Kernel Module Loading

The RDMA subsystem relies on the kernel, udev and systemd to load modules on demand when RDMA hardware is present. The RDMA subsystem is unique since it does not do not load the optional RDMA hardware modules unless the system has the rdma-core package installed.

This is to avoid exposing systems not using RDMA from having RDMA enabled, for instance if a system has a multi-protocol ethernet adapter, but is only using the net stack interface.

Boot ordering with systemd

systemd assumes everything is hot pluggable and runs in an event driven manner. This creates a chain of hot plug events as each part of the system autoloads based on earlier parts. The first step in the process is udev loading the physical hardware driver.

This can happen in several spots along the bootup:

From the initrd or built into the kernel. If hardware modules are present in the initrd then they are loaded into the kernel before booting the system. This is done largely synchronously with the boot process.
From udev when it auto detects PCI hardware or otherwise. This happens asynchronously in the boot process, systemd does not wait for udev to finish loading modules before it continues on.

This path makes it very likely the system will experience a RDMA 'hot plug' scenario.
From systemd's fixed module loader systemd-modules-load.service, e.g. from the list in /etc/modules-load.d/. In this case the modules load happens synchronously within systemd and it will hold off sysinit.target until modules are loaded

Once the hardware module is loaded it may be necessary to load a protocol module, e.g. to enable RDMA support on an ethernet device.

This is triggered automatically by udev rules that match the master devices and load the protocol module with udev's module loader. This happens asynchronously to the rest of the systemd startup.

Once a RDMA device is created by the kernel then udev will cause systemd to schedule ULP module loading services (e.g. rdma-load-modules@.service) specific to the plugged hardware. If sysinit.target has not yet been passed then these loaders will defer sysinit.target until they complete, otherwise this is a hot plug event and things will load asynchronously to the boot up process.

Finally udev will cause systemd to start RDMA specific daemons like srp_daemon, rdma-ndd and iwpmd. These starts are linked to the detection of the first RDMA hardware, and the daemons internally handle hot plug events for other hardware.

Hot Plug compatible services

Services using RDMA need to have device specific systemd dependencies in their unit files, either created by hand by the admin or by using udev rules.

For instance, a service that uses /dev/infiniband/umad0 requires:

After=dev-infiniband-umad0.device
BindsTo=dev-infiniband-umad0.device

Which will ensure the service will not run until the required umad device appears, and will be stopped if the umad device is unplugged.

This is similar to how systemd handles mounting filesystems and configuring ethernet devices.

Interaction with legacy non-hotplug services

Services that cannot handle hot plug must be ordered after systemd-udev-settle.service, which will wait for udev to complete loading modules and scheduling systemd services. This ensures that all RDMA hardware present at boot is setup before proceeding to run the le.g.acy service.

Admins using le.g.acy services can also place their RDMA hardware modules (e.g. mlx4_ib) directly in /etc/modules-load.d/ or in their initrd which will cause systemd to defer passing to sysinit.target until all RDMA hardware is setup, this is usually sufficient for le.g.acy services. This is probably the default behavior in many configurations.

Systemd Ordering

Within rdma-core we have a series of units which run in the pre basic.target world to setup kernel services:

iwpmd
rdma-ndd
rdma-load-modules@.service
ibacmd.socket

These special units use DefaultDependencies=no and order before any other unit that uses DefaultDependencies=yes. This will happen even in the case of hotplug.

Units for normal rdma-using daemons should use DefaultDependencies=yes, and either this pattern for 'any RDMA device':

[Unit]
# Order after rdma-hw.target has become active and setup the kernel services
Requires=rdma-hw.target
After=rdma-hw.target

[Install]
# Autostart when RDMA hardware is present
WantedBy=rdma-hw.target

Or this pattern for a specific RDMA device:

[Unit]
# Order after RDMA services are setup
After=rdma-hw.target
# Run only while a specific umad device is present
After=dev-infiniband-umad0.device
BindsTo=dev-infiniband-umad0.device

[Install]
# Schedual the unit to be runnable when RDMA hardware is present, but
# it will only start once the requested device actuall appears.
WantedBy=rdma-hw.target

Note, the above does explicitly reference After=rdma-hw.target even though all the current constituents of that target order before sysinit.target. This is to provide greater flexibility in the future.

rdma-hw.target

This target is Wanted automatically by udev as soon as any RDMA hardware is plugged in or becomes available at boot.

This may be used to pull in rdma management daemons dynamically when RDMA hardware is found. Such daemons should use:

[Install]
WantedBy=rdma-hw.target

In their unit files.

rdma-hw.target is also a synchronization point that orders after the low level, pre sysinit.target RDMA related units have been started.

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