OpenBMC & Systemd
===================
OpenBMC uses [systemd](https://www.freedesktop.org/wiki/Software/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](https://www.freedesktop.org/software/systemd/man/systemd.unit.html#) -
Units are the basic framework of all systemd work.
[Service](https://www.freedesktop.org/software/systemd/man/systemd.service.html) -
Services are a type of unit, that define the processes to be run and execute.
[Target](https://www.freedesktop.org/software/systemd/man/systemd.target.html) -
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](https://www.freedesktop.org/software/systemd/man/systemctl.html) 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`.

You'll find all the phosphor services associated with `multi-user.target`.

## Server Power On
When OpenBMC is used within a server, the [obmc-host-start@.target](https://github.com/openbmc/phosphor-state-manager/blob/master/target_files/obmc-host-start%40.target)
is what drives the boot of the system.

To start it you would run `systemctl start obmc-host-start@0.target`.

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

```
ls -1 /lib/systemd/system/obmc-host-start@0.target.requires/
obmc-host-startmin@0.target
phosphor-reset-host-reboot-attempts@0.service
```
The [obmc-host-startmin@.target](https://github.com/openbmc/phosphor-state-manager/blob/master/target_files/obmc-host-startmin%40.target)
represents the bare minimum of services and targets required to start the host.
This target is also utilized in host reboot scenarios. This distinction of a
host-start and a host-startmin target allows the user to put services in the
`obmc-host-start@.target` that should only be run on an initial host boot (and
not run on host reboots). For example, in the output above you can see the user
only wants to run the `phosphor-reset-host-reboot-attempts@0.service` on a fresh
host boot attempt.

Next if we look at the `obmc-host-startmin@0.target`, we see this:
```
ls -1 /lib/systemd/system/obmc-host-startmin@0.target.requires/
obmc-chassis-poweron@0.target
start_host@0.service
```

You can see within `obmc-host-startmin@0.target` that we have another target in
there, `obmc-chassis-poweron@0.target`, along with a service aptly named
`start_host@0.service`.

The `obmc-chassis-poweron@0.target` has corresponding services associated with
it:
```
ls -1 /lib/systemd/system/obmc-chassis-poweron@0.target.requires/
op-power-start@0.service
op-wait-power-on@0.service
```

If you run `systemctl start obmc-host-start@0.target` then systemd will start
execution of all of the above associated target and 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 (Soft)
The soft server power off function is encapsulated in the
`obmc-host-shutdown@.target`.  This target is soft in that it notifies the host
of the power off request and gives it a certain amount of time to shut itself
down.

## Server Power Off (Hard)
The hard server power off is encapsulated in the
`obmc-chassis-hard-poweroff@.target`. This target will force the stopping
of the soft power off service if running, and immediately cut power to the
system.

## Server Reboot
The reboot of the server is encapsulated in the `obmc-host-reboot@.target`.
This target will utilize the soft power off target and then, once that
completes, start the host power on target.

## Server Quiesce
The `obmc-host-quiesce@.target` is utilized in host error scenarios. When the
`obmc-host-quiesce@0.target` is entered, it puts the host state D-Bus
[object][https://github.com/openbmc/phosphor-dbus-interfaces/blob/master/xyz/openbmc_project/State/Host.interface.yaml]
in a `Quiesced` state.

## Server Emergency Power Off due to Error
The `obmc-chassis-emergency-poweroff@.target` is a wrapper target around the
`obmc-chassis-hard-poweroff@.target` and `obmc-host-quiesce@.target`. It is
utilized by applications that have detected critical thermal or power errors
which require an immediate shutdown of the system. It will turn the chassis off
and put the host into Quiesce (if the host is running). Certain non-critical
services in the shutdown path can conflict with this target to ensure only the
most critical services are run in this path.

Automated error recovery (i.e. host reboot) will not be done if this target is
started. User intervention is required to exit from it. The user could request
a power on if they wished or a host stop / power off if they wanted to get out
of quiesce.

## 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](https://github.com/openbmc/phosphor-dbus-interfaces/tree/master/xyz/openbmc_project/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 the server power on target.

## Systemd Services or Monitoring Applications
A common question when creating new OpenBMC applications which need to
execute some logic in the context of systemd targets is whether they should
be triggered by systemd services or by monitoring for the appropriate
D-Bus signal indicating the start/stop of the target they are interested in.

The basic guidelines for when to create a systemd service are the following:
- If your application logic depends on other systemd based services then
  make it a systemd service and utilize the Wants/After/Before service
  capabilities.
- If other applications depend on your application logic then it should be a
  systemd service.
- If your application failing during the target start could impact targets or
  services that run after it, then it should be a systemd service. This ensures
  dependent targets are not started if your application fails.

## 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)
        - *See more information below on [RemainAfterExit](#RemainAfterExit)

2. 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-chassis-poweron@0.target`, will have a
`OnFailure=obmc-chassis-poweroff@%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 a `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-poweron@.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.

## Target and Service Dependency Details
There are some tools available out there to visualize systemd service and
target dependencies (systemd-analyze) but due to the complexity of our design,
they do not generate anything very useful.

For now, document the current dependencies on a witherspoon system for
reference.

```
R = Requires
W = Wants
A = After
B = Before
S = Start (runs a command to start another target or service)
(S) = Synchronization Target
```

### Soft Power Off
```
obmc-host-shutdown.target
  R: xyz.openbmc_project.Ipmi.Internal.SoftPowerOff.service
     W: obmc-host-stopping.target (S)
     B: obmc-host-stopping.target (S)
  R: obmc-chassis-poweroff.target
     R: obmc-host-stop.target
        R: op-occ-disable.service
           B: obmc-host-stop-pre.target
     R: op-power-stop.service
        W: obmc-power-stop.target (S)
        B: obmc-power-stop.target (S)
        W: obmc-power-stop-pre.target (S)
        A: obmc-power-stop-pre.target (S)
        W: mapper-wait@-org-openbmc-control-power.service
        A: mapper-wait@-org-openbmc-control-power.service
     R: op-wait-power-off.service
        B: obmc-power-off.target (S)
        W: obmc-power-stop.target (S)
        B: obmc-power-stop.target (S)
        W: obmc-power-stop-pre.target (S)
        A: obmc-power-stop-pre.target (S)
        W: mapper-wait@-org-openbmc-control-power.service
        A: mapper-wait@-org-openbmc-control-power.service
     R: op-powered-off.service
        A: op-wait-power-off.service
        R: op-wait-power-off.service
        S: obmc-chassis-powered-off.target
     W: pcie-poweroff.service
        B: op-power-stop.service
        A: obmc-power-stop-pre@.target
```

#### Synchronization Target Dependencies
```
obmc-power-stop.target
  W: obmc-power-stop-pre.target
  A: obmc-power-stop-pre.target

obmc-power-stop-pre.target
  W: obmc-host-stopped.target
  A: obmc-host-stopped.target

obmc-host-stopped.target
  W: obmc-host-stopping.target
  A: obmc-host-stopping.target
  B: obmc-power-stop-pre.target

obmc-host-stopping.target
  W: obmc-host-stop-pre.target
  A: obmc-host-stop-pre.target
  B: obmc-host-stopped.target

obmc-host-stop-pre.target
  B: obmc-host-stopping.target
```