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    SubjectPer-CPU data as a structure

    At the moment, data specific to a CPU is stored in different, fixed-
    size separate arrays by means of the "percpu framework". I'm working
    on some changes to modify the way some CPUs are represented, and I'm
    wondering what's the rationale behind such a representation.

    At first sight, it'd seem more reasonable to have a structure holding
    all the information that is CPU-specific (as is done with any object
    represented within the system). After searching the mail archives, I
    see that similar changes were proposed before, but those threads did
    not seem to get any reply (so I'm assuming that the changes were not

    Similarly, and if I understood it correctly, the PDA (Per-processor
    Data Area) also aims to do the above, but at the moment it only
    contains some fields and is not defined in all platforms. There are
    still a lot of usages of the percpu functionality (such as, e.g., in

    Part of my changes introduce a new structure that is able to
    represent any kind of CPU (and which each platform can extend to add
    new information to it). It is supposed to supersede the per-cpu
    definitions. I bet this could also be redone by using percpu in some
    way... The thing is I am willing to share my work when I've finished
    it (it is still very much work-in-progress), but first I'm interested
    to know if adding this new structure is a crazy idea (meaning I
    should stick to percpu wherever possible) or something that can be
    accepted later on.

    Summarizing, my questions are:
    - Why is the code currently using multiple separate arrays (percpu)
    to hold CPU information instead of a structure?
    - Could this structure-based approach (instead of all these separate
    arrays) be considered for inclusion into the system?

    As far as I can tell, the advantage of percpu is that you can define
    new "fields" anywhere in the code and independently from the rest of
    the system. Also, I seem to understand that there are performance
    advantages related to this. But on the other hand, percpu seems like
    an unnatural approach to "reimplement" regular structures.

    Thank you very much.

    Julio M. Merino Vidal <>

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