That’s not to be minimized. Sounds like very much on the critical path, and definitely not some shitty infotainment sidecar. If that crashed, you couldn’t control core functions of the sub? Are there overrides?
Btw, thanks for sharing these details. I would never have learnt this. Love HN sometimes.
Sure, it's an important component of the entire system. Much as a steering wheel is an important component of a car, it still doesn't "run" the car. You need to interface, yes obviously; but if the machine crashes the sub operates merrily along like usual and any of the other control computers can continue to give it commands.
As to overrides, if you lost every single control computer and the direct interfaces (the analog control boards you see in crucial areas of naval vessels) then the overall system has a fallback procedure. It's not documented or public knowledge, but assumedly it would surface and activate an emergency beacon as the vessel is crippled (and likely damaged) at that point.
For US nuclear subs: All of the core components and electronics use a custom RTOS; older ones built in Ada, newer ones built in C++. The "control computers" (the computers that humans interface with to tell core components what to do) aren't running anything like Ubuntu or Fedora, but instead run a heavily ruggedized and customized Linux system based on the Titanium suite from Star Lab.
For UK subs, they run a similar custom OS for the modules and electronics and (until 2017 or so, at least) run Windows XP for their control computers.
I have no idea what other nations use; but I would assume they are similarly setup.
That's not to disavow the fact that clearly the US govt trusts those Linux computers to be reliable, thus their investment in them. So it is an accomplishment, they just don't "run" anything.