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

[Jeremy Fitzhardinge]

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.

   J
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