1OpenBMC & Systemd 2=================== 3OpenBMC uses [systemd](https://www.freedesktop.org/wiki/Software/systemd/) to 4manage all processes. It has its own set of target and service units to 5control which processes are started. There is a lot of documentation on systemd 6and to do OpenBMC state work, you're going to have to read some of it. Here's 7the highlights: 8 9[Unit](https://www.freedesktop.org/software/systemd/man/systemd.unit.html#) - 10Units are the basic framework of all systemd work. 11[Service](https://www.freedesktop.org/software/systemd/man/systemd.service.html) - 12Services are a type of unit, that define the processes to be run and execute. 13[Target](https://www.freedesktop.org/software/systemd/man/systemd.target.html) - 14Targets are another type of unit, they have two purposes: 15 161. Define synchronization points among services. 172. Define the services that get run for a given target. 18 19On an OpenBMC system, you can go to /lib/systemd/system/ and see all of the 20systemd units on the system. You can easily cat these files to start looking at 21the relationships among them. Service files can also be found in 22/etc/systemd/system and /run/systemd/system as well. 23 24[systemctl](https://www.freedesktop.org/software/systemd/man/systemctl.html) is 25the main tool you use to interact with systemd and its units. 26 27---------- 28## Initial Power 29When an OpenBMC system first has power applied, it starts the "default.target" 30unless an alternate target is specified on the kernel command line. In 31Phosphor OpenBMC, there is a link from `default.target` to `multi-user.target`. 32 33You'll find all the phosphor services associated with `multi-user.target`. 34 35## Server Power On 36When 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) 37is what drives the boot of the system. 38 39To start it you would run `systemctl start obmc-host-start@0.target`. 40 41If you dig into its .requires relationship, you'll see the following in the file 42system 43 44``` 45ls -1 /lib/systemd/system/obmc-host-start@0.target.requires/ 46obmc-host-startmin@0.target 47phosphor-reset-host-reboot-attempts@0.service 48``` 49The [obmc-host-startmin@.target](https://github.com/openbmc/phosphor-state-manager/blob/master/target_files/obmc-host-startmin%40.target) 50represents the bare minimum of services and targets required to start the host. 51This target is also utilized in host reboot scenarios. This distinction of a 52host-start and a host-startmin target allows the user to put services in the 53`obmc-host-start@.target` that should only be run on an initial host boot (and 54not run on host reboots). For example, in the output above you can see the user 55only wants to run the `phosphor-reset-host-reboot-attempts@0.service` on a fresh 56host boot attempt. 57 58Next if we look at the `obmc-host-startmin@0.target`, we see this: 59``` 60ls -1 /lib/systemd/system/obmc-host-startmin@0.target.requires/ 61obmc-chassis-poweron@0.target 62start_host@0.service 63``` 64 65You can see within `obmc-host-startmin@0.target` that we have another target in 66there, `obmc-chassis-poweron@0.target`, along with a service aptly named 67`start_host@0.service`. 68 69The `obmc-chassis-poweron@0.target` has corresponding services associated with 70it: 71``` 72ls -1 /lib/systemd/system/obmc-chassis-poweron@0.target.requires/ 73op-power-start@0.service 74op-wait-power-on@0.service 75``` 76 77If you run `systemctl start obmc-host-start@0.target` then systemd will start 78execution of all of the above associated target and services. 79 80The services have dependencies within them that control the execution of each 81service (for example, the op-power-start.service will run prior to the 82op-wait-power-on.service). These dependencies are set using targets and the 83Wants,Before,After keywords. 84 85## Server Power Off (Soft) 86The soft server power off function is encapsulated in the 87`obmc-host-shutdown@.target`. This target is soft in that it notifies the host 88of the power off request and gives it a certain amount of time to shut itself 89down. 90 91## Server Power Off (Hard) 92The hard server power off is encapsulated in the 93`obmc-chassis-hard-poweroff@.target`. This target will force the stopping 94of the soft power off service if running, and immediately cut power to the 95system. 96 97## Server Reboot 98The reboot of the server is encapsulated in the `obmc-host-reboot@.target`. 99This target will utilize the soft power off target and then, once that 100completes, start the host power on target. 101 102## Systemd Control in OpenBMC 103There are a collection of services within OpenBMC that interact with systemd and 104its unit files, providing somewhat of an abstraction layer from the user of the 105OpenBMC system and systemd. See the [state](https://github.com/openbmc/phosphor-dbus-interfaces/tree/master/xyz/openbmc_project/State) 106interfaces for more information on this function. 107 108For example, if you wanted to execute the server power on function, you would do 109the following: 110 111> busctl set-property xyz.openbmc_project.State.Host /xyz/openbmc_project/state/host0 112xyz.openbmc_project.State.Host RequestedHostTransition s 113xyz.openbmc_project.State.Host.Transition.On 114 115Underneath the covers, this is calling systemd with the server power on target. 116 117## Systemd Services or Monitoring Applications 118A common question when creating new OpenBMC applications which need to 119execute some logic in the context of systemd targets is whether they should 120be triggered by systemd services or by monitoring for the appropriate 121D-Bus signal indicating the start/stop of the target they are interested in. 122 123The basic guidelines for when to create a systemd service are the following: 124- If your application logic depends on other systemd based services then 125 make it a systemd service and utilize the Wants/After/Before service 126 capabilities. 127- If other applications depend on your application logic then it should be a 128 systemd service. 129- If your application failing during the target start could impact targets or 130 services that run after it, then it should be a systemd service. This ensures 131 dependent targets are not started if your application fails. 132 133## Error Handling of Systemd 134With great numbers of targets and services, come great chances for failures. 135To make OpenBMC a robust and productive system, it needs to be sure to have an 136error handling policy for when services and their targets fail. 137 138When a failure occurs, the OpenBMC software needs to notify the users of the 139system and provide mechanisms for either the system to automatically retry the 140failed operation (i.e. reboot the system) or to stay in a quiesced state so that 141error data can be collected and the failure can be investigated. 142 143There are two main failure scenarios when it comes to OpenBMC and systemd usage: 144 1451. A service within a target fails 146- If the service is a "oneshot" type, and the service is required 147(not wanted) by the target then the target will fail if the service 148fails 149 - Define a behavior for when the target fails using the 150 "OnFailure" option (i.e. go to a new failure target if any required 151 service fails) 152- If the service is not a "oneshot", then it can not fail the target 153(the target only knows that it started successfully) 154 - Define a behavior for when the service fails (OnFailure) 155 option. 156 - The service can not have "RemainAfterExit=yes" otherwise, the OnFailure 157 action does not occur until the service is stopped (instead of when it 158 fails) 159 - *See more information below on [RemainAfterExit](#RemainAfterExit) 160 1612. A failure outside of a normal systemd target/service (host watchdog expires, 162host checkstop detected) 163- The service which detects this failure is responsible for logging the 164appropriate error, and instructing systemd to go to the appropriate target 165 166Within OpenBMC, there is a host quiesce target. This is the target that other 167host related targets should go to when they hit a failure. Other software within 168OpenBMC can then monitor for the entry into this quiesce target and will handle 169the halt vs. automatic reboot functionality. 170 171Targets which are not host related, will need special thought in regards to 172their error handling. For example, the target responsible for applying chassis 173power, `obmc-chassis-poweron@0.target`, will have a 174`OnFailure=obmc-chassis-poweroff@%i.target` error path. That is, if the 175chassis power on target fails then power off the chassis. 176 177The above info sets up some general **guidelines** for our host related 178targets and services: 179 180- All targets should have a `OnFailure=obmc-quiesce-host@.target` 181- All services which are required for a target to achieve its function should 182be RequiredBy that target (not WantedBy) 183- All services should first try to be "Type=oneshot" so that we can just rely on 184the target failure path 185- If a service can not be "Type=oneshot", then it needs to have a 186`OnFailure=obmc-quiesce-host@.target` and ideally set "RemainAfterExit=no" 187(but see caveats on this below) 188- If a service can not be any of these then it's up to the service application 189to call systemd with the `obmc-quiesce-host@.target` on failures 190 191### RemainAfterExit 192This is set to "yes" for most OpenBMC services to handle the situation where 193someone starts the same target twice. If the associated service with that 194target is not running (i.e. RemainAfterExit=no), then the service will be 195executed again. Think about someone accidentally running the 196`obmc-chassis-poweron@.target` twice. If you execute it when the operating system 197is up and running, and the service which toggles the pgood pin is re-executed, 198you're going to crash your system. Given this info, the goal should always be 199to write "oneshot" services that have RemainAfterExit set to yes. 200 201## Target and Service Dependency Details 202There are some tools available out there to visualize systemd service and 203target dependencies (systemd-analyze) but due to the complexity of our design, 204they do not generate anything very useful. 205 206For now, document the current dependencies on a witherspoon system for 207reference. 208 209``` 210R = Requires 211W = Wants 212A = After 213B = Before 214S = Start (runs a command to start another target or service) 215(S) = Synchronization Target 216``` 217 218### Soft Power Off 219``` 220obmc-host-shutdown.target 221 R: xyz.openbmc_project.Ipmi.Internal.SoftPowerOff.service 222 W: obmc-host-stopping.target (S) 223 B: obmc-host-stopping.target (S) 224 R: obmc-chassis-poweroff.target 225 R: obmc-host-stop.target 226 R: op-occ-disable.service 227 B: obmc-host-stop-pre.target 228 R: op-power-stop.service 229 W: obmc-power-stop.target (S) 230 B: obmc-power-stop.target (S) 231 W: obmc-power-stop-pre.target (S) 232 A: obmc-power-stop-pre.target (S) 233 W: mapper-wait@-org-openbmc-control-power.service 234 A: mapper-wait@-org-openbmc-control-power.service 235 R: op-wait-power-off.service 236 B: obmc-power-off.target (S) 237 W: obmc-power-stop.target (S) 238 B: obmc-power-stop.target (S) 239 W: obmc-power-stop-pre.target (S) 240 A: obmc-power-stop-pre.target (S) 241 W: mapper-wait@-org-openbmc-control-power.service 242 A: mapper-wait@-org-openbmc-control-power.service 243 R: op-powered-off.service 244 A: op-wait-power-off.service 245 R: op-wait-power-off.service 246 S: obmc-chassis-powered-off.target 247 W: pcie-poweroff.service 248 B: op-power-stop.service 249 A: obmc-power-stop-pre@.target 250``` 251 252#### Synchronization Target Dependencies 253``` 254obmc-power-stop.target 255 W: obmc-power-stop-pre.target 256 A: obmc-power-stop-pre.target 257 258obmc-power-stop-pre.target 259 W: obmc-host-stopped.target 260 A: obmc-host-stopped.target 261 262obmc-host-stopped.target 263 W: obmc-host-stopping.target 264 A: obmc-host-stopping.target 265 B: obmc-power-stop-pre.target 266 267obmc-host-stopping.target 268 W: obmc-host-stop-pre.target 269 A: obmc-host-stop-pre.target 270 B: obmc-host-stopped.target 271 272obmc-host-stop-pre.target 273 B: obmc-host-stopping.target 274``` 275