Hey,
Hope you don't mind me chiming in, as someone responsible for some of the mess :-)
Testing my memory a bit but... sub-interpreters were originally used because some badly behaved dependencies (I think it was one of the web server libraries?) used global mutable state on their own module, and thereby caused a problem if two mgr modules were both using that dependency.
Using sub-interpreters put us on the fringes of CPython use cases, which isn't a great place to be. The other option is to use separate processes rather than multiple interpreters in the same process. At the time, that seemed too expensive (in terms of developer time), although process separation would be the preferable level of separation for functionally distinct units.
Some thoughts on options:
A) Isolate modules in processes, tethered to a central ceph-mgr process that provides a new RPC interface that mimics the existing MgrModule interface. Lots of work. Will introduce substantial runtime overhead.
B) Isolate modules in processes, where each is a first class RADOS client -- basically run N ceph-mgr daemons, each hosting a particular module (or group of modules, to reduce overhead for tiny modules). Much less work, but would need careful design to avoid adding user-facing complexity in managing many daemons.
C) Look at alternate python interpreters that might provide cleaner sandboxing than CPython (I am not up to date on the python world but perhaps something exists).
D) Stop using sub-interpreters, and forbid any python dependencies that use global state that could conflict between modules. Basically any time two modules use the same dependency, it needs some level of audit to ensure they aren't going to collide.
E) Re-write problematic mgr modules to C++. Perhaps if a module is numerically intensive enough to need numpy then it might be better off as native code to begin with. C++ with all the modern features is even quite a nice language :-)
Sorry for the wall of text, hope that's some help.
John