Sorry about the misleading reference to tracemalloc in my email - it was actually test_atexit I was debugging in the PEP 432 branch. tracemalloc only came up in that context because test.support.run_in_subinterp() automatically skips subinterpreter tests when tracemalloc is running. From the function comments:

# Issue #10915, #15751: PyGILState_*() functions don't work with
# sub-interpreters, the tracemalloc module uses these functions internally

As far as test coverage goes, what I would actually like to do is to run regrtest itself in a subinterpreter, and blacklist tests until it passes. This would double the length of a test run, so hiding it behind a resource option would probably be a good idea.