On Tuesday 25 September 2007 17:52:00 Ralf Baechle wrote:

When debugging a kernel using a logic analyzer (!) a colleague recently
noticed that because the <linux/cpumasks.h> functions are based on the
generic bitops which support arbitrary size bitfields we had a relativly
high overhead resulting from this.  Here's the chainsaw edition of a patch
to optimize this for CONFIG_NR_CPUS <= BITS_PER_LONG.  Comments?

The right thing to test is not CONFIG_NR_CPUS, but just
do __builtin_constant_p(x) && (x) <= BITS_PER_LONG ? fast case : external call
in find_*_bit()

x86-64 has done this already for some time. But one issue is that 
that the cpumask walk functions currently do (n = find_*_bit()) >= maxbit ? maxbit : n
which also creates more overhead because some architectures get this
wrong (including x86-64 I must admit) 

-Andi