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    SubjectRe: [RFC PATCH V4 5/5] cpuidle: cpuidle driver for apm


    On 03/24/2011 02:02 AM, Len Brown wrote:
    >>> Also wondering why you would ever have a different idle routine on
    >>> different cpus?
    >>
    >> Yes, this is an ongoing debate. Apparently it is a possibility
    >> because of ACPI bugs. CPU's can have asymmetric C-states
    >> and overall different idle routines on different cpus. Please
    >> refer to http://lkml.org/lkml/2009/9/24/132 and
    >> https://lkml.org/lkml/2011/2/10/37 for a discussion around this.
    >
    > Althought the ACPI specification allows the BIOS to tell the OS
    > about different C-states per-processor, I know of zero system
    > in the field and zero systems in development that require that
    > capability. That isn't a guarantee that capability will never
    > be used, but I'm not holding my breath.
    >
    > If there are systems with broken tables that make them
    > appear asymetric, then we should have a workaround that handles
    > that case, rather than complicating the normal code for
    > the broken case.
    >
    > So I recommend deleting the extra per-cpu registration stuff
    > unless there is some other architecture that requires it
    > and can't hadle the asymmetry in another way.

    Yes, lets go forward with removal of per-cpu registration
    and handle rare case of asymmetry in some other may.

    Using intersection or union of C-states for each cpu may
    be a solution. Using intersection or lowest common C-state
    has the corner case that we could have packages/cores
    supporting a new lower C-state in case of thermal limit and
    they would want OS to go to this state. Using intersection
    or lowest common C-state may prevent this.

    Another option is to use union of C-states;
    but I am not sure what happens if a CPU uses a state that
    is not reported for it???

    Maybe there is some other way to handle asymmetry ??

    >
    >> I have posted a patch series that does global registration
    >> i.e same idle routines for each cpu. Please check
    >> http://lkml.org/lkml/2011/3/22/161 . That series applies on
    >> top of this series. Global registration significantly
    >> simplifies the design, but still we are not sure about the
    >> direction to take.
    >
    > I'll review that.

    Thanks; please review especially the data structure changes
    https://lkml.org/lkml/2011/3/22/162

    -Trinabh


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