http://en.kioskea.net/faq/616-multicore-cpu-how-to-disable-a-core
During boot,
add maxcpus=X in the kernel command line
Without reboot,
(From http://www.cyberciti.biz/faq/debian-rhel-centos-redhat-suse-hotplug-cpu/)
# cd /sys/devices/system/cpu
# ls -l
# echo 0 > /sys/devices/system/cpu/cpu6/online
# grep “processor” /proc/cpuinfo
# echo 1 > /sys/devices/system/cpu/cpu6/online
# grep “processor” /proc/cpuinfo