[PATCH 18/35] cpumask: add nr_cpumask_bits

[Rusty Russell]

On Tuesday 21 October 2008 04:03:37 Mike Travis wrote:
> When nr_cpu_ids is set to CONFIG_NR_CPUS then references to nr_cpu_ids
> will return the maximum index of the configured NR_CPUS (+1) instead
> of the maximum index of the possible number of cpus (+1).  This results
> in extra unused memory being allocated by functions that are setting up
> arrays of structs to keep track of per cpu items.

1) I like the name in this context: it's a beacon of sanity after NR_CPUS and
   nr_cpu_ids.  But it's not so clearly a win when general code uses it:

	if (cpumask_first(mymask) == nr_cpumask_bits) ...

   vs:
   
	if (cpumask_first(mymask) == nr_cpu_ids) ...

2) This breaks anyone who tests that the iterators etc. return == nr_cpu_ids.
   One of the other patches tried to change them from NR_CPUS to nr_cpu_ids,
   that should now be revisited & reaudited.

3) Noone should be naively allocating "* nr_cpu_ids" arrays, they should be
   using per-cpu pointers.  Not doing so wastes memory on non-contiguous
   processor systems.

4) It should be a constant not be dependent on CONFIG_CPUMASK_OFFSTACK, but
   rather as it was on NR_CPUS > BITS_PER_LONG.  I think that's the sweet
   spot, and should also make your 2MB "gain" vanish.

That's why I suggested a max_possible_cpu() function, and using that for those 
who really want to do allocations, who should be audited anyway, see (3).  I 
don't want it as prominent as nr_cpu_ids, which is usually the Right Thing, 
and always safe.

Cheers,
Rusty.
PS.  I have part of a patch for this...