Look to /proc/[pid]/stat to make sure all processes are sleeping.

This is the best strategy so far - we first: make sure we think everyone's blocked, run sched_yield, sleep in select for 1us, and finally look at /proc/[pid]/stat to make sure everyone is indeed sleeping and kernel has nothing more for us.
trace_read() shall return a count and never block
Add next step: wait until we're the only process running.