I defined a cpuset to contain 12 of the 24 CPUs on this system.
I put a process into it. /proc/PID/cpuset gives the right answer.
top -p PID, followed by 1, shows lots of CPU time on CPUs not in the set.
As an additional oddity, the CPU time shows up in %ni instead of %us, even though no renicing is going on.
Ubuntu 11.10, a 2-socket machine with Intel(R) Xeon(R) CPU X5680 @ 3.33GHz chips.
my bad for thinking I had four sockets here.