[systemd-devel] When is a unit "loaded"?

[Zbigniew Jędrzejewski-Szmek]

On Wed, Jul 11, 2018 at 06:52:12PM -0700, Filipe Brandenburger wrote:
> Hey Daniel!
> 
> On Wed, Jul 11, 2018 at 5:16 PM Daniel Wang <wonderfly at google.com> wrote:
> > I have a unit, say foo.service, on my system that's in /usr/lib/systemd/system, but disabled by preset.
> 
> Not that it matters, but presets don't really matter here. The unit is
> disabled, period.
> 
> > On system boot, it doesn't show as "loaded" per `systemctl --all | grep foo`.
> 
> Because there's no reference to it in units systemd sees, so systemd
> doesn't need to load it.
> 
> At bootup, systemd will simply start with default.target and then
> recursively load the units it needs.
> 
> See this link for more details:
> https://www.freedesktop.org/software/systemd/man/bootup.html#System%20Manager%20Bootup
> 
> `systemctl --all` will only show the units in memory, so foo.service
> won't be listed since it's not loaded.
> 
> > So if I override it with a file with the same name but under /etc/systemd/system, `systemctl cat foo.service` will show the one under /etc without the need for a `systemctl daemon-reload`.
> 
> Yes, because it's not loaded.
> 
> > If I create another service unit, bar.service, which has a After= dependency on foo.service, and start bar, foo.service will show as loaded. And then if I try to override it, `systemctl cat foo.service` will print a warning saying a daemon-reload is needed.
> 
> Yes. If systemd sees a reference for that unit (even if it's an
> After=), it will need to load it, since it needs to record the
> dependency between the units in the internal memory structures, so it
> needs a reference to the unit, and it loads it to have a complete
> reference to it...
> 
> > Nothings seems incorrect, but I have a few questions:
> > - Which units are loaded on-boot and which are not?
> 
> Only default.target and recursively any unit referred to by the loaded units.

What you are saying is all correct, with two very minor nipticks:
- the "default" unit can be overridden with systemd.unit= and 1/2/3, so
  it's not always default.target,
- also, there are other units that'll be loaded, for example anything
  pulled in by SYSTEMD_WANTS= in udev rules (if the hardware is present),
  and .device/.mount/.swap units can also pull other stuff in, and such
  units may be created based on the state of the system. So it's
  not just the tree starting at the "default" unit, but also a bunch
  of other subtree.

> > - Is the After= dependency alone enough to have systemd load a unit? Are there any other dependency directives that will result in the same effect?
> 
> Yes, I believe any reference to units will trigger the unit to be
> loaded. As I mentioned, systemd wants to keep the state in memory, so
> loading the unit will allow it to keep complete state.
> 
> An exception (haven't checked it, but I expect it) is references in
> the [Install] section (such as Also=) since those are only used by
> `systemctl enable` and are not really loaded into memory as far as I
> can tell (but I might be wrong here, and these might trigger the unit
> to load as well.)
Yes, anything in [Install] doesn't matter (except that after the
installation is done, the symlinks that are created do matter).

Zbyszek