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

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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

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