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Previously this would manipulate the realization mask for invalidating
the realization. This is a bit ugly though as the realization mask's
primary purpose to is to reflect in which hierarchies a cgroup currently
exists, and it's probably a good idea to keep that in sync with
realities.
We nowadays have the an explicit fields for invalidating cgroup
controller information, the "cgroup_invalidated_mask", let's use this
one instead.
The effect is pretty much the same, as the main consumer of these masks
(unit_has_mask_realize()) checks both anyway.
This changes cg_enable_everywhere() to return which controllers are
enabled for the specified cgroup. This information is then used to
correctly track the enablement mask currently in effect for a unit.
Moreover, when we try to turn off a controller, and this works, then
this is indicates that the parent unit might succesfully turn it off
now, too as our unit might have kept it busy.
So far, when realizing cgroups, i.e. when syncing up the kernel
representation of relevant cgroups with our own idea we would strictly
work from the root to the leaves. This is generally a good approach, as
when controllers are enabled this has to happen in root-to-leaves order.
However, when controllers are disabled this has to happen in the
opposite order: in leaves-to-root order (this is because controllers can
only be enabled in a child if it is already enabled in the parent, and
if it shall be disabled in the parent then it has to be disabled in the
child first, otherwise it is considered busy when it is attempted to
remove it in the parent).
To make things complicated when invalidating a unit's cgroup membershup
systemd can actually turn off some controllers previously turned on at
the very same time as it turns on other controllers previously turned
off. In such a case we have to work up leaves-to-root *and*
root-to-leaves right after each other. With this patch this is
implemented: we still generally operate root-to-leaves, but as soon as
we noticed we successfully turned off a controller previously turned on
for a cgroup we'll re-enqueue the cgroup realization for all parents of
a unit, thus implementing leaves-to-root where necessary.
Let's tweak when precisely to apply cgroup attributes on the root
cgroup.
With this we now follow the following rules:
1. On cgroupsv2 we never apply any regular cgroups to the host root,
since the attributes generally do not exist there.
2. On cgroupsv1 we do not apply any "weight" or "shares" style
attributes to the host root cgroup, since they don't make much sense
on the top level where there's only one group, hence no need to
compare weights against each other. The other attributes are applied
to the host root cgroup however.
3. In any case we don't apply attributes to the root of container
environments (and --user roots), under the assumption that this is
managed by the manager further up. (Note that on cgroupsv2 this is
even enforced by the kernel)
4. BPF pseudo-attributes are applied in all cases (since we can have as
many of them as we want)
Let's emphasize that this function checks for the host root cgroup, i.e.
returns false for the root cgroup when we run in a container where
CLONE_NEWCGROUP is used. There has been some confusion around this
already, for example cgroup_context_apply() uses the function
incorrectly (which we'll fix in a later commit).
Just some refactoring, not change in behaviour.
If we run in a container we shouldn't patch around this, and most likely
we can't anyway, and there's not much point in complaining about this.
Hence let's strictly say: the agent is private property of the host's
system instance, nothing else.
The way this is used drifted a bit from the original intent. Let's update
the description and add some examples to inspire people to texts that look
less bad during initial boot.
