XELK-AN-008: How to use systemd on an Embedded system

Info Box Applies to Axel Ultra Applies to AXEL ESATTA Applies to Axel Lite

This application note has been validated starting from the XELK 4.0.0 kit version.

History

Version	Date	XELK version	Notes
1.0.0	Sep 2019	4.0.0

Introduction

Starting from XELK 4.0.0 the root file system generated by NXP Yocto recipes produces a root file system using systemd.

Systemd is a System and Service Manager which has enough different settings and configuration from systemV which was used on all XELK BSPs up to XELK 3.0.0.

This application note is not a complete systemd user's guide but collects some useful hints that can be used for getting familiar with Systemd. There is a plenty of documentation and User's Guide available for systemd, but some simple example - that can be found here below - may simplify the systemd approach for beginners.

Brief description

Systemd manages not only services but many different objects called Unit. Unit are related to the resources that systemd can manage. Unit configurations are defined into the Unit files.

Units categoris (identified by the file extension) are:

   .service
   .target
   .socket
   .device
   .mount
   .automount
   .swap
   .path
   .timer
   .snapshot
   .slice
   .scope

Major insteresting Units are services and targets. They will be analyzed in the following paragraphs.

Services

It is possible to display all started services with the following userspace command:

systemctl -t service

It is possible to display all services (including disabled and stopped services) with:

systemctl -t service --all

Other useful service commands

Starting a service from userspace:

systemctl start <service_name>

Stopping a service from userspace

systemctl stop <service_name>

Starting a service at boot time:

systemctl enable <service_name>

Disabling service (already started at boot time):

systemctl disable <service_name>

Targets

Targets are used by systemd for having a synchronization mechanism between different services at boot time or during runtime changes.

They can be used for set the system to a new state.

All services linked to a target are linked to the modification to the same target. These can be seen in a similar way of SystemV runlevels with many other added functionalities.

Target and runlevels

Here below there is a list of power on/off targets and related SystemV runlevels:

Description	SystemV (runlevel)	Systemd (target)
System halt	0	runlevel0.target, poweroff.target
Single user mode	1, s, single	runlevel1.target, rescue.target
Multi user	2	runlevel2.target, multi-user.target
Multi user with network	3	runlevel3.target, multi-user.target
Experimental	4	runlevel4.target, multi-user.target
Multi user with network, graphical mode	5	runlevel5.target, graphical.target
Reboot	6	runlevel6.target, reboot.target

multi-user target can be identified as the runlevel 3.

On the

/etc/systemd/system/<target_name>.target.wants

directory there is a list of services related to that target.

For example:

root@imx6qxelk:~# ls /etc/systemd/system/multi-user.target.wants/
atd.service	       busybox-syslog.service  gpuconfig.service  ofono.service		systemd-networkd.service
avahi-daemon.service   connman.service	       mytest.service	  psplash-quit.service	systemd-resolved.service
busybox-klogd.service  crond.service	       ntpdate.service	  remote-fs.target

Active targets

It is possible to display all active targets with:

systemctl -t target

Changing a target

systemctl isolate graphical

The actual target is shown with:

systemctl get-default

Changing the default target:

systemctl set-default multi-user

Unit files

For a complete information on Unit please look to the documentation page

Here below you can find an extract for the main used topics and configuration descriptions.

Location Path

Units are configured by systemd using configuration files that can be found in different directories. Each of them has different priority and behaviour:

• /lib/systemd/system

This directory stores a copy of configuration files. This is the default destination for new installed configuration file. Typically files in this directory should not be modified by the user.

• /etc/systemd/system

This is the directory where to store a new Unit or to modify an existing one. The files present in this directory have the highest priority.

• /run/systemd/system

The files present in this directory have higher priority only respect the ones on /lib/systemd/system. Systemd creates these configuration files dinamically at runtime; modification on this directory can be used for testing a runtime behaviour for a Unit but all modifications will be lost at next boot.

[Unit] section options

This section is used for defining the metadata and relations between different Unit

Please find below the main properties description:

Description=: 

Name and function

Documentation=: 

URI for the documentation

Requires=:

List of Units dependencies. For successfully executing this Unit, all listed dependency should be activated without errors, otherwise this Unit return fail.

Wants=:

Similar to a Requires but weaker. If the Unit listed are not found or return fail, this Unit are executed anyway. this is the recommended method to be used.

BindsTo=:

Similar to Requires but it does a Stop for the Unit when the listed Unit are terminated.

Before=:

The Unit listed will not be executed until this Unit will not change to started. This is used for an order of Units executions.

After=:

The Unit listed will be started before this Unit. This is used for an order of Units executions.

Conflicts=

The Unit listed cannot be executed simultaneously to this Unit.

[Install] section options

This section is optional but is commonly used for defining a Unit behaviour when it will be executed during enable or disable commands.

WantedBy=:

This is similar to the Wants= on [Unit] section but allows to mantain the top Unit more clean.

When the Unit will be enabled, a directory on /etc/systemd/system will be created with the Unit name adding .wants to the name. Inside this directoty a symbolic link to the Unit is created.

Example:

• current Unit has WantedBy=multi-user.target
• a directory /etc/systemd/system/multi-user.target.wants will be created
• the symbolic link to the Unit will be created inside the new directory
• disabling the Unit the symbolic link is deleted and the relation is then removed.

RequiredBy=:

This is similar to WantedBy= but a dependency cause a fail if not satisfied. When the Unit is enabled, a directory with added .requires will be created

Also=:

When the Unit is enabled, also the listed Units are enabled too.

Specific sections

Some Unit have specific sections based on their characteristic. The most important is the section Service related to the Unit .service

Please find more information at the documentation page

[Service] section

Used for providing configurations for the services.

Type

Type= uses one of the (main) following values:

simple:

Default configuration for a service when specified ExecStarts=

forking:

the process will call a fork() when starts causing the father to exit. This informs systemd that the process is still alive even if the father has been terminated.

oneshot:

the process has a very short execution time and then systemd should wait for its termination before continuing with other Units. this is the default configuration if ExecStarts= is not specified.

dbus:

the Unit will acquire the name on the D-Bus. systemd will continue to process the other Units

notify:

the service will notify when completely initialized. Systemd will wait for the notification before continuing with the following Units

idle:

the service will not be executed until all active jobs are dispatched.

Other options

ExecStarts=: 

Specifiy the full path and parameters for executing a service. If preceded by a "-" this inform that the command failure can be accepted.

ExecStartsPre=:

used for adding more commands to be executed before starting the main process. May be used multiple times specifying the complete path and command parameters.

ExecReload=:

commands to be executed for reloading the service configuration.

ExecStop=:

commands required for stopping the service. If missing, the service will be killed.

ExecStopPost=:

commands to be executed after the service has been stopped..

RestartSec=:

time to sleep (seconds) before restarting the service.

Restart=:

restart conditions for systemd to be checked before restarting the service (if terminated). Can be set to "always","on-success", "on-failure", "on-abnormal", "on-abort", or "on-watchdog".

TimeoutSec=:

time to sleep during start or stop before considering the process failed on start or stop. Start and stop timeout can be set with different values using TimeoutStartSec= nad e TimeoutStopSec=

Creating a new service

For creating a new service the following file has to be created:

/etc/systemd/system/<service_name>.service

Service example

The following paragraph shows how to create a new service called myservice executing a command (in our case iperf3):

/etc/systemd/system/myservice.service

[Unit]
Description=My Service
After=network.target
StartLimitIntervalSec=0

[Service]
Type=simple
Restart=0
RestartSec=1
User=root
ExecStart=/usr/bin/iperf3 -s

[Install]
WantedBy=multi-user.target

Basic settings

After 

The executed command (iperf3) requires the network interface to be already active, so we use After for this purpose.

Restart

This is configured with 0 for disabling the service after it has been run.

RestartSec

time sleep before restarting the service; default value is 100ms.

User

configures the user or group used for executing the service.

ExecStart

command to be executed when the service will be started (in our case iperf3).

WantedBy

defines which target is used related to the service started.

running a service

Starting the service from userspace:

systemctl start myservice

Starting the service at boot time:

systemctl enable myservice

From SystemV to Systemd

start

Considering a SystemV script executing the start() function as in the following example:

start() {
  echo "Starting My Custom Service..."
  /usr/bin/myservice -D
}

The related command is executed in the custom service /usr/bin/myservice with the same -D parameter. It is possibile to use the ExecStart=:

[Service]
ExecStart=/usr/bin/myservice -D

restart

The same SystemV script may use special commands for restarting the service like reboot() function:

reboot() {
  echo "Reloading My Custom Service..."
  /usr/bin/myservice reload
}

which is equivalent to use ExecReload=:

[Service]
ExecReload=/usr/bin/myservice reload

stop

The stop() function in the script will become ExecStop=:

SystemV:

stop() {
  echo "Stopping My Custom Service..."
  /usr/bin/myservice shutdown
}

Systemd:

[Service]
ExecStop=/usr/bin/myservice shutdown

Network Configuration

One of the most systemd configuration used is the Network configuration.

wired interface

systemd uses a slightly different configuration mechanism than SystemV. The configuration file is the following one with an example of configuration:

/etc/systemd/network/eth0.network

[Match]
Name=eth0

# Prevent the interface loading if the kernel boots from nfs
KernelCommandLine=!nfsroot

[Network]
Address=192.168.0.120
Gateway=192.168.0.254
DNS=192.168.0.1
#DNS=8.8.8.8

Note:

The DNS is used only if the systemd-resolved service is enabled and the /etc/resolv.conf has a symbolic link to /run/systemd/resolve/stub-resolv.conf

ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf

wireless interface

wpa_supplicant

wpa_supplicant provides different services on systemd:

• wpa_supplicant.service uses D-Bus, recommended with the NetworkManager
• wpa_supplicant@interface.service uses the interface name (like wlan0) as parameter and executes the wpa_supplicant daemon on that interface. The configuration file is /etc/wpa_supplicant/wpa_supplicant-interface.conf

For enabling the interface at boot time it is required to enable the service

systemctl enable wpa_supplicant@interface

Configuration example

Assuming wlan0 as the wireless interface name, the configuration file examples are the following one:

/etc/systemd/network/wlan0.network

[Match]
Name=wlan0
[Network]
# Uncomment for DHCP
#DHCP=yes
Address=192.168.1.120
Gateway=192.168.1.254
DNS=8.8.8.8

/etc/wpa_supplicant/wpa_supplicant-wlan0.conf

ctrl_interface=/var/run/wpa_supplicant
eapol_version=1
ap_scan=1
fast_reauth=1

network={
    ssid="SSID1"
    psk="password1"
    priority=1
}
network={
    ssid="SSID2"
    psk="password2"
    priority=2
}

For automatically creating the network configuration, the following command can be used:

wpa_passphrase <ESSID> <passphrase> >> /etc/wpa_supplicant/wpa_supplicant-wlan0.conf

Then, the sergice should be enabled on the wlan0 interface for instructing systemd to start it using the (just) created configuration file wpa_supplicant-wlan0.conf:

systemctl enable wpa_supplicant@wlan0