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    SubjectRe: [git pull] cpus4096 fixes


    On Mon, 28 Jul 2008, Mike Travis wrote:
    >
    > Sorry, I didn't know that was the protocol. And yes, the clever idea of
    > compacting the memory is a good one (wish I would have thought of it... ;-)
    > But, and it's a big but, if you really have 4096 cpus present (not NR_CPUS,
    > but nr_cpu_ids), then 2MB is pretty much chump change.

    Umm. Yes, it's chump change, but if you compile a kernel to be generic,
    and you actually only have a few CPU's, it's no longer chump change.

    > But I'll redo the patch again.

    Here's a trivial setup, that is even tested. It's _small_ too.

    /* cpu_bit_bitmap[0] is empty - so we can back into it */
    #define MASK_DECLARE_1(x) [x+1][0] = 1ul << (x)
    #define MASK_DECLARE_2(x) MASK_DECLARE_1(x), MASK_DECLARE_1(x+1)
    #define MASK_DECLARE_4(x) MASK_DECLARE_2(x), MASK_DECLARE_2(x+2)
    #define MASK_DECLARE_8(x) MASK_DECLARE_4(x), MASK_DECLARE_4(x+4)

    static const unsigned long cpu_bit_bitmap[BITS_PER_LONG+1][BITS_TO_LONGS(NR_CPUS)] = {
    MASK_DECLARE_8(0), MASK_DECLARE_8(8),
    MASK_DECLARE_8(16), MASK_DECLARE_8(24),
    #if BITS_PER_LONG > 32
    MASK_DECLARE_8(32), MASK_DECLARE_8(40),
    MASK_DECLARE_8(48), MASK_DECLARE_8(56),
    #endif
    };

    static inline const cpumask_t *get_cpu_mask(unsigned int nr)
    {
    const unsigned long *p = cpu_bit_bitmap[1 + nr % BITS_PER_LONG];
    p -= nr / BITS_PER_LONG;
    return (const cpumask_t *)p;
    }

    that should be all you need to do.

    Honesty in advertizing: my "testing" was some trivial user-space harness,
    maybe I had some bug in it. But at least it's not _horribly_ wrong.

    And yes, this has the added optimization from Viro of overlapping the
    cpumask_t's internally too, rather than making them twice the size. So
    with 4096 CPU's, this should result 32.5kB of static const data.

    Linus


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