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diff --git a/docs/CGROUP_DELEGATION.md b/docs/CGROUP_DELEGATION.md new file mode 100644 index 0000000..4210a75 --- /dev/null +++ b/docs/CGROUP_DELEGATION.md @@ -0,0 +1,502 @@ +--- +title: Control Group APIs and Delegation +category: Interfaces +layout: default +SPDX-License-Identifier: LGPL-2.1-or-later +--- + +# 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 the +[unified cgroup hierarchy](https://docs.kernel.org/admin-guide/cgroup-v2.html). +This document then adds in the higher-level view from systemd. + +This document augments the existing documentation we already have: + +* [The New Control Group Interfaces](https://www.freedesktop.org/wiki/Software/systemd/ControlGroupInterface) +* [Writing VM and Container Managers](https://www.freedesktop.org/wiki/Software/systemd/writing-vm-managers) + +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 cgroup v2 (which we however try to adapt as far as we can to +cgroup v1 too). Specifically two cgroup v2 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 cgroup v2 is used: as +soon as you add a process to a cgroup it is ensured the rule is not +violated. On cgroup v1 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 cgroup v1 nor cgroup v2 (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 cgroup v1 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. cgroup v2 is where things are going, and most new +kernel features in this area are only added to cgroup v2, and not cgroup v1 +anymore. For example, cgroup v2 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 cgroup v1. + +## 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 cgroup v2 +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 cgroup v1 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 cgroup v2 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 cgroup v2 +functionality and not about resource management.) In this mode compatibility +with cgroup v1 is retained while some cgroup v2 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 cgroup v2 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 distinguish 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. +From a shell, you can check the `Type` in `stat -f /sys/fs/cgroup` and +`stat -f /sys/fs/cgroup/unified`. + +## 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 cgroup v2. + +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 cgroup v2, +but we expose it on cgroup v1 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 privileged processes is not safe + in cgroup v1 (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 that 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 services 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. + +The service manager sets the `user.delegate` extended attribute (readable via +`getxattr(2)` and related calls) to the character `1` on cgroup directories +where delegation is enabled (and removes it on those cgroups where it is +not). This may be used by service programs to determine whether a cgroup tree +was delegated to them. Note that this is only supported on kernels 5.6 and +newer in combination with systemd 251 and newer. + +(OK, here's one caveat: if you turn on delegation for a service, and that +service has `ExecStartPost=`, `ExecReload=`, `ExecStop=` or `ExecStopPost=` +set, then these commands will be executed within the `.control/` sub-cgroup of +your service's cgroup. This is necessary because by turning on delegation we +have to assume that the cgroup delegated to your service is now an *inner* +cgroup, which means that it may not directly contain any processes. Hence, if +your service has any of these four settings set, you must be prepared that a +`.control/` subcgroup might appear, managed by the service manager. This also +means that your service code should have moved itself further down the cgroup +tree by the time it notifies the service manager about start-up readiness, so +that the service's main cgroup is definitely an inner node by the time the +service manager might start `ExecStartPost=`. Starting with systemd 254 you may +also use `DelegateSubgroup=` to let the service manager put your initial +service process into a subgroup right away.) + +(Also note, if you intend to use "threaded" cgroups — as added in Linux 4.14 —, +then you should do that *two* levels down from the main service cgroup your +turned delegation on for. Why that? You need one level so that systemd can +properly create the `.control` subgroup, as described above. But that one +cannot be threaded, since that would mean `.control` has to be threaded too — +this is a requirement of threaded cgroups: either a cgroup and all its siblings +are threaded or none –, but systemd expects it to be a regular cgroup. Thus you +have to nest a second cgroup beneath it which then can be threaded.) + +## 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 you start your first managed process (a container, for example) you would + register a new scope unit with systemd, and that scope unit would have + `Delegate=` turned on, and it would contain the PID of this process; all + your managed processes subsequently created should also be moved into this + scope. From systemd's PoV there'd be two units: your manager service and the + big scope that contains all your managed processes 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: + +* on cgroup v1: `cpu`, `cpuacct`, `blkio`, `memory`, `devices`, `pids` +* on cgroup v2: `cpu`, `io`, `memory`, `pids` + +It is our intention to natively support all cgroup v2 controllers as they are +added to the kernel. However, regarding cgroup v1: 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 cgroup v1: +`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 cgroup v2, and we +won't add new code for stuff that clearly has no future. + +Effectively this means that all those mentioned cgroup v1 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. One 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 cgroup v1 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 cgroup v2 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 cgroup v1, 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 cgroup v1 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 cgroup v2 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 |