We propose PKDB, the first interactive debugger for GPU and
multithreaded low-level kernels written in Python. Python is widely
used in high performance computing (HPC), with frameworks such as
PyKokkos translating Python-embedded domain-specific languages to
native code that runs across OpenMP-threaded CPUs and various
GPUs. Yet inter- active debugging support for such code is absent:
developers resort to print statements, framework-specific assertions,
or CPU-only execution, the last of which requires altering the program
or its data and can mask device-specific bugs. PKDB enables standard
interactive debugging like breakpoints, stepping, and variable
inspection while preserving actual on-device execution without source
modification. Beyond these fundamentals, PKDB introduces two advanced
capabilities that exploit the dynamic nature of Python and PyKokkos:
(i) Live code evaluation, which lets developers execute arbitrary
Python expressions or entire kernels in the middle of a paused kernel
without restarting the process; (ii) Kernel call site substitution,
which allows an actively running kernel to be updated and reloaded on
the fly, so only the kernel is recompiled and re-executed without
restarting the application. Our performance evaluation on Intel, AMD,
and NVIDIA CPUs and NVIDIA and AMD GPUs shows that PKDB introduces
limited overhead and is practical for everyday use while introducing
critical debugging features to the Python HPC ecosystem.