[Bug 880049] Re: 100% CPU usage in init if /dev/console is not available

[Cedric Schieli]

I totally agree the proper fix would be that OpenVZ provides a /dev/console. My patch is rather a workaround.
Concerning jobs specifying console access, they already fallback to CONSOLE_NONE in job_process_spawn so they are working (maybe not exactly with the same behaviour though).
Concerning jobs specifying console ownership, they will fail indeed, but those jobs (rcS, mountall-shell, ...) really can't be used in a OpenVZ environment anyway.

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Title:
  100% CPU usage in init if /dev/console is not available

Status in “upstart” package in Ubuntu:
  Confirmed

Bug description:
  When /dev/console is not available (for example when running as an OpenVZ guest), init consumes 100% time of one CPU core.
  Here is a strace excerpt captured on the OpenVZ host :

  ...
  waitid(P_ALL, 0, {}, WNOHANG|WEXITED|WSTOPPED|WCONTINUED, NULL) = 0
  clock_gettime(CLOCK_MONOTONIC, {3903451, 826182267}) = 0
  select(5, [0 3 4], [], [4], NULL) = 1 (in [0])
  read(0, "", 1) = 0
  waitid(P_ALL, 0, {}, WNOHANG|WEXITED|WSTOPPED|WCONTINUED, NULL) = 0
  clock_gettime(CLOCK_MONOTONIC, {3903451, 826467596}) = 0
  select(5, [0 3 4], [], [4], NULL) = 1 (in [0])
  read(0, "", 1) = 0
  waitid(P_ALL, 0, {}, WNOHANG|WEXITED|WSTOPPED|WCONTINUED, NULL) = 0
  clock_gettime(CLOCK_MONOTONIC, {3903451, 827157279}) = 0
  select(5, [0 3 4], [], [4], NULL) = 1 (in [0])
  read(0, "", 1) = 0
  ...

  Here is what happens :

  - in init's main function, system_setup_console(CONSOLE_OUTPUT) fails, leaving file descriptors 0 and 1 uninitialized
  - nih_main_loop_init eventually get called, initializing its interrupt_pipe on file descriptors 0 and 1
  - we end up calling system_setup_console(CONSOLE_NONE), resetting file descriptors 0 and 1 to /dev/null
  - in nih_main_loop, select is called on 0 (/dev/null instead of the pipe) which repeatedly appears to have changed

  I'm attaching a small patch which fixes this.

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