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    SubjectRe: [PATCH 18/35] cpumask: add nr_cpumask_bits
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    On Tuesday 21 October 2008 04:03:37 Mike Travis wrote:
    > When nr_cpu_ids is set to CONFIG_NR_CPUS then references to nr_cpu_ids
    > will return the maximum index of the configured NR_CPUS (+1) instead
    > of the maximum index of the possible number of cpus (+1). This results
    > in extra unused memory being allocated by functions that are setting up
    > arrays of structs to keep track of per cpu items.

    1) I like the name in this context: it's a beacon of sanity after NR_CPUS and
    nr_cpu_ids. But it's not so clearly a win when general code uses it:

    if (cpumask_first(mymask) == nr_cpumask_bits) ...

    vs:

    if (cpumask_first(mymask) == nr_cpu_ids) ...

    2) This breaks anyone who tests that the iterators etc. return == nr_cpu_ids.
    One of the other patches tried to change them from NR_CPUS to nr_cpu_ids,
    that should now be revisited & reaudited.

    3) Noone should be naively allocating "* nr_cpu_ids" arrays, they should be
    using per-cpu pointers. Not doing so wastes memory on non-contiguous
    processor systems.

    4) It should be a constant not be dependent on CONFIG_CPUMASK_OFFSTACK, but
    rather as it was on NR_CPUS > BITS_PER_LONG. I think that's the sweet
    spot, and should also make your 2MB "gain" vanish.

    That's why I suggested a max_possible_cpu() function, and using that for those
    who really want to do allocations, who should be audited anyway, see (3). I
    don't want it as prominent as nr_cpu_ids, which is usually the Right Thing,
    and always safe.

    Cheers,
    Rusty.
    PS. I have part of a patch for this...


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