Which logical core does MPI background threads are pinned?

I program in C++ which parallelizes using both MPI and OpenMP (1 MPI process per machine, Multiple threads are exploited by each MPI process)

I carefully pin the worker threads of OpenMP and others in my application. I know that MPI spawns some background threads for its internal uses. But I don't know to which logical cores the threads are attached.

How can I find and control it?

Thanks