i misread the code a bit and was confused with some stuff i did a while ago with cpusets and similar parsing.
but you can't trust this (cpusets and cgroup are optional to start with)

eg, on my laptop (dual core with HT) (with cgroups active!)
{{{
$ cat /proc/$$/status |grep Cpus_allowed_list
Cpus_allowed_list:	0-15
$ taskset -cp $$
pid 4956's current affinity list: 0-3
$ python -c 'from vsc.utils.affinity import sched_getaffinity; print sum(sched_getaffinity().cpus)' 
4
}}}

the initial mask that is reported in status is not the actual value (after setting it, it becomes correct when using eg cgroups)