systemd

Control Group APIs and Delegation

Intended audience: hackers working on userspace subsystems that require direct cgroup access, such as container managers and similar.

So you are wondering about resource management with systemd, you know Linux control groups (cgroups) a bit and are trying to integrate your software with what systemd has to offer there. Here’s a bit of documentation about the concepts and interfaces involved with this.

What’s described here has been part of systemd and documented since v205 times. However, it has been updated and improved substantially, even though the concepts stayed mostly the same. This is an attempt to provide more comprehensive up-to-date information about all this, particular in light of the poor implementations of the components interfacing with systemd of current container managers.

Before you read on, please make sure you read the low-level kernel documentation about cgroupsv2. This documentation then adds in the higher-level view from systemd.

This document augments the existing documentation we already have:

These wiki documents are not as up to date as they should be, currently, but the basic concepts still fully apply. You should read them too, if you do something with cgroups and systemd, in particular as they shine more light on the various D-Bus APIs provided. (That said, sooner or later we should probably fold that wiki documentation into this very document, too.)

Two Key Design Rules

Much of the philosophy behind these concepts is based on a couple of basic design ideas of cgroupsv2 (which we however try to adapt as far as we can to cgroupsv1 too). Specifically two cgroupsv2 rules are the most relevant:

  1. The no-processes-in-inner-nodes rule: this means that it’s not permitted to have processes directly attached to a cgroup that also has child cgroups and vice versa. A cgroup is either an inner node or a leaf node of the tree, and if it’s an inner node it may not contain processes directly, and if it’s a leaf node then it may not have child cgroups. (Note that there are some minor exceptions to this rule, though. E.g. the root cgroup is special and allows both processes and children — which is used in particular to maintain kernel threads.)

  2. The single-writer rule: this means that each cgroup only has a single writer, i.e. a single process managing it. It’s OK if different cgroups have different processes managing them. However, only a single process should own a specific cgroup, and when it does that ownership is exclusive, and nothing else should manipulate it at the same time. This rule ensures that various pieces of software don’t step on each other’s toes constantly.

These two rules have various effects. For example, one corollary of this is: if your container manager creates and manages cgroups in the system’s root cgroup you violate rule #2, as the root cgroup is managed by systemd and hence off limits to everybody else.

Note that rule #1 is generally enforced by the kernel if cgroupsv2 is used: as soon as you add a process to a cgroup it is ensured the rule is not violated. On cgroupsv1 this rule didn’t exist, and hence isn’t enforced, even though it’s a good thing to follow it then too. Rule #2 is not enforced on either cgroupsv1 nor cgroupsv2 (this is UNIX after all, in the general case root can do anything, modulo SELinux and friends), but if you ignore it you’ll be in constant pain as various pieces of software will fight over cgroup ownership.

Note that cgroupsv1 is currently the most deployed implementation, even though it’s semantically broken in many ways, and in many cases doesn’t actually do what people think it does. cgroupsv2 is where things are going, and most new kernel features in this area are only added to cgroupsv2, and not cgroupsv1 anymore. For example cgroupsv2 provides proper cgroup-empty notifications, has support for all kinds of per-cgroup BPF magic, supports secure delegation of cgroup trees to less privileged processes and so on, which all are not available on cgroupsv1.

Three Different Tree Setups 🌳

systemd supports three different modes how cgroups are set up. Specifically:

  1. Unified — this is the simplest mode, and exposes a pure cgroupsv2 logic. In this mode /sys/fs/cgroup is the only mounted cgroup API file system and all available controllers are exclusively exposed through it.

  2. Legacy — this is the traditional cgroupsv1 mode. In this mode the various controllers each get their own cgroup file system mounted to /sys/fs/cgroup/<controller>/. On top of that systemd manages its own cgroup hierarchy for managing purposes as /sys/fs/cgroup/systemd/.

  3. Hybrid — this is a hybrid between the unified and legacy mode. It’s set up mostly like legacy, except that there’s also an additional hierarchy /sys/fs/cgroup/unified/ that contains the cgroupsv2 hierarchy. (Note that in this mode the unified hierarchy won’t have controllers attached, the controllers are all mounted as separate hierarchies as in legacy mode, i.e. /sys/fs/cgroup/unified/ is purely and exclusively about core cgroupsv2 functionality and not about resource management.) In this mode compatibility with cgroupsv1 is retained while some cgroupsv2 features are available too. This mode is a stopgap. Don’t bother with this too much unless you have too much free time.

To say this clearly, legacy and hybrid modes have no future. If you develop software today and don’t focus on the unified mode, then you are writing software for yesterday, not tomorrow. They are primarily supported for compatibility reasons and will not receive new features. Sorry.

Superficially, in legacy and hybrid modes it might appear that the parallel cgroup hierarchies for each controller are orthogonal from each other. In systemd they are not: the hierarchies of all controllers are always kept in sync (at least mostly: sub-trees might be suppressed in certain hierarchies if no controller usage is required for them). The fact that systemd keeps these hierarchies in sync means that the legacy and hybrid hierarchies are conceptually very close to the unified hierarchy. In particular this allows us to talk of one specific cgroup and actually mean the same cgroup in all available controller hierarchies. E.g. if we talk about the cgroup /foo/bar/ then we actually mean /sys/fs/cgroup/cpu/foo/bar/ as well as /sys/fs/cgroup/memory/foo/bar/, /sys/fs/cgroup/pids/foo/bar/, and so on. Note that in cgroupsv2 the controller hierarchies aren’t orthogonal, hence thinking about them as orthogonal won’t help you in the long run anyway.

If you wonder how to detect which of these three modes is currently used, use statfs() on /sys/fs/cgroup/. If it reports CGROUP2_SUPER_MAGIC in its .f_type field, then you are in unified mode. If it reports TMPFS_MAGIC then you are either in legacy or hybrid mode. To distuingish these two cases, run statfs() again on /sys/fs/cgroup/unified/. If that succeeds and reports CGROUP2_SUPER_MAGIC you are in hybrid mode, otherwise not.

systemd’s Unit Types

The low-level kernel cgroups feature is exposed in systemd in three different “unit” types. Specifically:

  1. 💼 The .service unit type. This unit type is for units encapsulating processes systemd itself starts. Units of these types have cgroups that are the leaves of the cgroup tree the systemd instance manages (though possibly they might contain a sub-tree of their own managed by something else, made possible by the concept of delegation, see below). Service units are usually instantiated based on a unit file on disk that describes the command line to invoke and other properties of the service. However, service units may also be declared and started programmatically at runtime through a D-Bus API (which is called transient services).

  2. 👓 The .scope unit type. This is very similar to .service. The main difference: the processes the units of this type encapsulate are forked off by some unrelated manager process, and that manager asked systemd to expose them as a unit. Unlike services, scopes can only be declared and started programmatically, i.e. are always transient. That’s because they encapsulate processes forked off by something else, i.e. existing runtime objects, and hence cannot really be defined fully in ‘offline’ concepts such as unit files.

  3. 🔪 The .slice unit type. Units of this type do not directly contain any processes. Units of this type are the inner nodes of part of the cgroup tree the systemd instance manages. Much like services, slices can be defined either on disk with unit files or programmatically as transient units.

Slices expose the trunk and branches of a tree, and scopes and services are attached to those branches as leaves. The idea is that scopes and services can be moved around though, i.e. assigned to a different slice if needed.

The naming of slice units directly maps to the cgroup tree path. This is not the case for service and scope units however. A slice named foo-bar-baz.slice maps to a cgroup /foo.slice/foo-bar.slice/foo-bar-baz.slice/. A service quux.service which is attached to the slice foo-bar-baz.slice maps to the cgroup /foo.slice/foo-bar.slice/foo-bar-baz.slice/quux.service/.

By default systemd sets up four slice units:

  1. -.slice is the root slice. i.e. the parent of everything else. On the host system it maps directly to the top-level directory of cgroupsv2.

  2. system.slice is where system services are by default placed, unless configured otherwise.

  3. user.slice is where user sessions are placed. Each user gets a slice of its own below that.

  4. machines.slice is where VMs and containers are supposed to be placed. systemd-nspawn makes use of this by default, and you’re very welcome to place your containers and VMs there too if you hack on managers for those.

Users may define any amount of additional slices they like though, the four above are just the defaults.

Delegation

Container managers and suchlike often want to control cgroups directly using the raw kernel APIs. That’s entirely fine and supported, as long as proper delegation is followed. Delegation is a concept we inherited from cgroupsv2, but we expose it on cgroupsv1 too. Delegation means that some parts of the cgroup tree may be managed by different managers than others. As long as it is clear which manager manages which part of the tree each one can do within its sub-graph of the tree whatever it wants.

Only sub-trees can be delegated (though whoever decides to request a sub-tree can delegate sub-sub-trees further to somebody else if they like). Delegation takes place at a specific cgroup: in systemd there’s a Delegate= property you can set for a service or scope unit. If you do, it’s the cut-off point for systemd’s cgroup management: the unit itself is managed by systemd, i.e. all its attributes are managed exclusively by systemd, however your program may create/remove sub-cgroups inside it freely, and those then become exclusive property of your program, systemd won’t touch them — all attributes of those sub-cgroups can be manipulated freely and exclusively by your program.

By turning on the Delegate= property for a scope or service you get a few guarantees:

  1. systemd won’t fiddle with your sub-tree of the cgroup tree anymore. It won’t change attributes of any cgroups below it, nor will it create or remove any cgroups thereunder, nor migrate processes across the boundaries of that sub-tree as it deems useful anymore.

  2. If your service makes use of the User= functionality, then the sub-tree will be chown()ed to the indicated user so that it can correctly create cgroups below it. Note however that systemd will do that only in the unified hierarchy (in unified and hybrid mode) as well as on systemd’s own private hierarchy (in legacy and hybrid mode). It won’t pass ownership of the legacy controller hierarchies. Delegation to less privileges processes is not safe in cgroupsv1 (as a limitation of the kernel), hence systemd won’t facilitate access to it.

  3. Any BPF IP filter programs systemd installs will be installed with BPF_F_ALLOW_MULTI so that your program can install additional ones.

In unit files the Delegate= property is superficially exposed as boolean. However, since v236 it optionally takes a list of controller names instead. If so, delegation is requested for listed controllers specifically. Note hat this only encodes a request. Depending on various parameters it might happen that your service actually will get fewer controllers delegated (for example, because the controller is not available on the current kernel or was turned off) or more. If no list is specified (i.e. the property simply set to yes) then all available controllers are delegated.

Let’s stress one thing: delegation is available on scope and service units only. It’s expressly not available on slice units. Why? Because slice units are our inner nodes of the cgroup trees and we freely attach service and scopes to them. If we’d allow delegation on slice units then this would mean that both systemd and your own manager would create/delete cgroups below the slice unit and that conflicts with the single-writer rule.

So, if you want to do your own raw cgroups kernel level access, then allocate a scope unit, or a service unit (or just use the service unit you already have for your service code), and turn on delegation for it.

Three Scenarios

Let’s say you write a container manager, and you wonder what to do regarding cgroups for it, as you want your manager to be able to run on systemd systems.

You basically have three options:

  1. 😊 The integration-is-good option. For this, you register each container you have either as a systemd service (i.e. let systemd invoke the executor binary for you) or a systemd scope (i.e. your manager executes the binary directly, but then tells systemd about it. In this mode the administrator can use the usual systemd resource management and reporting commands individually on those containers. By turning on Delegate= for these scopes or services you make it possible to run cgroup-enabled programs in your containers, for example a nested systemd instance. This option has two sub-options:

    a. You transiently register the service or scope by directly contacting systemd via D-Bus. In this case systemd will just manage the unit for you and nothing else.

    b. Instead you register the service or scope through systemd-machined (also via D-Bus). This mini-daemon is basically just a proxy for the same operations as in a. The main benefit of this: this way you let the system know that what you are registering is a container, and this opens up certain additional integration points. For example, journalctl -M can then be used to directly look into any container’s journal logs (should the container run systemd inside), or systemctl -M can be used to directly invoke systemd operations inside the containers. Moreover tools like “ps” can then show you to which container a process belongs (ps -eo pid,comm,machine), and even gnome-system-monitor supports it.

  2. 🙁 The i-like-islands option. If all you care about is your own cgroup tree, and you want to have to do as little as possible with systemd and no interest in integration with the rest of the system, then this is a valid option. For this all you have to do is turn on Delegate= for your main manager daemon. Then figure out the cgroup systemd placed your daemon in: you can now freely create sub-cgroups beneath it. Don’t forget the no-processes-in-inner-nodes rule however: you have to move your main daemon process out of that cgroup (and into a sub-cgroup) before you can start further processes in any of your sub-cgroups.

  3. 🙁 The i-like-continents option. In this option you’d leave your manager daemon where it is, and would not turn on delegation on its unit. However, as first thing you register a new scope unit with systemd, and that scope unit would have Delegate= turned on, and then you place all your containers underneath it. From systemd’s PoV there’d be two units: your manager service and the big scope that contains all your containers in one.

BTW: if for whatever reason you say “I hate D-Bus, I’ll never call any D-Bus API, kthxbye”, then options #1 and #3 are not available, as they generally involve talking to systemd from your program code, via D-Bus. You still have option #2 in that case however, as you can simply set Delegate= in your service’s unit file and you are done and have your own sub-tree. In fact, #2 is the one option that allows you to completely ignore systemd’s existence: you can entirely generically follow the single rule that you just use the cgroup you are started in, and everything below it, whatever that might be. That said, maybe if you dislike D-Bus and systemd that much, the better approach might be to work on that, and widen your horizon a bit. You are welcome.

Controller Support

systemd supports a number of controllers (but not all). Specifically, supported are:

It is our intention to natively support all cgroupsv2 controllers as they are added to the kernel. However, regarding cgroupsv1: at this point we will not add support for any other controllers anymore. This means systemd currently does not and will never manage the following controllers on cgroupsv1: freezer, cpuset, net_cls, perf_event, net_prio, hugetlb. Why not? Depending on the case, either their API semantics or implementations aren’t really usable, or it’s very clear they have no future on cgroupsv2, and we won’t add new code for stuff that clearly has no future.

Effectively this means that all those mentioned cgroupsv1 controllers are up for grabs: systemd won’t manage them, and hence won’t delegate them to your code (however, systemd will still mount their hierarchies, simply because it mounts all controller hierarchies it finds available in the kernel). If you decide to use them, then that’s fine, but systemd won’t help you with it (but also not interfere with it). To be nice to other tenants it might be wise to replicate the cgroup hierarchies of the other controllers in them too however, but of course that’s between you and those other tenants, and systemd won’t care. Replicating the cgroup hierarchies in those unsupported controllers would mean replicating the full cgroup paths in them, and hence the prefixing .slice components too, otherwise the hierarchies will start being orthogonal after all, and that’s not really desirable. On more thing: systemd will clean up after you in the hierarchies it manages: if your daemon goes down, its cgroups will be removed too. You basically get the guarantee that you start with a pristine cgroup sub-tree for your service or scope whenever it is started. This is not the case however in the hierarchies systemd doesn’t manage. This means that your programs should be ready to deal with left-over cgroups in them — from previous runs, and be extra careful with them as they might still carry settings that might not be valid anymore.

Note a particular asymmetry here: if your systemd version doesn’t support a specific controller on cgroupsv1 you can still make use of it for delegation, by directly fiddling with its hierarchy and replicating the cgroup tree there as necessary (as suggested above). However, on cgroupsv2 this is different: separately mounted hierarchies are not available, and delegation has always to happen through systemd itself. This means: when you update your kernel and it adds a new, so far unseen controller, and you want to use it for delegation, then you also need to update systemd to a version that groks it.

systemd as Container Payload

systemd can happily run as a container payload’s PID 1. Note that systemd unconditionally needs write access to the cgroup tree however, hence you need to delegate a sub-tree to it. Note that there’s nothing too special you have to do beyond that: just invoke systemd as PID 1 inside the root of the delegated cgroup sub-tree, and it will figure out the rest: it will determine the cgroup it is running in and take possession of it. It won’t interfere with any cgroup outside of the sub-tree it was invoked in. Use of CLONE_NEWCGROUP is hence optional (but of course wise).

Note one particular asymmetry here though: systemd will try to take possession of the root cgroup you pass to it in full, i.e. it will not only create/remove child cgroups below it, it will also attempt to manage the attributes of it. OTOH as mentioned above, when delegating a cgroup tree to somebody else it only passes the rights to create/remove sub-cgroups, but will insist on managing the delegated cgroup tree’s top-level attributes. Or in other words: systemd is greedy when accepting delegated cgroup trees and also greedy when delegating them to others: it insists on managing attributes on the specific cgroup in both cases. A container manager that is itself a payload of a host systemd which wants to run a systemd as its own container payload instead hence needs to insert an extra level in the hierarchy in between, so that the systemd on the host and the one in the container won’t fight for the attributes. That said, you likely should do that anyway, due to the no-processes-in-inner-cgroups rule, see below.

When systemd runs as container payload it will make use of all hierarchies it has write access to. For legacy mode you need to make at least /sys/fs/cgroup/systemd/ available, all other hierarchies are optional. For hybrid mode you need to add /sys/fs/cgroup/unified/. Finally, for fully unified you (of course, I guess) need to provide only /sys/fs/cgroup/ itself.

Some Dos

  1. ⚡ If you go for implementation option 1a or 1b (as in the list above), then each of your containers will have its own systemd-managed unit and hence cgroup with possibly further sub-cgroups below. Typically the first process running in that unit will be some kind of executor program, which will in turn fork off the payload processes of the container. In this case don’t forget that there are two levels of delegation involved: first, systemd delegates a group sub-tree to your executor. And then your executor should delegate a sub-tree further down to the container payload. Oh, and because of the no-process-in-inner-nodes rule, your executor needs to migrate itself to a sub-cgroup of the cgroup it got delegated, too. Most likely you hence want a two-pronged approach: below the cgroup you got started in, you want one cgroup maybe called supervisor/ where your manager runs in and then for each container a sibling cgroup of that maybe called payload-xyz/.

  2. ⚡ Don’t forget that the cgroups you create have to have names that are suitable as UNIX file names, and that they live in the same namespace as the various kernel attribute files. Hence, when you want to allow the user arbitrary naming, you might need to escape some of the names (for example, you really don’t want to create a cgroup named tasks, just because the user created a container by that name, because tasks after all is a magic attribute in cgroupsv1, and your mkdir() will hence fail with EEXIST. In systemd we do escaping by prefixing names that might collide with a kernel attribute name with an underscore. You might want to do the same, but this is really up to you how you do it. Just do it, and be careful.

Some Don’ts

  1. 🚫 Never create your own cgroups below arbitrary cgroups systemd manages, i.e cgroups you haven’t set Delegate= in. Specifically: 🔥 don’t create your own cgroups below the root cgroup 🔥. That’s owned by systemd, and you will step on systemd’s toes if you ignore that, and systemd will step on yours. Get your own delegated sub-tree, you may create as many cgroups there as you like. Seriously, if you create cgroups directly in the cgroup root, then all you do is ask for trouble.

  2. 🚫 Don’t attempt to set Delegate= in slice units, and in particular not in -.slice. It’s not supported, and will generate an error.

  3. 🚫 Never write to any of the attributes of a cgroup systemd created for you. It’s systemd’s private property. You are welcome to manipulate the attributes of cgroups you created in your own delegated sub-tree, but the cgroup tree of systemd itself is out of limits for you. It’s fine to read from any attribute you like however. That’s totally OK and welcome.

  4. 🚫 When not using CLONE_NEWCGROUP when delegating a sub-tree to a container payload running systemd, then don’t get the idea that you can bind mount only a sub-tree of the host’s cgroup tree into the container. Part of the cgroup API is that /proc/$PID/cgroup reports the cgroup path of every process, and hence any path below /sys/fs/cgroup/ needs to match what /proc/$PID/cgroup of the payload processes reports. What you can do safely however, is mount the upper parts of the cgroup tree read-only (or even replace the middle bits with an intermediary tmpfs — but be careful not to break the statfs() detection logic discussed above), as long as the path to the delegated sub-tree remains accessible as-is.

  5. ⚡ Currently, the algorithm for mapping between slice/scope/service unit naming and their cgroup paths is not considered public API of systemd, and may change in future versions. This means: it’s best to avoid implementing a local logic of translating cgroup paths to slice/scope/service names in your program, or vice versa — it’s likely going to break sooner or later. Use the appropriate D-Bus API calls for that instead, so that systemd translates this for you. (Specifically: each Unit object has a ControlGroup property to get the cgroup for a unit. The method GetUnitByControlGroup() may be used to get the unit for a cgroup.)

  6. ⚡ Think twice before delegating cgroupsv1 controllers to less privileged containers. It’s not safe, you basically allow your containers to freeze the system with that and worse. Delegation is a strongpoint of cgroupsv2 though, and there it’s safe to treat delegation boundaries as privilege boundaries.

And that’s it for now. If you have further questions, refer to the systemd mailing list.

— Berlin, 2018-04-20