Messages in this thread | | | Date | Mon, 28 Jul 2008 11:32:33 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [git pull] cpus4096 fixes |
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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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