | 83 ++
    init/Kconfig | 37 +
    init/main.c | 10
    kernel/Makefile | 3
    kernel/sched.c | 1313
    kernel/scheduler.c | 1201
    mm/oom_kill.c | 2
    11 files changed, 1599 insertions(+), 1110 deletions(-)

    Thanks to William Lee Irwin III for design help, Alex Nyberg for testing
    and a bootload of others for ideas and help with the coding.


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