Purpose of init.scope

Purpose of init.scope

[Demi Marie Obenour]

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What is the purpose of having PID 1 in init.scope?  Is it functionally
necessary, or is it solely for consistency with when systemd is
running in a container or as a user service manager?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)


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Re: [systemd-devel] Purpose of init.scope

[Barry Scott]

> On 29 Jul 2026, at 21:13, Demi Marie Obenour <demiobenour@gmail.com> wrote:
> 
> What is the purpose of having PID 1 in init.scope?  Is it functionally
> necessary, or is it solely for consistency with when systemd is
> running in a container or as a user service manager?

I assume it so that every process is in a cgroup container.
But that is a guess.

Barry


> -- 
> Sincerely,
> Demi Marie Obenour (she/her/hers)
> 

Re: [systemd-devel] Purpose of init.scope

[Lennart Poettering]

On Mi, 29.07.26 16:13, Demi Marie Obenour (demiobenour@gmail.com) wrote:

> What is the purpose of having PID 1 in init.scope?  Is it functionally
> necessary, or is it solely for consistency with when systemd is
> running in a container or as a user service manager?

PID 1 needs to live somehwere, so that's where we put it. Where else
would you put it?

Lennart

--
Lennart Poettering, Berlin

RE: [systemd-devel] Purpose of init.scope

[SCOTT FIELDS]

For anyone interested:

https://github.com/systemd/systemd/pull/1116

"This moves PID 1 into a new "init.scope" implicit scope unit in the root

slice. This is necessary since on the unified hierarchy cgroups may

either contain subgroups or processes but not both. PID 1 hence has to

move out of the root cgroup (strictly speaking the root cgroup is the

only one where processes and subgroups are still allowed, but in order

to support containers nicey, we move PID 1 into the new scope in all

cases.) This new unit is also used on legacy hierarchy setups. It's

actually pretty useful on all systems, as it can then be used to filter

journal messages coming from PID 1, and so on."


________________________________________
From: systemd-devel <systemd-devel-bounces@lists.freedesktop.org> on behalf of Lennart Poettering <lennart@poettering.net>
Sent: Sunday, August 16, 2026 11:02 AM
To: Demi Marie Obenour <demiobenour@gmail.com>
Cc: systemd development <systemd-devel@lists.freedesktop.org>; Linux cgroups development <cgroups@vger.kernel.org>; Alyssa Ross <hi@alyssa.is>
Subject: [EXTERNAL] Re: [systemd-devel] Purpose of init.scope
 
On Mi, 29.07.26 16:13, Demi Marie Obenour (demiobenour@gmail.com) wrote:

> What is the purpose of having PID 1 in init.scope?  Is it functionally
> necessary, or is it solely for consistency with when systemd is
> running in a container or as a user service manager?

PID 1 needs to live somehwere, so that's where we put it. Where else
would you put it?

Lennart

--
Lennart Poettering, Berlin

Re: [systemd-devel] Purpose of init.scope

[Demi Marie Obenour]

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On 8/17/26 07:32, SCOTT FIELDS wrote:
> For anyone interested:
> 
> https://github.com/systemd/systemd/pull/1116
> 
> "This moves PID 1 into a new "init.scope" implicit scope unit in the root
> slice. This is necessary since on the unified hierarchy cgroups may
> either contain subgroups or processes but not both. PID 1 hence has to
> move out of the root cgroup (strictly speaking the root cgroup is the
> only one where processes and subgroups are still allowed, but in order
> to support containers nicey, we move PID 1 into the new scope in all
> cases.) This new unit is also used on legacy hierarchy setups. It's
> actually pretty useful on all systems, as it can then be used to filter
> journal messages coming from PID 1, and so on."

Oh, that explains why PID 1 is not just left in the root cgroup.  I missed
the container use-case.

The project I am working on will never run in a container so that is not
a concern.

> On Mi, 29.07.26 16:13, Demi Marie Obenour (demiobenour@gmail.com) wrote:
> 
>> What is the purpose of having PID 1 in init.scope?  Is it functionally
>> necessary, or is it solely for consistency with when systemd is
>> running in a container or as a user service manager?
> 
> PID 1 needs to live somehwere, so that's where we put it. Where else
> would you put it?
> 
> Lennart
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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