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To: Rusty Russell <rusty@rustcorp.com.au>
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Subject: Re: Optimisation for smp_num_cpus loop in hotplug 
In-Reply-To: Message from Rusty Russell <rusty@rustcorp.com.au> 
   of "Sat, 22 Jun 2002 01:14:46 +1000." <E17LQ7b-0000Iz-00@wagner.rustcorp.com.au> 
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Date: Fri, 21 Jun 2002 11:31:44 -0400
From: James Bottomley <James.Bottomley@steeleye.com>
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rusty@rustcorp.com.au said:
> Yeah, it's simple, and none of the current ones are really critical.
> But I think we're better off with:
> 	for (i = first_cpu(); i < NR_CPUS; i = next_cpu(i)) {

> Which is simple enough not to need an iterator macro, and also has the
> bonus of giving irq-balancing et al. an efficient, portable way of
> looking for the "next" cpu. 

So you're thinking that next_cpu(i) is something like

__ffs((~(unsigned)((1<<i)-1) & cpu_online_map)

plus an extra exception piece to take next_cpu(i) above NR_CPUS if we have no 
remaining CPUs (because __ffs would be undefined)?  It's the exception piece 
that I don't see how to do really efficiently.

James