Poor man's profiler, or profiling a running program with a cheap combination of gdb, awk and scripting:

Sampling tools like oprofile or dtrace's profile provider don't really provide methods to see what [multithreaded] programs are blocking on - only where they spend CPU time. Though there exist advanced techniques (such as systemtap and dtrace call level probes), it is overkill to build upon that [...]
[...] one needs to improvise, like.. use debuggers - they can walk threads and provide stacks.

http://poormansprofiler.org/