OpenBMC & Systemd

OpenBMC uses systemd to manage all processes. It has its own set of target and service units to control which processes are started. There is a lot of documentation on systemd and to do OpenBMC state work, you're going to have to read some of it. Here's the highlights:

Unit - Units are the basic framework of all systemd work. Service - Services are a type of unit, that define the processes to be run and execute. Target - Targets are another type of unit, they have two purposes:

  1. Define synchronization points among services.
  2. Define the services that get run for a given target.

On an OpenBMC system, you can go to /lib/systemd/system/ and see all of the systemd units on the system. You can easily cat these files to start looking at the relationships among them. Service files can also be found in /etc/systemd/system and /run/systemd/system as well.

systemctl is the main tool you use to interact with systemd and its units.

Initial Power

When an OpenBMC system first has power applied, it starts the "default.target" unless an alternate target is specified on the kernel command line. In Phosphor OpenBMC, there is a link from default.target to multi-user.target. Within Phosphor OpenBMC, obmc-standby.target is wanted by multi-user.target and is where you'll find all Phosphor services associated.

Server Power On

When OpenBMC is used within a server, the obmc-chassis-start@.target is what drives the boot of the system.

If you dig into its .wants relationship, you'll see the following in the file system

ls -1 /lib/systemd/system/obmc-chassis-start@0.target.wants/
obmc-power-chassis-on@0.target
obmc-start-host@0.service
...

You can see we have another target in here, obmc-power-chassis-on@0.target, along with some services that will all be started by systemd when you do a "systemctl start obmc-chassis-start@0.target".

The target has corresponding services associated with it:

ls -1 /lib/systemd/system/obmc-power-chassis-on\@0.target.wants/
op-power-start@0.service
op-wait-power-on@0.service

So basically, if you run systemctl start obmc-chassis-start@0.target then systemd will start execution of all associated services.

The services have dependencies within them that control the execution of each service (for example, the op-power-start.service will run prior to the op-wait-power-on.service). These dependencies are set using targets and the Wants,Before,After keywords.

Server Power Off

The server power off function is encapsulated in the obmc-chassis-stop@0.target. You can do a similar exercise with the power on to see its dependent services.

Systemd Control in OpenBMC

There are a collection of services within OpenBMC that interact with systemd and its unit files, providing somewhat of an abstraction layer from the user of the OpenBMC system and systemd. See the state interfaces for more information on this function.

For example, if you wanted to execute the server power on function, you would do the following:

busctl set-property xyz.openbmc_project.State.Host /xyz/openbmc_project/state/host0 xyz.openbmc_project.State.Host RequestedHostTransition s xyz.openbmc_project.State.Host.Transition.On

Underneath the covers, this is calling systemd with obmc-chassis-start@0.target

Error Handling of Systemd

With great numbers of targets and services, come great chances for failures. To make OpenBMC a robust and productive system, it needs to be sure to have an error handling policy for when services and their targets fail.

When a failure occurs, the OpenBMC software needs to notify the users of the system and provide mechanisms for either the system to automatically retry the failed operation (i.e. reboot the system) or to stay in a quiesced state so that error data can be collected and the failure can be investigated.

There are two main failure scenarios when it comes to OpenBMC and systemd usage:

  1. A service within a target fails
  • If the service is a "oneshot" type, and the service is required (not wanted) by the target then the target will fail if the service fails
    • Define a behavior for when the target fails using the "OnFailure" option (i.e. go to a new failure target if any required service fails)
  • If the service is not a "oneshot", then it can not fail the target (the target only knows that it started successfully)
    • Define a behavior for when the service fails (OnFailure) option.
    • The service can not have "RemainAfterExit=yes" otherwise, the OnFailure action does not occur until the service is stopped (instead of when it fails)
  1. A failure outside of a normal systemd target/service (host watchdog expires, host checkstop detected)
  • The service which detects this failure is responsible for logging the appropriate error, and instructing systemd to go to the appropriate target

Within OpenBMC, there is a host quiesce target. This is the target that other host related targets should go to when they hit a failure. Other software within OpenBMC can then monitor for the entry into this quiesce target and will handle the halt vs. automatic reboot functionality.

Targets which are not host related, will need special thought in regards to their error handling. For example, the target responsible for applying chassis power, obmc-power-chassis-on@0.target, will have a "OnFailure=obmc-power-chassis-off@%i.target" error path. That is, if the chassis power on target fails then power off the chassis.

The above info sets up some general guidelines for our host related targets and services:

  • All targets should have an "OnFailure=obmc-quiesce-host@.target"
  • All services which are required for a target to achieve its function should be RequiredBy that target (not WantedBy)
  • All services should first try to be "Type=oneshot" so that we can just rely on the target failure path
  • If a service can not be "Type=oneshot", then it needs to have a "OnFailure=obmc-quiesce-host@.target" and ideally set "RemainAfterExit=no" (but see caveats on this below)
  • If a service can not be any of these then it's up to the service application to call systemd with the obmc-quiesce-host@.target on failures

RemainAfterExit

This is set to "yes" for most OpenBMC services to handle the situation where someone starts the same target twice. If the associated service with that target is not running (i.e. RemainAfterExit=no), then the service will be executed again. Think about someone accidentally running the obmc-chassis-start@.target twice. If you execute it when the operating system is up and running, and the service which toggles the pgood pin is re-executed, you're going to crash your system. Given this info, the goal should always be to write "oneshot" services that have RemainAfterExit set to yes.