TIL: gcc’s -H flag to untangle header files

While backporting upstream patches to an older distro kernel:

  f6ac18fafcf6 sched: Improve try_invoke_on_locked_down_task()
  9b3c4ab3045e sched,rcu: Rework try_invoke_on_locked_down_task()
  00619f7c650e sched,livepatch: Use task_call_func()
  8850cb663b5c sched: Simplify wake_up_*idle*()
  5de62ea84abd sched,livepatch: Use wake_up_if_idle()

I ran into compilation error:

  kernel/livepatch/transition.c:434:33: error: implicit declaration of function ‘wake_up_if_idle’

A closer look at the upstream commits shows (“sched,livepatch: Use task_call_func()”) removed “../sched/sched.h” from kernel/livepatch/transition.c. wake_up_if_idle() is defined in include/linux/sched/idle.h so there is probably a connection – but how does this even build upstream and how to untangle the rat’s nest of kernel includes?

Enter gcc and the -H flag.

I rebuilt the upstream kernel kernel/livepatch/transition.o with V=1 and snarfed its gcc compilation line. Pasted into the terminal, added -H, and gcc generates a long header file stack. The header of interest was six includes deep:

  . ./include/linux/cpu.h
  .. ./include/linux/node.h
  ... ./include/linux/device.h
  .... ./include/linux/energy_model.h
  ..... ./include/linux/sched/topology.h
  ...... ./include/linux/sched/idle.h

Moving back to my distro kernel, its device.h did not include energy_model.h (introduced upstream by 1bc138c62295 (“PM / EM: add support for other devices than CPUs in Energy Model”).

Mystery solved and a new trick to remember for a future problem.