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SubjectRe: [PATCH 18/35] cpumask: add nr_cpumask_bits
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) ...


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.

PS. I have part of a patch for this...

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