What is an example of an "external user process"?

Question

I'm currently studying for my LFCS and have reached a section regarding the difference betwen user-created processes and kernel-created processes. Inside kernel-created processes, they define two subtypes of process.

1. Internal kernel processes
2. External user processes

They have given examples for internal kernel processes, such as maintenance work like flushing buffers to disk and ensuring that load is evenly spread across CPUs. But have only specified for external user processes that:

These are processes which run in user space like normal applications but which the kernel started. There are very few of these and they are usually short lived.

My first thought was that these would be things run under an init system like systemd, but my gut tells me that this probably isn't the case due to it mentioning they are usually short lived.

What would be an example of external user processes?

Answer 1

1. The program named in /proc/sys/kernel/hotplug.
2. The /sbin/v86d helper used by the uvesafb kernel module.

Further reading

• Jonathan de Boyne Pollard (2017). plug-and-play-event-handler. nosh Guide. Softwares.

Answer 2

An example of an "external user process" would be init. The kernel starts the init process without any request from userspace (i.e., not in response a fork() or related system call). Your thought about systemd is reasonable, since systemd plays the role of init.

The kernel can also trigger the execution of userspace processes on certain events. For example, historically the kernel would execute a hotplug userspace executable when devices were added or removed. Such processes are short-lived. (While that is a possible example, I don't think that approach is still used today for that use case.)