CUDA-GDB debugger does not break on launch

[complexitysoftware]

uv/pixi CLI Version

0.49.0

Mojo Version

Mojo 25.5.0.dev2025072605 (b53163aa)

GPU Information

NVIDIA GeForce RTX 4090 - Compute Capability 8.9

Operating System

Ubuntu under WSL Windows

Related Puzzle

Puzzle 9 Essentials

What happened?

1. In Tutorial step 3: Debugging the GPU kernel (https://puzzles.modular.com/puzzle_09/essentials.html) I run the command: pixi run mojo debug --cuda-gdb --break-on-launch solutions/p01/p01.mojo and get the cuda-gbd prompt
2. There I run the run command which should run execution to the kernel entry and stop due to the --break-on-launch flag
3. In fact the code runs to completion:
   (mojo-gpu-puzzles) mark@Basil:~/mojo-gpu-puzzles$ pixi run mojo debug --cuda-gdb --break-on-launch solutions/p01/p01.mojo
   This will run cuda-gdb using the Unified Debugger backend.
   If the debugger shows errors mentioning CUDBG_ERROR_COMMUNICATION_FAILURE,
   retry this command prepending the environment variable CUDBG_USE_LEGACY_DEBUGGER=1
   NVIDIA (R) CUDA Debugger
   12.0 release
   Portions Copyright (C) 2007-2022 NVIDIA Corporation
   GNU gdb (GDB) 12.1
   Copyright (C) 2022 Free Software Foundation, Inc.
   License GPLv3+: GNU GPL version 3 or later http://gnu.org/licenses/gpl.html
   This is free software: you are free to change and redistribute it.
   There is NO WARRANTY, to the extent permitted by law.
   Type "show copying" and "show warranty" for details.
   This GDB was configured as "x86_64-linux-gnu".
   Type "show configuration" for configuration details.
   For bug reporting instructions, please see:
   https://www.gnu.org/software/gdb/bugs/.
   Find the GDB manual and other documentation resources online at:
   http://www.gnu.org/software/gdb/documentation/.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /home/mark/mojo-gpu-puzzles/.pixi/envs/default/bin/mojo...

This GDB supports auto-downloading debuginfo from the following URLs:
https://debuginfod.ubuntu.com
Enable debuginfod for this session? (y or [n]) y
Debuginfod has been enabled.
To make this setting permanent, add 'set debuginfod enabled on' to .gdbinit.
(No debugging symbols found in /home/mark/mojo-gpu-puzzles/.pixi/envs/default/bin/mojo)
(cuda-gdb) run
Starting program: /home/mark/mojo-gpu-puzzles/.pixi/envs/default/bin/mojo run -O0 --debug-level=full /home/mark/mojo-gpu-puzzles/solutions/p01/p01.mojo
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff27ff6c0 (LWP 51360)]
[New Thread 0x7ffff15e06c0 (LWP 51361)]
[Detaching after fork from child process 51362]
[New Thread 0x7ffff09ff6c0 (LWP 51371)]
[Thread 0x7ffff09ff6c0 (LWP 51371) exited]
[New Thread 0x7ffff09ff6c0 (LWP 51372)]
[Detaching after vfork from child process 51373]
WARNING: Debug interface is not enabled. Please see https://docs.nvidia.com/cuda/cuda-gdb/index.html#supported-platforms for more details.
[Detaching after fork from child process 51382]
[New Thread 0x7fffd9ce56c0 (LWP 51386)]
[New Thread 0x7fffd94e46c0 (LWP 51387)]
[New Thread 0x7fffd8ce36c0 (LWP 51388)]
[New Thread 0x7fffcbfff6c0 (LWP 51389)]
[New Thread 0x7fffcb7fe6c0 (LWP 51390)]
[New Thread 0x7fffcaffd6c0 (LWP 51391)]
[New Thread 0x7fffca7fc6c0 (LWP 51392)]
[New Thread 0x7fffc9ffb6c0 (LWP 51393)]
[New Thread 0x7fffc97fa6c0 (LWP 51394)]
[New Thread 0x7fffc8ff96c0 (LWP 51395)]
[New Thread 0x7fffabfff6c0 (LWP 51396)]
[New Thread 0x7fffab7fe6c0 (LWP 51397)]
[New Thread 0x7fffaaffd6c0 (LWP 51398)]
[New Thread 0x7fffaa7fc6c0 (LWP 51399)]
[New Thread 0x7fffa9ffb6c0 (LWP 51400)]
[New Thread 0x7fffa97fa6c0 (LWP 51401)]
[New Thread 0x7fffa8ff96c0 (LWP 51402)]
[New Thread 0x7fff8bfff6c0 (LWP 51403)]
[Thread 0x7fff8bfff6c0 (LWP 51403) exited]
out: HostBuffer([10.0, 11.0, 12.0, 13.0])
expected: HostBuffer([10.0, 11.0, 12.0, 13.0])
[Thread 0x7fffa97fa6c0 (LWP 51401) exited]
[Thread 0x7fffcb7fe6c0 (LWP 51390) exited]
[Thread 0x7fffd94e46c0 (LWP 51387) exited]
[Thread 0x7fffcaffd6c0 (LWP 51391) exited]
[Thread 0x7fffa9ffb6c0 (LWP 51400) exited]
[Thread 0x7fffaaffd6c0 (LWP 51398) exited]
[Thread 0x7fffc97fa6c0 (LWP 51394) exited]
[Thread 0x7fffc9ffb6c0 (LWP 51393) exited]
[Thread 0x7fffca7fc6c0 (LWP 51392) exited]
[Thread 0x7fffd9ce56c0 (LWP 51386) exited]
[Thread 0x7fffc8ff96c0 (LWP 51395) exited]
[Thread 0x7fffd8ce36c0 (LWP 51388) exited]
[Thread 0x7fffab7fe6c0 (LWP 51397) exited]
[Thread 0x7fffaa7fc6c0 (LWP 51399) exited]
[Thread 0x7fffabfff6c0 (LWP 51396) exited]
[Thread 0x7fffcbfff6c0 (LWP 51389) exited]
[Thread 0x7fffa8ff96c0 (LWP 51402) exited]
[Thread 0x7ffff15e06c0 (LWP 51361) exited]
[Thread 0x7ffff09ff6c0 (LWP 51372) exited]
[Thread 0x7ffff4a54e00 (LWP 51350) exited]
[Thread 0x7ffff27ff6c0 (LWP 51360) exited]
[New process 51350]
[Inferior 1 (process 51350) exited normally]
(cuda-gdb)

I cannot see that I am missing anything. Is there a step left out of the instructions?

Steps to Reproduce

See above

Relevant Log Output

See above

Additional Context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[ehsanmok]

Thanks for the filing the issue and sorry it didn't work for RTX 4090! Currently I'm testing the puzzles on Tier I GPUs. Our tooling team will be looking into the it.

[complexitysoftware]

I just noticed the message in the log:
WARNING: Debug interface is not enabled. Please see https://docs.nvidia.com/cuda/cuda-gdb/index.html#supported-platforms for more details.

Regarding Windows Subsystem for Linux (WSL) this NVIDIA link says:
cuda-gdb supports debugging CUDA application on WSL2.
Make sure this capability is enabled via the registry key >HKEY_LOCAL_MACHINE\SOFTWARE\NVIDIA Corporation\GPUDebugger\EnableInterface set to (DWORD) 1.

I made this registry change and restarted the machine. I now get the error below.

(cuda-gdb) run
Starting program: /home/mark/mojo-gpu-puzzles/.pixi/envs/default/bin/mojo run -O0 --debug-level=full /home/mark/mojo-gpu-puzzles/solutions/p01/p01.mojo
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff27ff6c0 (LWP 4062)]
[New Thread 0x7ffff15e06c0 (LWP 4063)]
[Detaching after fork from child process 4064]
[New Thread 0x7ffff09ff6c0 (LWP 4073)]
[Thread 0x7ffff09ff6c0 (LWP 4073) exited]
[New Thread 0x7ffff09ff6c0 (LWP 4074)]
[Detaching after vfork from child process 4075]
Warning:
Cannot insert breakpoint -107.
Cannot access memory at address 0xb00cf8000

(cuda-gdb)

Thanks