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    Subject[RFC PATCH v3 0/9] CPU hotplug: stop_machine()-free CPU hotplug

    This patchset removes CPU hotplug's dependence on stop_machine() from the CPU
    offline path and provides an alternative (set of APIs) to preempt_disable() to
    prevent CPUs from going offline, which can be invoked from atomic context.

    This is an RFC patchset with only a few call-sites of preempt_disable()
    converted to the new APIs for now, and the main goal is to get feedback on the
    design of the new atomic APIs and see if it serves as a viable replacement for
    stop_machine()-free CPU hotplug. A brief description of the algorithm is
    available in the "Changes in vN" section.

    Overview of the patches:

    Patch 1 introduces the new APIs that can be used from atomic context, to
    prevent CPUs from going offline.

    Patch 2 is a cleanup; it converts preprocessor macros to static inline

    Patches 3 to 8 convert various call-sites to use the new APIs.

    Patch 9 is the one which actually removes stop_machine() from the CPU
    offline path.

    Changes in v3:
    * Dropped the _light() and _full() variants of the APIs. Provided a single
    interface: get/put_online_cpus_atomic().

    * Completely redesigned the synchronization mechanism again, to make it
    fast and scalable at the reader-side in the fast-path (when no hotplug
    writers are active). This new scheme also ensures that there is no
    possibility of deadlocks due to circular locking dependency.
    In summary, this provides the scalability and speed of per-cpu rwlocks
    (without actually using them), while avoiding the downside (deadlock
    possibilities) which is inherent in any per-cpu locking scheme that is
    meant to compete with preempt_disable()/enable() in terms of flexibility.

    The problem with using per-cpu locking to replace preempt_disable()/enable
    was explained here:

    Basically we use per-cpu counters (for scalability) when no writers are
    active, and then switch to global rwlocks (for lock-safety) when a writer
    becomes active. It is a slightly complex scheme, but it is based on
    standard principles of distributed algorithms.

    Changes in v2:
    * Completely redesigned the synchronization scheme to avoid using any extra

    * Provided APIs for 2 types of atomic hotplug readers: "light" (for
    light-weight) and "full". We wish to have more "light" readers than
    the "full" ones, to avoid indirectly inducing the "stop_machine effect"
    without even actually using stop_machine().

    And the patches show that it _is_ generally true: 5 patches deal with
    "light" readers, whereas only 1 patch deals with a "full" reader.

    Also, the "light" readers happen to be in very hot paths. So it makes a
    lot of sense to have such a distinction and a corresponding light-weight

    Links to previous versions:

    Comments and suggestions welcome!

    Paul E. McKenney (1):
    cpu: No more __stop_machine() in _cpu_down()

    Srivatsa S. Bhat (8):
    CPU hotplug: Provide APIs to prevent CPU offline from atomic context
    CPU hotplug: Convert preprocessor macros to static inline functions
    smp, cpu hotplug: Fix smp_call_function_*() to prevent CPU offline properly
    smp, cpu hotplug: Fix on_each_cpu_*() to prevent CPU offline properly
    sched, cpu hotplug: Use stable online cpus in try_to_wake_up() & select_task_rq()
    kick_process(), cpu-hotplug: Prevent offlining of target CPU properly
    yield_to(), cpu-hotplug: Prevent offlining of other CPUs properly
    kvm, vmx: Add atomic synchronization with CPU Hotplug

    arch/x86/kvm/vmx.c | 8 +-
    include/linux/cpu.h | 8 +-
    kernel/cpu.c | 254 ++++++++++++++++++++++++++++++++++++++++++++++++++-
    kernel/sched/core.c | 22 ++++
    kernel/smp.c | 63 ++++++++-----
    5 files changed, 321 insertions(+), 34 deletions(-)

    Srivatsa S. Bhat
    IBM Linux Technology Center

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