Number of threads in haskell program

Hi devs, Recently I became interested in obtaining the rough number of (non-GC) threads that are alive in a Haskell program. Intuitively this seemed like something that the RTS would expose in some way - but I couldn't find any such exposition in base. I then thought that I could simply total the size of each run_queue_hd in each capability, but the machinery for doing that doesn't seem to be exposed. Any thoughts? Thanks