[Buildroot] [PATCH 1/1] package/systemd: hybrid cgroupfs hierarchy for docker compatibility

[Jérémy ROSEN]

It's about what implementation of cgroup is used by systemd to maintain the
cgroup-tree of processes

* legacy is v1
* hybrid is both v1 and v2
* unified is v2 only
(note that only the default is changed, this can be changed via the kernel
command-line at boot time)

v1 is deprecated on the kernel side because it was... a mess.
The systemd people are migrating stuff from v1 to v2 as new controllers are
added to v2, but at this point there is little left that is v1 only thus
the change of the default value

Very few user-space projects care about cgroups : mainly systemd and
container-managers. Container-manager need to interact with systemd as
described in
https://systemd.io/CGROUP_DELEGATION.html which allows to drop v1 and to
correctly spread the responsibility between systemd and the container
managers without
stepping on each other's toes.
Also note that Fedora31 will use the unified cgroup hierarchy.
A good summary of adoption status :
https://medium.com/nttlabs/cgroup-v2-596d035be4d7

At that point, I think staying in hybrid mode is the safest bet :
everything in user-space hasn't migrated yet.
Let Fedora take the fire and force everybody to migrate, and let's revisit
the subject once (at least) Docker has migrated...

Le lun. 11 nov. 2019 à 15:39, Thomas Petazzoni <thomas.petazzoni at bootlin.com>
a écrit :

> Hello,
>
> On Mon, 11 Nov 2019 11:09:27 +0100
> Jérémy ROSEN <jeremy.rosen at smile.fr> wrote:
>
> > Keeping a list like that is going to be tricky...
>
> You don't really have to keep a list. You can have a blind option:
>
> config BR2_PACKAGE_SYSTEMD_NEEDS_HYBRID_HIERARCHY
>         bool
>
> systemd.mk would observe this boolean to enable (or not) the hybrid
> hierarchy, and packages that need it would select this boolean.
>
> That being said, is there any drawback in unconditionally using the
> hybrid hierarchy? James, since you suggested to make it conditional, do
> you have some arguments/feedback?
>
> Thanks,
>
> Thomas
> --
> Thomas Petazzoni, CTO, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com
>


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