Chrome OS Shutdown and Reboot

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This document describes why and how Chrome OS devices shut down or reboot.

“Shut down” is a verb. “Shutdown” is a noun.

Why we shut down

Requests from other processes

Shutdown and reboot are initiated on Chrome OS devices through RequestShutdown or RequestRestart D-Bus method calls to powerd. These methods are typically called by Chrome in response to requests from the user, but they can also be invoked by other processes in some cases (e.g. by update_engine to reboot to apply a system update, or by shell scripts in the lab to reboot devices that don't have working network connections).

Both D-Bus methods accept two arguments:

  • an int32 power_manager::RequestShutdownReason or power_manager::RequestRestartReason value from the powerd constants file in system_api
  • a string containing a human-readable description of the reason for the request

powerd logs both arguments to the current log file within /var/log/power_manager before handling the request; look for messages like these near the end of the file:

[0429/] Got RequestShutdown message from :1.62 with reason user-request (UI request from ash)
[0425/] Got RequestRestart message from :1.33 with reason system-update (update_engine applying update)

Requests within powerd

powerd may also decide to shut the system down without input from other processes in some cases, including:

  • the battery charge dropping below its shutdown threshold
  • the lid being closed while Chrome has sent a policy requesting a lid-closed action of “shut down” (e.g. because the login screen is displayed)
  • the kernel repeatedly failing to suspend
  • the power button being pressed while no displays are connected

In all cases, powerd will also log a terse message just before it shuts down or reboots:

[0429/] Shutting down, reason: user-request
[0425/] Restarting, reason: system-update

How we shut down

powerd's Daemon class runs powerd_setuid_helper, its setuid-root helper binary, with --action=shut_down or --action=reboot. powerd_setuid_helper runs:

initctl emit --no-wait runlevel RUNLEVEL=<runlevel> SHUTDOWN_REASON=<reason>

to instruct Upstart to either shut down (runlevel 0) or reboot (runlevel 6) the system. The SHUTDOWN_REASON argument contains a short dash-separated string describing the reason for the request.

The Upstart halt job or reboot job is triggered by the runlevel change. This triggers a cascade of other jobs within Upstart's /etc/init directory. The pre-shutdown job logs a message to /var/log/messages to make it easier to see when and why the system shut down or rebooted while examining logs:

2018-04-24T21:30:08.513032-07:00 NOTICE pre-shutdown[22132]: Shutting down for reboot: not-via-powerd

In the above case, not-via-powerd indicates that this clean reboot was initiated by the reboot command being run directly rather through powerd. Requests should always go through powerd when possible, both for consistency and for correctness (e.g. powerd knows to defer shutting down if a firmware update is in progress).

After other jobs have completed, the halt or reboot job executes the chromeos_shutdown script. chromeos_shutdown handles various tasks:

  • Remaining processes are killed.
  • Partitions are unmounted.
  • If powerd passed a SHUTDOWN_REASON argument with value low-battery, the display_low_battery_alert script is executed to use frecon to display a brief animation before shutting down.

Finally, the halt or reboot job executes the halt or reboot command with --force to instruct the kernel to immediately halt or reboot the system.