cpu masks: optimize and clean up cpumask_of_cpu()

Clean up and optimize cpumask_of_cpu(), by sharing all the zero words.

Instead of stupidly generating all possible i=0...NR_CPUS 2^i patterns
creating a huge array of constant bitmasks, realize that the zero words
can be shared.

In other words, on a 64-bit architecture, we only ever need 64 of these
arrays - with a different bit set in one single world (with enough zero
words around it so that we can create any bitmask by just offsetting in
that big array). And then we just put enough zeroes around it that we
can point every single cpumask to be one of those things.

So when we have 4k CPU's, instead of having 4k arrays (of 4k bits each,
with one bit set in each array - 2MB memory total), we have exactly 64
arrays instead, each 8k bits in size (64kB total).

And then we just point cpumask(n) to the right position (which we can
calculate dynamically). Once we have the right arrays, getting
"cpumask(n)" ends up being:

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

This brings other advantages and simplifications as well:

 - we are not wasting memory that is just filled with a single bit in
   various different places

 - we don't need all those games to re-create the arrays in some dense
   format, because they're already going to be dense enough.

if we compile a kernel for up to 4k CPU's, "wasting" that 64kB of memory
is a non-issue (especially since by doing this "overlapping" trick we
probably get better cache behaviour anyway).

[ mingo@elte.hu:

  Converted Linus's mails into a commit. See:

     http://lkml.org/lkml/2008/7/27/156
     http://lkml.org/lkml/2008/7/28/320

  Also applied a family filter - which also has the side-effect of leaving
  out the bits where Linus calls me an idio... Oh, never mind ;-)
]

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Mike Travis <travis@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 23 insertions, 3 deletions

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 8fa3b6d4a320..96d0509fb8d8 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -265,10 +265,30 @@ static inline void __cpus_shift_left(cpumask_t *dstp,
         bitmap_shift_left(dstp->bits, srcp->bits, n, nbits);
 }
 
+/*
+ * Special-case data structure for "single bit set only" constant CPU masks.
+ *
+ * We pre-generate all the 64 (or 32) possible bit positions, with enough
+ * padding to the left and the right, and return the constant pointer
+ * appropriately offset.
+ */
+extern const unsigned long
+        cpu_bit_bitmap[BITS_PER_LONG+1][BITS_TO_LONGS(NR_CPUS)];
+
+static inline const cpumask_t *get_cpu_mask(unsigned int cpu)
+{
+        const unsigned long *p = cpu_bit_bitmap[1 + cpu % BITS_PER_LONG];
+        p -= cpu / BITS_PER_LONG;
+        return (const cpumask_t *)p;
+}
+
+/*
+ * In cases where we take the address of the cpumask immediately,
+ * gcc optimizes it out (it's a constant) and there's no huge stack
+ * variable created:
+ */
+#define cpumask_of_cpu(cpu) ({ *get_cpu_mask(cpu); })
 
-/* cpumask_of_cpu_map[] is in kernel/cpu.c */
-extern const cpumask_t *cpumask_of_cpu_map;
-#define cpumask_of_cpu(cpu)     (cpumask_of_cpu_map[cpu])
 
 #define CPU_MASK_LAST_WORD BITMAP_LAST_WORD_MASK(NR_CPUS)