During young-gc, `PSThreadRootsTaskClosure` is applied on every thread, and it visits all oops and nmethods within a thread.

However, the actual set of nmethods that needs visiting are more accurately visited already:

```
    case ParallelRootType::code_cache:
      {
        MarkingNMethodClosure code_closure(&roots_to_old_closure, NMethodToOopClosure::FixRelocations, false /* keepalive nmethods */);
        ScavengableNMethods::nmethods_do(&code_closure);
      }
```

There is no need to visit nmethods reachable from threads.