Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

[Jes Sorensen <jes@sgi.com>]

Jeremy Fitzhardinge wrote:
> Jens Axboe wrote:
>> On Thu, Apr 24 2008, Russell King wrote:
>>> On Tue, Apr 22, 2008 at 08:50:17PM +0200, Jens Axboe wrote:
>>>> +        data->csd.func(data->csd.info);
>>>> +
>>>> +        spin_lock(&data->lock);
>>>> +        cpu_clear(cpu, data->cpumask);
>>>> +        WARN_ON(data->refs == 0);
>>>> +        data->refs--;
>>>> +        refs = data->refs;
>>>>       
>>> Probably a silly question, but what does data->refs do that
>>> cpus_empty(data->cpumask) wouldn't do?  (as indeed ARM presently does.)
>>
>> I guess it can be marginally slower for NR_CPUS > BITS_PER_LONG,
>> otherwise there's absolutely no reason to have a seperate ref counter.
> 
> Jes was concerned about scanning bitmasks on a 4096 CPU Altix.  I'm not 
> sure its all that important, but a refcount check would definitely be 
> quicker.

I just felt it was silly to do a bigger test if it wasn't necessary.
Even on a 4096 CPU box it's probably barely noticeable, but if it adds
cost then I'd be in favor of keeping the slightly faster version. Maybe
it would be worth doing a branched version, one for
NR_CPUS <= BITS_PER_LONG and one for the other case?

Cheers,
Jes