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SubjectRe: [RFC] CPUMASK: proposal for replacing cpumask_t
Rusty Russell wrote:
> On Thursday 11 September 2008 08:47:58 Mike Travis wrote:
>> Here's an initial proposal for abstracting cpumask_t to be either
>> an array of 1 or a pointer to an array... Hopefully this will
>> minimize the amount of code changes while providing the capabilities
>> this change is attempting to do.
>> Comments most welcome. ;-)
> I think this is still "wrong way go back".
> I'm yet to be convinced that we really need to allocate cpumasks in any fast
> paths. And if not, we should simply allocate them everywhere. I'd rather
> see one #ifdef around a place where we can show a perf issue.
> Get rid of CPU_MASK_ALL et al in favour of cpu_mask_all. And cpu_mask_any_one
> instead of CPU_MASK_CPU0 since that's usually what they want.
> API looks like so (look Ma, no typedefs!)
> struct cpumask *cpus;
> cpus = cpumask_alloc();
> if (!cpus)
> return -ENOMEM;
> cpumask_init_single(cpunum);
> OR
> cpumask_init(cpu_mask_all);
> ...
> cpumask_free(cpus);
> Unmistakable and really hard to screw up. You can even be clever and not
> reveal the struct cpumask definition so noone can declare one by accident...
> Cheers,
> Rusty.

Using a typedef came from Linus, and the idea is basically if NR_CPUS fits
into a long, then it's carried as an array of one (ie., local variable).
If it's bigger, then it's a pointer to a remote array. The references can
all be pointers (*cpumask), though most of the references use the cpu_XXX
operators which already treat the references correctly (in my proposal,
that is). That way, small systems can optimize out the indirect reference
and the overhead becomes zero.

Also, cpumask_alloc/free() becomes nop's for small systems.

But I like the idea of dumping some of the initializers. I should have
made CPU0 "cpumask_of_cpu(0)". I'll have to look at where they are used to
see if this is feasible.


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