Flight Profiler Stack

flight-profiler-stack

Inspect stack frames of the current running process. Essential for diagnosing hangs, deadlocks, and understanding what the process is doing right now. This is a non-streaming command — it captures a snapshot and returns immediately.

Prerequisites: Read the flight-profiler-attach skill first for platform requirements, installation, permissions, and connection details.

When to Use

• The process appears to hang or is unresponsive — see what each thread is blocked on
• You suspect a deadlock between threads
• You need to identify which thread holds the GIL
• You want to inspect async coroutine/task stacks to find stuck asyncio tasks
• You need native C-level frames (Linux only) for debugging C extension issues or GIL contention
• You want to locate the business code file paths and module names in the target process — stack frames reveal the exact files being executed, which helps determine the correct module, watch, and trace command arguments

Usage

flight_profiler <pid> --cmd "stack [options]" --no-color

Options

Output Format

Thread Stacks (default, Linux — pystack)

Each thread produces a traceback block:

Traceback for thread <tid> (<interpreter>) [<GIL status>] (most recent call last):
    (Python) File "<filename>", line <lineno>, in <funcname>
        <source_line>

Output Fields

Thread Stacks (macOS — CPython dump via server)

On macOS, thread stacks are fetched via the injected server using CPython's _Py_DumpTracebackThreads. Thread names are automatically added:

(<thread_name>) 0x<hex_tid> (most recent call first):
  File "<filename>", line <lineno> in <funcname>

Note: macOS output uses "most recent call first" (reverse order), while Linux pystack uses "most recent call last" (Python traceback convention).

Async Coroutine Stacks (-a)

When -a is used, the output shows all active asyncio tasks grouped by the thread running their event loop:

============================================================
Async Coroutine/Task Stacks
============================================================

Thread: <thread_name> (tid: 0x<hex_tid>)
--------------------------------------------------

  Task #1: <task_name>
    State: <state>
    Coroutine: <qualname>
    Stack (<N> frames):
      File "<filename>", line <lineno>, in <funcname>
        <source_line>

============================================================

Async Output Fields

If no active coroutines are found:

No active coroutines/async tasks found.

Note: Coroutines are only visible when an event loop is running.

Examples

Show all thread stacks — macOS

flight_profiler <pid> --cmd "stack" --no-color

Output — each thread shows its name, hex TID, and Python call stack (most recent call first):

(cpu-worker) 0x0000000170da3000 (most recent call first):
  File "stack_demo_script.py", line 28 in cpu_worker
  File "/Users/zy/miniforge3/envs/py39/lib/python3.9/threading.py", line 888 in run
  File "/Users/zy/miniforge3/envs/py39/lib/python3.9/threading.py", line 950 in _bootstrap_inner
  File "/Users/zy/miniforge3/envs/py39/lib/python3.9/threading.py", line 908 in _bootstrap

(lock-holder) 0x000000016fd97000 (most recent call first):
  File "stack_demo_script.py", line 20 in lock_holder
  File "/Users/zy/miniforge3/envs/py39/lib/python3.9/threading.py", line 888 in run
  ...

(io-worker) 0x000000016ed8b000 (most recent call first):
  File "stack_demo_script.py", line 13 in io_worker
  File "/Users/zy/miniforge3/envs/py39/lib/python3.9/threading.py", line 888 in run
  ...

(async-loop) 0x0000000171daf000 (most recent call first):
  File "/Users/zy/miniforge3/envs/py39/lib/python3.9/selectors.py", line 562 in select
  File "/Users/zy/miniforge3/envs/py39/lib/python3.9/asyncio/base_events.py", line 1854 in _run_once
  File "/Users/zy/miniforge3/envs/py39/lib/python3.9/asyncio/base_events.py", line 596 in run_forever
  ...
  File "stack_demo_script.py", line 57 in start_async_thread
  ...

(MainThread) 0x0000000207ede240 (most recent call first):
  File "stack_demo_script.py", line 74 in <module>

Reading this output:

• (cpu-worker) — currently at cpu_worker line 28, doing computation
• (lock-holder) — at lock_holder line 20, sleeping while holding a lock
• (io-worker) — at io_worker line 13, sleeping in I/O wait
• (async-loop) — the event loop thread, blocked in selectors.select (normal for async — use -a to see coroutine details)
• (MainThread) — main thread at line 74, the while True: time.sleep(1) loop
• Note: macOS output also includes flight-profiler-agent and flight-profiler-recv-* threads (PyFlightProfiler's internal threads) — these can be ignored

Show all thread stacks — Linux

flight_profiler <pid> --cmd "stack" --no-color

Output — Linux uses pystack, showing GIL status and "most recent call last" order:

Traceback for thread 170938 (python) [Has the GIL] (most recent call last):
* - Unable to merge native stack due to insufficient native information - *
    (Python) File "/home/admin/miniforge3/lib/python3.10/threading.py", line 973, in _bootstrap
        self._bootstrap_inner()
    (Python) File "/home/admin/miniforge3/lib/python3.10/threading.py", line 1016, in _bootstrap_inner
        self.run()
    (Python) File "/home/admin/miniforge3/lib/python3.10/threading.py", line 953, in run
        self._target(*self._args, **self._kwargs)
    (Python) File "stack_demo_script.py", line 40, in cpu_worker
        total += i

Traceback for thread 170937 (python) [] (most recent call last):
    ...
    (Python) File "stack_demo_script.py", line 32, in lock_holder
        time.sleep(5)

Traceback for thread 170936 (python) [] (most recent call last):
    ...
    (Python) File "stack_demo_script.py", line 22, in io_worker
        time.sleep(2)

Traceback for thread 170934 (python) [] (most recent call last):
    ...
    (Python) File "stack_demo_script.py", line 97, in main
        time.sleep(1)

Reading this output:

• Thread 170938 (cpu_worker) [Has the GIL] — this thread currently holds the GIL, actively computing
• Thread 170937 (lock_holder) [] — sleeping in time.sleep(5), holding a lock
• Thread 170936 (io_worker) [] — sleeping in time.sleep(2)
• Thread 170934 (main) [] — the main thread, sleeping in time.sleep(1)

Key differences between macOS and Linux output

Show native C-level frames (--native, Linux only)

flight_profiler <pid> --cmd "stack --native" --no-color

Output — mixed Python + C frames, useful for debugging C extensions or GIL contention:

Traceback for thread 170938 (python) [Has the GIL] (most recent call last):
    (C) File "???", line 0, in __clone (/usr/lib64/libc-2.32.so)
    (C) File "???", line 0, in start_thread (/usr/lib64/libpthread-2.32.so)
    (C) File ".../Python/thread_pthread.h", line 248, in pythread_wrapper (python3.10)
    (C) File ".../Modules/_threadmodule.c", line 1100, in thread_run (python3.10)
    (Python) File "/home/admin/miniforge3/lib/python3.10/threading.py", line 973, in _bootstrap
        self._bootstrap_inner()
    ...
    (Python) File "stack_demo_script.py", line 40, in cpu_worker
        total += i

Traceback for thread 170937 (python) [] (most recent call last):
    (C) File "???", line 0, in __clone (/usr/lib64/libc-2.32.so)
    (C) File "???", line 0, in start_thread (/usr/lib64/libpthread-2.32.so)
    ...
    (Python) File "stack_demo_script.py", line 32, in lock_holder
        time.sleep(5)
    (C) File ".../Modules/timemodule.c", line 370, in time_sleep (python3.10)
    (C) File ".../Modules/timemodule.c", line 2076, in pysleep (inlined) (python3.10)
    (C) File "???", line 0, in __select (/usr/lib64/libc-2.32.so)

Traceback for thread 170934 (python) [] (most recent call last):
    (C) File "???", line 0, in _start (python3.10)
    (C) File "???", line 0, in __libc_start_main (/usr/lib64/libc-2.32.so)
    (C) File ".../Modules/main.c", line 1094, in Py_BytesMain (python3.10)
    ...
    (Python) File "stack_demo_script.py", line 97, in main
        time.sleep(1)
    (C) File ".../Modules/timemodule.c", line 370, in time_sleep (python3.10)
    (C) File ".../Modules/timemodule.c", line 2076, in pysleep (inlined) (python3.10)
    (C) File "???", line 0, in __select (/usr/lib64/libc-2.32.so)

Reading this output:

• (C) frames show the native call stack — you can see time.sleep going into CPython's pysleep then into the OS __select syscall
• [Has the GIL] vs [Waiting for the GIL] helps identify GIL contention
• Native frames are interleaved with Python frames to show the full execution path

Show async coroutine stacks (-a)

flight_profiler <pid> --cmd "stack -a" --no-color

Output — all active asyncio tasks grouped by thread, with their coroutine call chain (same format on both macOS and Linux):

============================================================
Async Coroutine/Task Stacks
============================================================

Thread: async-loop (tid: 0x171daf000)
--------------------------------------------------

  Task #1: periodic-heartbeat
    State: WAITING
    Coroutine: periodic_task
    Stack (2 frames):
      File "stack_demo_script.py", line 44, in periodic_task
        await asyncio.sleep(1)
      File ".../asyncio/tasks.py", line 649, in sleep
        return await future

  Task #2: Task-1
    State: WAITING
    Coroutine: run_async_loop
    Stack (1 frames):
      File "stack_demo_script.py", line 53, in run_async_loop
        await asyncio.gather(*tasks)

  Task #3: process-item-42
    State: WAITING
    Coroutine: process_item
    Stack (2 frames):
      File "stack_demo_script.py", line 38, in process_item
        await asyncio.sleep(3600)
      File ".../asyncio/tasks.py", line 649, in sleep
        return await future

  Task #4: fetch-api-data
    State: WAITING
    Coroutine: fetch_data
    Stack (2 frames):
      File "stack_demo_script.py", line 33, in fetch_data
        await asyncio.sleep(3600)
      File ".../asyncio/tasks.py", line 649, in sleep
        return await future

============================================================

Reading this output:

• All 4 tasks are WAITING — they are suspended at an await point
• fetch-api-data and process-item-42 are waiting on asyncio.sleep(3600) — long-running waits, potential indicators of stuck tasks
• periodic-heartbeat is waiting on asyncio.sleep(1) — a healthy periodic task
• Task-1 (the run_async_loop gather) is waiting for its child tasks to complete
• Task names come from asyncio.create_task(name=...) — unnamed tasks show as Task-N

Save stack output to file (-f)

flight_profiler <pid> --cmd "stack -f ./stack.log" --no-color

Output:

✓ Write stack to ./stack.log successfully!

The file contains the same thread stack output as the default stack command. Useful when:

• The output is too long to read in the terminal
• You want to save snapshots for comparison over time
• You need to share the output with others

Troubleshooting: No Output or Errors

No output at all

• Process exited — the target process may have terminated. Verify with ps -p <pid>
• Wrong PID — for multi-process applications (gunicorn, multiprocessing), make sure you're targeting the right worker process

symbol _Py_DumpTracebackThreads not found

• This occurs on macOS when the CPython binary doesn't export the required symbol. Ensure you're using a standard CPython build (not PyPy or a stripped build)

No async tasks shown with -a

• No event loop running — coroutines are only visible when an event loop is actively running in the target process
• All tasks completed — finished tasks are filtered out. Only pending/waiting tasks are shown
• flight_profiler internal tasks are filtered — tasks belonging to flight_profiler's own agent are hidden automatically

Permission errors

• Check target process ownership with ps -p <pid> -o user=
• If the target runs as root, use sudo flight_profiler <pid> ...
• On Linux, verify ptrace_scope: cat /proc/sys/kernel/yama/ptrace_scope (should be 0)

Tips

• Take multiple stack snapshots a few seconds apart to distinguish between a thread that is stuck vs one that is just slow
• Look for [Has the GIL] in thread stacks to identify which thread is holding the GIL — if a CPU-bound thread holds it for too long, other threads will show [Waiting for the GIL]
• For async applications, always use `-a` to see coroutine stacks — thread stacks alone will just show the event loop waiting in epoll_wait/select
• Use --native when debugging C extension crashes or hangs — it reveals the C-level call stack beneath Python frames
• Save snapshots with -f before and after a suspected issue for comparison
• The * - Unable to merge native stack due to insufficient native information - * line in default mode is normal — it means native frames are available but not requested (use --native to see them)
• Use `stack` as the first step to locate business code — before running watch, trace, or module, run stack to discover the file paths and module names active in the target process. The stack frames show exactly which files are being executed (e.g., File "myapp/services/order.py", line 42, in process_order), so you can derive the correct module name for subsequent commands

Platform Notes

• Linux: uses pystack (Bloomberg) for stack analysis, supports native C frames via --native
• macOS: fetches thread stacks via the injected agent server using CPython's _Py_DumpTracebackThreads
• Async stacks (-a): always fetched via server communication on both platforms (requires access to the target's event loop)

Related Commands

• watch — observe function arguments, return values, and timing at each invocation
• perf — if you need a statistical view of where time is spent (flamegraph), use perf instead of repeated stack snapshots
• gilstat — if threads are contending on the GIL, use gilstat to confirm and measure contention rate

Source Files

• CLI plugin: flight_profiler/plugins/stack/cli_plugin_stack.py
• Parser: flight_profiler/plugins/stack/stack_parser.py
• Server plugin: flight_profiler/plugins/stack/server_plugin_stack.py