linux.conf.au
Optimization

From: Rusty Russell <rusty@rustcorp.com.au>
To: Andrew Morton <akpm@zip.com.au>
Cc: torvalds@transmeta.com, linux-kernel@vger.kernel.org, k-suganuma@mvj.biglobe.ne.jp
Subject: Re: [PATCH] 2.5.21 Nonlinear CPU support 

In message <3D05A9E8.FF0DA223@zip.com.au> you write:
> and slowdown:

ARGH!  STOP IT!  I realize it's 'leet to be continually worrying about
possible microoptimizations, but I challenge you to *measure* the
slowdown between:

> > -       for (i = 0; i < smp_num_cpus; i++) {
> > -               int logical = cpu_logical_map(i);
and
> > +       for (i = 0; i < NR_CPUS; i++) {
> > +               if (!cpu_online(i))
> > +                       continue;

*Especially* in this context.  Sure, a new "max_cpu_number" or "cpu_for_each(i)" macro would fix this, but at the expense of using up additional stack in the reader's brain.

Let's not perpetuate the myth that everything in the kernel needs to be tuned to the last cycle at all costs, hm?

Yes, you stepped on a sore point 8)
Rusty.
PS.  Of course, you know the correct answer, anyway.
--
  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.