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    SubjectRe: [PATCH v4] acpi: Fix CPU hot removal problem
    On Fri, Sep 23, 2011 at 1:49 AM, canquan.shen <shencanquan@huawei.com> wrote:
    > On 2011/9/23 0:53, Bjorn Helgaas wrote:
    >>
    >> On Wed, Sep 14, 2011 at 8:56 PM, Bjorn Helgaas<bhelgaas@google.com>
    >>  wrote:
    >>>
    >>> On Wed, Sep 14, 2011 at 7:06 PM, canquan.shen<shencanquan@huawei.com>
    >>>  wrote:
    >>>>
    >>>> We run linux as a guest in Xen environment. When we used the xen tools
    >>>> (xm vcpu-set<n>) to hot add and remove vcpu to and from the guest, we
    >>>> encountered the failure on vcpu removal. We found the reason is that it
    >>>> didn't go to really remove cpu in the cpu removal code path.
    >>>>
    >>>> This patch adds acpi_bus_trim in acpi_process_hotplug_notify to fix this
    >>>> issue. With this patch, it works fine for us.
    >>>>
    >>>> Signed-off-by:Canquan Shen<shencanquan@huawei.com>
    >>>
    >>> Reviewed-by: Bjorn Helgaas<bhelgaas@google.com>
    >>
    >> On second thought, let's think about this a bit more.
    >>
    >> As I mentioned before, I have a long-term goal to move the hotplug
    >> flow out of drivers and into the ACPI core.  That will be easier if
    >> the code in the drivers is as generic as possible.
    >>
    >> The dock and acpiphp hot-remove code calls acpi_bus_trim(), then
    >> evaluates _EJ0.  The core acpi_bus_hot_remove_device() function
    >> already does both acpi_bus_trim() and _EJ0.  This function is
    >> currently only used when we write to sysfs "eject" files, but I wonder
    >> if we should use it in acpi_processor_hotplug_notify() as well.
    >>
    >> That would get us one step closer to removing this gunk from the
    >> drivers and having acpi_bus_notify() look something like this:
    >>
    >>     case ACPI_NOTIFY_EJECT_REQUEST:
    >>         driver->ops.remove(device);
    >>         acpi_bus_hot_remove_device(device);
    >>         break;
    >>
    >> There is a description of a CPU hot-remove that does include _EJ0
    >> methods in the "DIG64 Hot-Plug&  Partitioning Flows Specification"
    >> [1], sec 2.2.4.  I know this document is Itanium-oriented, but this
    >> part seems fairly generic and it's the only description of the process
    >> I've seen so far.
    >>
    >> So would using acpi_bus_hot_remove_device() instead of acpi_bus_trim()
    >> also solve your problem, Canquan?
    >>
    > Yes. It can solve my problem.
    > I fully aggree to replace acpi_bus_hot_remove_device() to acpi_bus_trim().
    > Initially I insert the acpi_bus_hot_remove_device() in acpi_bus_notify
    > function . lately I think I should give a chance for user,and so send
    >  KOBJ_OFFLINE message to the udvev module.
    >
    > But why add the  driver->ops.remove(device) before
    > acpi_bus_hot_remove_device(device).  it can be called in
    > acpi_bus_hot_remove_device code path as bellowing:
    >    acpi_bus_trim
    >        acpi_bus_remove
    >            device_release_driver
    >                __device_release_driver
    >                        acpi_device_remove
    >                                acpi_drv->ops.remove

    OK. Maybe the future acpi_bus_notify() code would be even simpler.
    The point is that the ACPI core should handle the notification, call
    the driver's .remove() method, and do whatever namespace cleanup is
    required (i.e., acpi_bus_trim()). None of this should be in the
    driver itself.

    Can you re-post your patch, using acpi_bus_hot_remove_device() instead
    of acpi_bus_trim()? Please include Khalid's tweak, too, so we don't
    print warnings for CPUs that don't supply _EJ0 methods.

    Bjorn
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