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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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