Messages in this thread | | | Date | Mon, 28 Jul 2008 12:04:38 -0700 | From | Mike Travis <> | Subject | Re: [git pull] cpus4096 fixes |
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Linus Torvalds wrote: > > 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.
The 2Mb's of initdata is released, I just meant that if you really have 4k cpus in the system, you'll probably have 4k * [2 .. 32 Gb (or more?)] of memory. The Nahalem memory limit is (iirc) 44 bits.
Originally, I only had the constant for cpu(0) but since it _was_ originally a constant (alibi, rvalue only), then it might be thought that it's valid to use any cpu# before setup_per_cpu_areas is called.
> >> 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.
Very cool, thanks!!
> > 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
Don't worry, I'll beat it to death... ;-) [and try not to screw up the acknowledgments this time... ;-)]
Thanks again, Mike
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