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    Subject[RFC/PATCH 0/4] cpus, nodes, and the device model: dynamic cpu registration
    Hi there-

    I know of at least two platforms (ppc64 and ia64) which allow cpus to
    be physically or logically added and removed from a running system.
    These are distinct operations from onlining or offlining, which is
    well supported already. Right now there is little support in the core
    cpu "driver" for dynamic addition or removal. The patch series which
    follows implements support for this in a way which will (hopefully)
    reduce code duplication and enforce some uniformity across the
    relevant architectures.

    For starters, the current situation is that cpu sysdevs are registered
    from architecture code at boot. Already we have inconsistencies
    betweeen the arches -- ia64 registers only online cpus, ppc64
    registers all "possible" cpus. I propose to move the initial cpu
    sysdev registrations to the cpu "driver" itself (drivers/base/cpu.c),
    and to register only "present" cpus at boot.

    But that breaks all the arch code which explicitly registers cpu
    sysdevs. For instance, ppc64 wants to hang all kinds of attributes
    off of the cpu devices for performance counter stuff. So code such as
    this needs to be converted to register a sysdev_driver with the cpu
    device class, which will allow the ppc64 code to be notified when a
    cpu is added or removed. In the patches that follow I include the
    changes necessary for ppc64, as an example. (An arch sweep or
    temporary compatibility hack can come later if I get positive
    responses to this approach.)

    Also, there is the matter of the base numa "node" driver. Currently
    the cpu driver makes symlinks from nodes to their cpus. This seems
    backwards to me, so I have changed the node driver to create or remove
    the symlinks upon cpu addition or removal, respectively, also using
    the sysdev_driver approach. I've also converted base/drivers/node.c
    to doing the boot-time node registration itself, like the cpu code.

    Finally, I've added two new interfaces which wrap all this up --
    cpu_add() and cpu_remove(). These carry out the necessary update to
    cpu_present_map and take care of the cpu device registration. These
    are meant to be invoked from the platform-specific code which
    discovers and removes processors.

    This is the first real device model-related hacking I've done. I'm
    hoping Greg or Patrick will tell me whether I'm on the right track or
    abusing the APIs :)

    These patches have been boot-tested on ppc64. I haven't gotten to
    test the removal paths yet.

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