Re: [PATCH] sched.c

[Andrew Morton <akpm@zip.com.au>]

Lahti Oy wrote:
> 
> Small patch that makes NR_CPUS loops decrement from 31 to 0 in sched.c to
> squeeze out some cycles (of course only on SMP machines). Also deprecated a
> macro that was only used once in the code and changed one if-conditional to
> else if.
> 
> ...
> 
> - for (i = 0; i < NR_CPUS; i++)
> + for (i = NR_CPUS; i; i--)
>    sum += cpu_rq(i)->nr_running;

Off-by-one there.  You'd want

	for (i = NR_CPUS; --i >= 0; )

or something similarly foul ;)

But these are not performance-critical functions.  And by far the
most inefficient part of them is that they're reading data for
CPUs which cannot exist.   That can be fixed with a `cpu_possible(i)'
test in there, but Rusty was going to give us a `for_each_cpu' macro.
We haven't seen that yet.