info > CGROUPS

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) MUST be escaped as < and >. So `` should be escaped to `<` and `>` UNLESS they're part of one of those allowed tags. Inside a `
` block, the `` characters appearing in code content ARE NOT part of an HTML tag, so they need to be escaped. But wait — `` and `` ARE allowed tags. The `` that OPEN and CLOSE those tags are part of allowed tags. But the content within `...` could have `` that aren't tag delimiters. Looking at the original code blocks: - `echo $$ > /sys/fs/cgroup/cpu/cg1/cgroup.procs` — the `>` here is a redirection, needs escaping - `echo '+pids -memory' > x/y/cgroup.subtree_control` — same, `>` needs escaping So in code blocks I should use `>` for `>`. Now let me start the output. I should be careful about character count. The original is quite long, so I need to be efficient. Let me structure this: ```html CGROUPS(7) Linux Programmer's Manual 📛 NAME cgroups - Linux control groups 🚀 Quick Reference ... Use Case Command Description 📖 DESCRIPTION ... 🆅 CGROUPS VERSION 1 ... 🆅2 CGROUPS VERSION 2 ... 🧵 CGROUPS VERSION 2 THREAD MODE ... 🤝 CGROUPS DELEGATION ... ❌ ERRORS ... 📝 NOTES ... 🔗 SEE ALSO ... 📘 COLOPHON ``` For controllers in v1, I'll use h3 with emoji: - 🖥️ cpu - 📊 cpuacct - 📌 cpuset - 💾 memory - 🔌 devices - ❄️ freezer - 🌐 net_cls - 💿 blkio - 📈 perf_event - 🎯 net_prio - 📄 hugetlb - 🔢 pids - 🚀 rdma Let me build it now. I should make sure not to use . Looking at the rules again: > Comments in code examples are NOT headings — do NOT wrap them in / / Good. > ALL code MUST be wrapped in ... Yes. > Code includes anything with: $variable, ->method, use Module;, function(), flags like -f --long Got it. `$ cat`, `$ >` etc. are code. Let me start writing. For the Quick Reference table, I need commands in `` tags (not `` since that breaks table layout). Let me build common ones: | Use Case | Command | Description | |----------|---------|-------------| | Mount v1 controller | `mount -t cgroup -o cpu none /sys/fs/cgroup/cpu` | Mount a single v1 controller | | Mount multiple v1 controllers | `mount -t cgroup -o cpu,cpuacct none /sys/fs/cgroup/cpu,cpuacct` | Comount controllers | | Mount all v1 controllers | `mount -t cgroup -o all cgroup /sys/fs/cgroup` | Mount all available v1 controllers | | Mount v2 unified hierarchy | `mount -t cgroup2 none /mnt/cgroup2` | Mount the unified v2 filesystem | | Mount v2 with nsdelegate | `mount -t cgroup2 -o remount,nsdelegate none /sys/fs/cgroup/unified` | Enable namespace delegation | | Create a cgroup | `mkdir /sys/fs/cgroup/cpu/cg1` | Create new empty cgroup under cpu controller | | Move process to cgroup | `echo $$ > /sys/fs/cgroup/cpu/cg1/cgroup.procs` | Move current shell to cgroup | | Enable v2 controllers | `echo '+pids -memory' > x/y/cgroup.subtree_control` | Enable pids, disable memory in subtree | | Boot with v1 disabled | `cgroup_no_v1=all systemd.legacy_systemd_cgroup_controller` | Boot kernel without v1 controllers | | List processes in cgroup | `cat /proc/[pid]/cgroup` | Show cgroup membership of a process | Now let me build the entire HTML output. I need to be careful about size — let me aim for being thorough but efficient. Let me consider the structure of the description and controllers. Each controller section is a definition list-like structure. I think I can keep them as paragraphs with `` for the controller names, since that maintains the original style. Actually, looking at the original more carefully, the controllers are formatted like: ``` cpu (since Linux 2.6.24; CONFIG_CGROUP_SCHED) Cgroups can be guaranteed... ``` This is a definition list style. I'll preserve this with `` for the term and indented paragraphs. Let me write this out now. I'll be careful with escaping. The `>` characters in echo commands and the `<` for comparisons etc. The original document has `> /sys/fs/cgroup/...` which needs to be `>`. Looking again: the original uses `>` for shell redirection. These are NOT inside allowed HTML tags (they're content), so they must be escaped to `>`. OK, let me write the full output now. I'll aim to keep it comprehensive but tight. One more consideration: the description of the "Tasks (threads) versus processes" section and other subsections within DESCRIPTION - these should be h3. Sub-sections I can identify: - Terminology - Cgroups version 1 and version 2 Within "CGROUPS VERSION 1": - Tasks (threads) versus processes - Mounting v1 controllers - Unmounting v1 controllers - Cgroups version 1 controllers (then each controller) - Creating cgroups and moving processes - Removing cgroups - Cgroups v1 release notification - Cgroup v1 named hierarchies Within "CGROUPS VERSION 2": - (new behaviors list) - Cgroups v2 unified hierarchy - Cgroups v2 mount options - Cgroups v2 controllers - Cgroups v2 subtree control - Cgroups v2 "no internal processes" rule - Cgroups v2 cgroup.events file - Cgroup v2 release notification - Cgroups v2 cgroup.stat file - Limiting the number of descendant cgroups Within "CGROUPS VERSION 2 THREAD MODE": - Threaded versus domain controllers - Creating a threaded subtree - Using a threaded subtree - Rules for writing to cgroup.type and creating threaded subtrees - The "domain threaded" cgroup type - Exceptions for the root cgroup - The cgroups v2 "cpu" controller and realtime threads Within "CGROUPS DELEGATION": - Cgroups v2 delegation: nsdelegate and cgroup namespaces - Cgroup delegation containment rules Within "NOTES": - /proc files - /sys/kernel/cgroup files This is a LOT of h3 sections. Let me proceed. I need to be mindful of character count. Let me write efficiently. One more thing - looking at "Cgroups v2 delegation: nsdelegate and cgroup namespaces" - this looks like it has its own section with a long heading that combines as a section. Looking at the original: ``` Cgroups v2 delegation: nsdelegate and cgroup namespaces Starting with Linux 4.13... ``` This is a subsection (h3) within "CGROUPS DELEGATION" h2. Let me now write the complete output. Actually, I should consider that the cgroup v1 controllers section has many controllers listed with their descriptions. To save space and improve scannability, I could use a list format. But the rules say preserve original structure. I'll keep them as definition-style entries with ``. Let me also note: the rules state: > Preserve ALL original technical information — do NOT create new sections or content So I shouldn't add new content beyond the original. But I should add emoji to enhance readability. > Add descriptive emoji to option descriptions and list item text Good. Let me also consider that there's a list within a paragraph (bullet-style with `*`) in the original. Let me check: ``` The cgroups v2 release-notification mechanism offers the following advantages over the cgroups v1 release_agent mechanism: * It allows for cheaper notification, since a single process can monitor multiple cgroup.events files (using the techniques described earlier). By contrast, the cgroups v1 mechanism requires the expense of creating a process for each notification. * Notification for different cgroup subhierarchies can be delegated to different processes. By contrast, the cgroups v1 mechanism allows only one release agent for an entire hierarchy. ``` This should be a `` list. OK let me finalize and write it out. Also need to make sure the file paths/code blocks are correct. Let me proceed with writing the output now. For the `` tags - I'll keep them as-is from the original. The original has: - `CGROUPS(7)` - `tmpfs(5)` - etc. These should be preserved. Let me also think about the `` tags. The original uses `` for headings like NAME, DESCRIPTION, etc. — but I'm transforming those into proper `` tags. The `` tags within text (like `cpu`, `cpuacct`) should be preserved. OK, time to write the full HTML. Let me start: ```html CGROUPS(7) Linux Programmer's Manual 📛 NAME cgroups - Linux control groups 🚀 Quick Reference Use Case Command Description 🔧 Mount v1 controller mount -t cgroup -o cpu none /sys/fs/cgroup/cpu Mount single v1 controller on tmpfs 🔗 Comount v1 controllers mount -t cgroup -o cpu,cpuacct none /sys/fs/cgroup/cpu,cpuacct Mount multiple controllers on one hierarchy 📦 Mount all v1 controllers mount -t cgroup -o all cgroup /sys/fs/cgroup Mount every available v1 controller 🆕 Mount v2 unified hierarchy mount -t cgroup2 none /mnt/cgroup2 Mount the unified v2 filesystem 🔐 Mount v2 with nsdelegate mount -t cgroup2 -o remount,nsdelegate none /sys/fs/cgroup/unified Enable cgroup namespace delegation ➕ Create a cgroup mkdir /sys/fs/cgroup/cpu/cg1 Create new empty cgroup under cpu controller ➡️ Move process to cgroup echo $$ > /sys/fs/cgroup/cpu/cg1/cgroup.procs Move current shell into cgroup 🎛️ Enable v2 controllers echo '+pids -memory' > x/y/cgroup.subtree_control Enable pids, disable memory in subtree 🚫 Boot with v1 disabled cgroup_no_v1=all systemd.legacy_systemd_cgroup_controller Boot kernel without any v1 controllers 🔍 Inspect cgroup membership cat /proc/[pid]/cgroup Show cgroups a process belongs to ``` Now DESCRIPTION section. Let me work through it. For the term definitions like "A cgroup is...", I could use a definition list style or just paragraphs. Let me use paragraphs since the original uses indented paragraphs. Actually wait - the rules say: > For function, method, and class reference sections (pydoc modules, ruby ri classes, perldoc function lists): use name(args) — description format WITHOUT per-item emoji. This is for function references. The "Terminology" section is similar to that — defining terms. I could use a `` or just bold the term. Let me use `` for proper semantics. Hmm, looking more carefully, the cgroup v1 controllers also fit this pattern. They have names and descriptions like function references. Let me consider using `` for those too, OR keeping them as defined with ``. Actually, I'll keep the v1 controllers as the original style with `` for the term — this preserves the structure better. The function reference rule is more for languages like Python where each item is a method/function. Let me write the rest now. I'll go ahead and produce the full output. I need to balance comprehensiveness with brevity. Let me also reconsider — the "Cgroups v2 controllers" section is shorter and looks like a simple list. I'll keep that as a simple list with `` or just paragraphs. Actually let me use the dl/dt/dd approach for both Terminology and v1 controllers since they're definition-like. That makes it scannable. Hmm, but the original uses definition list style with the term and explanation. Let me convert these to `` blocks: ```html cgroup A collection of processes that are bound to a set of limits or parameters defined via the cgroup filesystem. subsystem ... ``` This works. Let me do this. OK, final plan. Let me write out the complete output now. One more thing — I should be mindful about the length. The original is quite long. Let me see if I can summarize some redundant sections while preserving key technical info. The rules say: > Condense your output to under 128,000 characters. Preserve key sections but summarize/combine verbatim repetition. Prefer tight formatting over verbosity. So I can condense, but I should preserve all key sections. Let me proceed. Let me start writing the complete output now. For cgroup v2 controllers — this is just a list of controllers. I can use `` for that. For the numbered behaviors in cgroups v2: ``` 1. Cgroups v2 provides a unified hierarchy against which all controllers are mounted. 2. "Internal" processes are not permitted. ... 3. Active cgroups must be specified via the files cgroup.controllers and cgroup.subtree_control. 4. The tasks file has been removed. ... 5. An improved mechanism for notification of empty cgroups is provided by the cgroup.events file. ``` This is an ordered list, use ``. Let me start writing. I'm going to write the full HTML now. I need to be careful about escape characters and tag consistency. CGROUPS(7) Linux Programmer's Manual 📛 NAME cgroups - Linux control groups 🚀 Quick Reference Use Case Command Description 🔧 Mount a single v1 controller mount -t cgroup -o cpu none /sys/fs/cgroup/cpu Mount cpu controller under tmpfs 🔗 Comount v1 controllers mount -t cgroup -o cpu,cpuacct none /sys/fs/cgroup/cpu,cpuacct Bind controllers to one hierarchy 📦 Mount all v1 controllers mount -t cgroup -o all cgroup /sys/fs/cgroup Mount every available v1 controller 🆕 Mount v2 unified hierarchy mount -t cgroup2 none /mnt/cgroup2 Mount the single unified v2 filesystem 🔐 Remount v2 with nsdelegate mount -t cgroup2 -o remount,nsdelegate none /sys/fs/cgroup/unified Treat cgroup namespaces as delegation boundaries ➕ Create a new cgroup mkdir /sys/fs/cgroup/cpu/cg1 Create empty cgroup under cpu controller ➡️ Move process to cgroup echo $$ > /sys/fs/cgroup/cpu/cg1/cgroup.procs Move current shell's PID into the cgroup 🎛️ Toggle v2 subtree controllers echo '+pids -memory' > x/y/cgroup.subtree_control Enable pids, disable memory in subtree 🚫 Disable all v1 controllers at boot cgroup_no_v1=all systemd.legacy_systemd_cgroup_controller Force v2-only mode for experimentation 🔍 Inspect process cgroup membership cat /proc/[pid]/cgroup Show hierarchies and paths a PID belongs to 📜 List available v2 features cat /sys/kernel/cgroup/features Discover kernel-supported v2 features 🗑️ Remove a cgroup rmdir /sys/fs/cgroup/cpu/cg1 Delete a cgroup (must be empty) 📖 DESCRIPTION 🐧 Control groups, usually referred to as cgroups, are a Linux kernel feature which allow processes to be organized into hierarchical groups whose usage of various types of resources can then be limited and monitored. The kernel's cgroup interface is provided through a pseudo-filesystem called cgroupfs. Grouping is implemented in the core cgroup kernel code, while resource tracking and limits are implemented in a set of per-resource-type subsystems (memory, CPU, and so on). 📚 Terminology cgroup 📦 A collection of processes that are bound to a set of limits or parameters defined via the cgroup filesystem. subsystem (resource controller / controller) ⚙️ A kernel component that modifies the behavior of the processes in a cgroup. Various subsystems have been implemented, making it possible to do things such as limiting the amount of CPU time and memory available to a cgroup, accounting for the CPU time used by a cgroup, and freezing and resuming execution of the processes in a cgroup. hierarchy 🌳 The cgroups for a controller are arranged in a hierarchy, defined by creating, removing, and renaming subdirectories within the cgroup filesystem. At each level of the hierarchy, attributes (e.g., limits) can be defined. The limits, control, and accounting provided by cgroups generally have effect throughout the sub-hierarchy underneath the cgroup where the attributes are defined. Limits placed at a higher level in the hierarchy cannot be exceeded by descendant cgroups. 🆚 Cgroups version 1 and version 2 📅 The initial release of the cgroups implementation was in Linux 2.6.24. Over time, various cgroup controllers have been added to allow the management of various types of resources. However, the development of these controllers was largely uncoordinated, with the result that many inconsistencies arose between controllers and management of the cgroup hierarchies became rather complex. A longer description of these problems can be found in the kernel source file Documentation/admin-guide/cgroup-v2.rst (or Documentation/cgroup-v2.txt in Linux 4.17 and earlier). 🛠️ Because of the problems with the initial cgroups implementation (cgroups version 1), starting in Linux 3.10, work began on a new, orthogonal implementation to remedy these problems. Initially marked experimental, and hidden behind the -o __DEVEL__sane_behavior mount option, the new version (cgroups version 2) was eventually made official with the release of Linux 4.5. Differences between the two versions are described in the text below. The file cgroup.sane_behavior, present in cgroups v1, is a relic of this mount option: it always reports "0" and is only retained for backward compatibility. 🤝 Although cgroups v2 is intended as a replacement for cgroups v1, the older system continues to exist (and for compatibility reasons is unlikely to be removed). Currently, cgroups v2 implements only a subset of the controllers available in cgroups v1. The two systems are implemented so that both v1 controllers and v2 controllers can be mounted on the same system. Thus, for example, it is possible to use those controllers that are supported under version 2, while also using version 1 controllers where version 2 does not yet support those controllers. The only restriction here is that a controller can't be simultaneously employed in both a cgroups v1 hierarchy and in the cgroups v2 hierarchy. 🆅 CGROUPS VERSION 1 🗂️ Under cgroups v1, each controller may be mounted against a separate cgroup filesystem that provides its own hierarchical organization of the processes on the system. It is also possible to comount multiple (or even all) cgroups v1 controllers against the same cgroup filesystem, meaning that the comounted controllers manage the same hierarchical organization of processes. 📂 For each mounted hierarchy, the directory tree mirrors the control group hierarchy. Each control group is represented by a directory, with each of its child control cgroups represented as a child directory. For instance, /user/joe/1.session represents control group 1.session, which is a child of cgroup joe, which is a child of /user. Under each cgroup directory is a set of files which can be read or written to, reflecting resource limits and a few general cgroup properties. 🧵 Tasks (threads) versus processes 🔀 In cgroups v1, a distinction is drawn between processes and tasks. In this view, a process can consist of multiple tasks (more commonly called threads, from a user-space perspective, and called such in the remainder of this man page). In cgroups v1, it is possible to independently manipulate the cgroup memberships of the threads in a process. ⚠️ The cgroups v1 ability to split threads across different cgroups caused problems in some cases. For example, it made no sense for the memory controller, since all of the threads of a process share a single address space. Because of these problems, the ability to independently manipulate the cgroup memberships of the threads in a process was removed in the initial cgroups v2 implementation, and subsequently restored in a more limited form (see the discussion of "thread mode" below). 🔩 Mounting v1 controllers ⚙️ The use of cgroups requires a kernel built with the CONFIG_CGROUP option. In addition, each of the v1 controllers has an associated configuration option that must be set in order to employ that controller. 📁 In order to use a v1 controller, it must be mounted against a cgroup filesystem. The usual place for such mounts is under a tmpfs(5) filesystem mounted at /sys/fs/cgroup. Thus, one might mount the cpu controller as follows: mount -t cgroup -o cpu none /sys/fs/cgroup/cpu 🔗 It is possible to comount multiple controllers against the same hierarchy. For example, here the cpu and cpuacct controllers are comounted against a single hierarchy: mount -t cgroup -o cpu,cpuacct none /sys/fs/cgroup/cpu,cpuacct 🤝 Comounting controllers has the effect that a process is in the same cgroup for all of the comounted controllers. Separately mounting controllers allows a process to be in cgroup /foo1 for one controller while being in /foo2/foo3 for another. 📦 It is possible to comount all v1 controllers against the same hierarchy: mount -t cgroup -o all cgroup /sys/fs/cgroup 💡 (One can achieve the same result by omitting -o all, since it is the default if no controllers are explicitly specified.) 🚫 It is not possible to mount the same controller against multiple cgroup hierarchies. For example, it is not possible to mount both the cpu and cpuacct controllers against one hierarchy, and to mount the cpu controller alone against another hierarchy. It is possible to create multiple mount points with exactly the same set of comounted controllers; however, in this case all that results is multiple mount points providing a view of the same hierarchy. 🐉 Note that on many systems, the v1 controllers are automatically mounted under /sys/fs/cgroup; in particular, systemd(1) automatically creates such mount points. 🔌 Unmounting v1 controllers 🚪 A mounted cgroup filesystem can be unmounted using the umount(8) command, as in the following example: umount /sys/fs/cgroup/pids ⚠️ But note well: a cgroup filesystem is unmounted only if it is not busy, that is, it has no child cgroups. If this is not the case, then the only effect of the umount(8) is to make the mount invisible. Thus, to ensure that the mount point is really removed, one must first remove all child cgroups, which in turn can be done only after all member processes have been moved from those cgroups to the root cgroup. 🎛️ Cgroups version 1 controllers 🧩 Each of the cgroups version 1 controllers is governed by a kernel configuration option (listed below). Additionally, the availability of the cgroups feature is governed by the CONFIG_CGROUPS kernel configuration option. 🖥️ cpu (since Linux 2.6.24; CONFIG_CGROUP_SCHED) 📊 Cgroups can be guaranteed a minimum number of "CPU shares" when a system is busy. This does not limit a cgroup's CPU usage if the CPUs are not busy. See Documentation/scheduler/sched-design-CFS.rst (Documentation/scheduler/sched-design-CFS.txt in Linux 5.2 and earlier). ⏱️ In Linux 3.2, this controller was extended to provide CPU "bandwidth" control. If the kernel is configured with CONFIG_CFS_BANDWIDTH, then within each scheduling period (defined via a file in the cgroup directory), it is possible to define an upper limit on the CPU time allocated to the processes in a cgroup. This upper limit applies even if there is no other competition for the CPU. Further information: Documentation/scheduler/sched-bwc.rst (Documentation/scheduler/sched-bwc.txt in Linux 5.2 and earlier). 📊 cpuacct (since Linux 2.6.24; CONFIG_CGROUP_CPUACCT) 🧮 Provides accounting for CPU usage by groups of processes. See Documentation/admin-guide/cgroup-v1/cpuacct.rst (Documentation/cgroup-v1/cpuacct.txt in Linux 5.2 and earlier). 📌 cpuset (since Linux 2.6.24; CONFIG_CPUSETS) 🧷 This cgroup can be used to bind the processes in a cgroup to a specified set of CPUs and NUMA nodes. See Documentation/admin-guide/cgroup-v1/cpusets.rst (Documentation/cgroup-v1/cpusets.txt in Linux 5.2 and earlier). 💾 memory (since Linux 2.6.25; CONFIG_MEMCG) 🧠 Supports reporting and limiting of process memory, kernel memory, and swap used by cgroups. See Documentation/admin-guide/cgroup-v1/memory.rst (Documentation/cgroup-v1/memory.txt in Linux 5.2 and earlier). 🔌 devices (since Linux 2.6.26; CONFIG_CGROUP_DEVICE) 🛡️ Controls which processes may create (mknod) devices as well as open them for reading or writing. Policies may be specified as allow-lists and deny-lists. Hierarchy is enforced, so new rules must not violate existing rules for the target or ancestor cgroups. See Documentation/admin-guide/cgroup-v1/devices.rst (Documentation/cgroup-v1/devices.txt in Linux 5.2 and earlier). ❄️ freezer (since Linux 2.6.28; CONFIG_CGROUP_FREEZER) 🥶 Can suspend and restore (resume) all processes in a cgroup. Freezing a cgroup /A also causes its children (e.g., /A/B) to be frozen. See Documentation/admin-guide/cgroup-v1/freezer-subsystem.rst (Documentation/cgroup-v1/freezer-subsystem.txt in Linux 5.2 and earlier). 🌐 net_cls (since Linux 2.6.29; CONFIG_CGROUP_NET_CLASSID) 🏷️ Places a classid, specified for the cgroup, on network packets created by a cgroup. These classids can be used in firewall rules and to shape traffic using tc(8). Applies only to packets leaving the cgroup, not to traffic arriving at it. See Documentation/admin-guide/cgroup-v1/net_cls.rst (Documentation/cgroup-v1/net_cls.txt in Linux 5.2 and earlier). 💿 blkio (since Linux 2.6.33; CONFIG_BLK_CGROUP) 📀 Controls and limits access to specified block devices by applying IO control in the form of throttling and upper limits against leaf nodes and intermediate nodes in the storage hierarchy. Two policies are available: a proportional-weight time-based division of disk implemented with CFQ (in effect for leaf nodes using CFQ), and a throttling policy which specifies upper I/O rate limits on a device. See Documentation/admin-guide/cgroup-v1/blkio-controller.rst (Documentation/cgroup-v1/blkio-controller.txt in Linux 5.2 and earlier). 📈 perf_event (since Linux 2.6.39; CONFIG_CGROUP_PERF) 🔬 Allows perf monitoring of the set of processes grouped in a cgroup. See the kernel source files. 🎯 net_prio (since Linux 3.3; CONFIG_CGROUP_NET_PRIO) 🚦 Allows priorities to be specified, per network interface, for cgroups. See Documentation/admin-guide/cgroup-v1/net_prio.rst (Documentation/cgroup-v1/net_prio.txt in Linux 5.2 and earlier). 📄 hugetlb (since Linux 3.5; CONFIG_CGROUP_HUGETLB) 📃 Supports limiting the use of huge pages by cgroups. See Documentation/admin-guide/cgroup-v1/hugetlb.rst (Documentation/cgroup-v1/hugetlb.txt in Linux 5.2 and earlier). 🔢 pids (since Linux 4.3; CONFIG_CGROUP_PIDS) 👶 Permits limiting the number of processes that may be created in a cgroup (and its descendants). See Documentation/admin-guide/cgroup-v1/pids.rst (Documentation/cgroup-v1/pids.txt in Linux 5.2 and earlier). 🚀 rdma (since Linux 4.11; CONFIG_CGROUP_RDMA) 📡 Permits limiting the use of RDMA/IB-specific resources per cgroup. See Documentation/admin-guide/cgroup-v1/rdma.rst (Documentation/cgroup-v1/rdma.txt in Linux 5.2 and earlier). ➕ Creating cgroups and moving processes 🌱 A cgroup filesystem initially contains a single root cgroup, /, which all processes belong to. A new cgroup is created by creating a directory in the cgroup filesystem: mkdir /sys/fs/cgroup/cpu/cg1 📭 This creates a new empty cgroup. ➡️ A process may be moved to this cgroup by writing its PID into the cgroup's cgroup.procs file: echo $$ > /sys/fs/cgroup/cpu/cg1/cgroup.procs 🔢 Only one PID at a time should be written to this file. 0️⃣ Writing the value 0 to a cgroup.procs file causes the writing process to be moved to the corresponding cgroup. 🧵 When writing a PID into the cgroup.procs, all threads in the process are moved into the new cgroup at once. 📍 Within a hierarchy, a process can be a member of exactly one cgroup. Writing a process's PID to a cgroup.procs file automatically removes it from the cgroup of which it was previously a member. 📋 The cgroup.procs file can be read to obtain a list of the processes that are members of a cgroup. The returned list of PIDs is not guaranteed to be in order. Nor is it guaranteed to be free of duplicates (e.g., a PID may be recycled while reading from the list). 🪡 In cgroups v1, an individual thread can be moved to another cgroup by writing its thread ID (i.e., the kernel thread ID returned by clone(2) and gettid(2)) to the tasks file in a cgroup directory. This file can be read to discover the set of threads that are members of the cgroup. 🗑️ Removing cgroups 🚫 To remove a cgroup, it must first have no child cgroups and contain no (nonzombie) processes. So long as that is the case, one can simply remove the corresponding directory pathname. Note that files in a cgroup directory cannot and need not be removed. 📢 Cgroups v1 release notification 🔔 Two files can be used to determine whether the kernel provides notifications when a cgroup becomes empty. A cgroup is considered to be empty when it contains no child cgroups and no member processes. 📝 A special file in the root directory of each cgroup hierarchy, release_agent, can be used to register the pathname of a program that may be invoked when a cgroup in the hierarchy becomes empty. The pathname of the newly empty cgroup (relative to the cgroup mount point) is provided as the sole command-line argument when the release_agent program is invoked. The release_agent program might remove the cgroup directory, or perhaps repopulate it with a process. ⛔ The default value of the release_agent file is empty, meaning that no release agent is invoked. 🗂️ The content of the release_agent file can also be specified via a mount option when the cgroup filesystem is mounted: mount -o release_agent=pathname ... 🚦 Whether or not the release_agent program is invoked when a particular cgroup becomes empty is determined by the value in the notify_on_release file in the corresponding cgroup directory. If this file contains the value 0, the release_agent program is not invoked. If it contains the value 1, the release_agent program is invoked. The default value for this file in the root cgroup is 0. At the time when a new cgroup is created, the value in this file is inherited from the corresponding file in the parent cgroup. 🏷️ Cgroup v1 named hierarchies 🔖 In cgroups v1, it is possible to mount a cgroup hierarchy that has no attached controllers: mount -t cgroup -o none,name=somename none /some/mount/point 📚 Multiple instances of such hierarchies can be mounted; each hierarchy must have a unique name. The only purpose of such hierarchies is to track processes. (See the discussion of release notification below.) An example is the name=systemd cgroup hierarchy used by systemd(1) to track services and user sessions. 🛑 Since Linux 5.0, the cgroup_no_v1 kernel boot option (described below) can be used to disable cgroup v1 named hierarchies, by specifying cgroup_no_v1=named. 🆅2 CGROUPS VERSION 2 🌳 In cgroups v2, all mounted controllers reside in a single unified hierarchy. While (different) controllers may be simultaneously mounted under the v1 and v2 hierarchies, it is not possible to mount the same controller simultaneously under both the v1 and the v2 hierarchies. 🆕 The new behaviors in cgroups v2 are summarized here, and in some cases elaborated in the following subsections: 🌐 Cgroups v2 provides a unified hierarchy against which all controllers are mounted. 🍃 "Internal" processes are not permitted. With the exception of the root cgroup, processes may reside only in leaf nodes (cgroups that do not themselves contain child cgroups). The details are somewhat more subtle than this, and are described below. 🎛️ Active cgroups must be specified via the files cgroup.controllers and cgroup.subtree_control. 🚫 The tasks file has been removed. In addition, the cgroup.clone_children file that is employed by the cpuset controller has been removed. 🔔 An improved mechanism for notification of empty cgroups is provided by the cgroup.events file. 📚 For more changes, see the Documentation/admin-guide/cgroup-v2.rst file in the kernel source (or Documentation/cgroup-v2.txt in Linux 4.17 and earlier). 🧬 Some of the new behaviors listed above saw subsequent modification with the addition in Linux 4.14 of "thread mode" (described below). 🌐 Cgroups v2 unified hierarchy 🧩 In cgroups v1, the ability to mount different controllers against different hierarchies was intended to allow great flexibility for application design. In practice, though, the flexibility turned out to be less useful than expected, and in many cases added complexity. Therefore, in cgroups v2, all available controllers are mounted against a single hierarchy. The available controllers are automatically mounted, meaning that it is not necessary (or possible) to specify the controllers when mounting the cgroup v2 filesystem using a command such as: mount -t cgroup2 none /mnt/cgroup2 ⚠️ A cgroup v2 controller is available only if it is not currently in use via a mount against a cgroup v1 hierarchy. Or, to put things another way, it is not possible to employ the same controller against both a v1 hierarchy and the unified v2 hierarchy. This means that it may be necessary first to unmount a v1 controller (as described above) before that controller is available in v2. Since systemd(1) makes heavy use of some v1 controllers by default, it can in some cases be simpler to boot the system with selected v1 controllers disabled. To do this, specify the cgroup_no_v1=list option on the kernel boot command line; list is a comma-separated list of the names of the controllers to disable, or the word all to disable all v1 controllers. (This situation is correctly handled by systemd(1), which falls back to operating without the specified controllers.) 🐉 Note that on many modern systems, systemd(1) automatically mounts the cgroup2 filesystem at /sys/fs/cgroup/unified during the boot process. 🛠️ Cgroups v2 mount options 🎛️ The following options (mount -o) can be specified when mounting the cgroup v2 filesystem: 🔐 nsdelegate (since Linux 4.15) 🛡️ Treat cgroup namespaces as delegation boundaries. For details, see below. 📊 memory_localevents (since Linux 5.2) 🧮 The memory.events file should show statistics only for the cgroup itself, and not for any descendant cgroups. This was the behavior before Linux 5.2. Starting in Linux 5.2, the default behavior is to include statistics for descendant cgroups in memory.events, and this mount option can be used to revert to the legacy behavior. This option is system wide and can be set on mount or modified through remount only from the initial mount namespace; it is silently ignored in noninitial namespaces. 🎮 Cgroups v2 controllers 📚 The following controllers, documented in the kernel source file Documentation/admin-guide/cgroup-v2.rst (or Documentation/cgroup-v2.txt in Linux 4.17 and earlier), are supported in cgroups version 2: 🖥️ cpu (since Linux 4.15) — successor to the v1 cpu and cpuacct controllers. 📌 cpuset (since Linux 5.0) — successor to the v1 cpuset controller. ❄️ freezer (since Linux 5.2) — successor to the v1 freezer controller. 📄 hugetlb (since Linux 5.6) — successor to the v1 hugetlb controller. 💿 io (since Linux 4.5) — successor to the v1 blkio controller. 💾 memory (since Linux 4.5) — successor to the v1 memory controller. 📈 perf_event (since Linux 4.11) — same as the v1 perf_event controller. 🔢 pids (since Linux 4.5) — same as the v1 pids controller. 🚀 rdma (since Linux 4.11) — same as the v1 rdma controller. 🚫 There is no direct equivalent of the net_cls and net_prio controllers from cgroups version 1. Instead, support has been added to iptables(8) to allow eBPF filters that hook on cgroup v2 pathnames to make decisions about network traffic on a per-cgroup basis. 🔌 The v2 devices controller provides no interface files; instead, device control is gated by attaching an eBPF (BPF_CGROUP_DEVICE) program to a v2 cgroup. 🌿 Cgroups v2 subtree control 🌳 Each cgroup in the v2 hierarchy contains the following two files: cgroup.controllers 📋 Read-only. Exposes a list of the controllers that are available in this cgroup. The contents match the contents of the cgroup.subtree_control file in the parent cgroup. cgroup.subtree_control 🎛️ A list of controllers that are active (enabled) in the cgroup. The set of controllers in this file is a subset of the set in the cgroup.controllers of this cgroup. The set of active controllers is modified by writing strings to this file containing space-delimited controller names, each preceded by + (to enable a controller) or - (to disable a controller), as in the following example: echo '+pids -memory' > x/y/cgroup.subtree_control ❌ An attempt to enable a controller that is not present in cgroup.controllers leads to an ENOENT error when writing to the cgroup.subtree_control file. 🔒 Because the list of controllers in cgroup.subtree_control is a subset of those in cgroup.controllers, a controller that has been disabled in one cgroup in the hierarchy can never be re-enabled in the subtree below that cgroup. ⛓️ A cgroup's cgroup.subtree_control file determines the set of controllers that are exercised in the child cgroups. When a controller (e.g., pids) is present in the cgroup.subtree_control file of a parent cgroup, then the corresponding controller-interface files (e.g., pids.max) are automatically created in the children of that cgroup and can be used to exert resource control in the child cgroups. 🍃 Cgroups v2 "no internal processes" rule 🌿 Cgroups v2 enforces a so-called "no internal processes" rule. Roughly speaking, this rule means that, with the exception of the root cgroup, processes may reside only in leaf nodes (cgroups that do not themselves contain child cgroups). This avoids the need to decide how to partition resources between processes which are members of cgroup A and processes in child cgroups of A. 🌱 For instance, if cgroup /cg1/cg2 exists, then a process may reside in /cg1/cg2, but not in /cg1. This is to avoid an ambiguity in cgroups v1 with respect to the delegation of resources between processes in /cg1 and its child cgroups. The recommended approach in cgroups v2 is to create a subdirectory called leaf for any nonleaf cgroup which should contain processes, but no child cgroups. Thus, processes which previously would have gone into /cg1 would now go into /cg1/leaf. This has the advantage of making explicit the relationship between processes in /cg1/leaf and /cg1's other children. 🔍 The "no internal processes" rule is in fact more subtle than stated above. More precisely, the rule is that a (nonroot) cgroup can't both (1) have member processes, and (2) distribute resources into child cgroups — that is, have a nonempty cgroup.subtree_control file. Thus, it is possible for a cgroup to have both member processes and child cgroups, but before controllers can be enabled for that cgroup, the member processes must be moved out of the cgroup (e.g., perhaps into the child cgroups). 🧵 With the Linux 4.14 addition of "thread mode" (described below), the "no internal processes" rule has been relaxed in some cases. 📅 Cgroups v2 cgroup.events file 📄 Each nonroot cgroup in the v2 hierarchy contains a read-only file, cgroup.events, whose contents are key-value pairs (delimited by newline characters, with the key and value separated by spaces) providing state information about the cgroup: $ cat mygrp/cgroup.events populated 1 frozen 0 🔑 The following keys may appear in this file: populated 👥 The value is 1 if this cgroup or any of its descendants has member processes, or otherwise 0. frozen (since Linux 5.2) ❄️ The value is 1 if this cgroup is currently frozen, or 0 if it is not. 👀 The cgroup.events file can be monitored, in order to receive notification when the value of one of its keys changes. Such monitoring can be done using inotify(7), which notifies changes as IN_MODIFY events, or poll(2), which notifies changes by returning the POLLPRI and POLLERR bits in the revents field. 📢 Cgroup v2 release notification 🔔 Cgroups v2 provides a new mechanism for obtaining notification when a cgroup becomes empty. The cgroups v1 release_agent and notify_on_release files are removed, and replaced by the populated key in the cgroup.events file. This key either has the value 0, meaning that the cgroup (and its descendants) contain no (nonzombie) member processes, or 1, meaning that the cgroup (or one of its descendants) contains member processes. ✨ The cgroups v2 release-notification mechanism offers the following advantages over the cgroups v1 release_agent mechanism: 💰 It allows for cheaper notification, since a single process can monitor multiple cgroup.events files (using the techniques described earlier). By contrast, the cgroups v1 mechanism requires the expense of creating a process for each notification. 🪪 Notification for different cgroup subhierarchies can be delegated to different processes. By contrast, the cgroups v1 mechanism allows only one release agent for an entire hierarchy. 📊 Cgroups v2 cgroup.stat file 📈 Each cgroup in the v2 hierarchy contains a read-only cgroup.stat file (first introduced in Linux 4.14) that consists of lines containing key-value pairs. The following keys currently appear in this file: nr_descendants 🌳 The total number of visible (i.e., living) descendant cgroups underneath this cgroup. nr_dying_descendants 💀 The total number of dying descendant cgroups underneath this cgroup. A cgroup enters the dying state after being deleted. It remains in that state for an undefined period (which will depend on system load) while resources are freed before the cgroup is destroyed. Note that the presence of some cgroups in the dying state is normal, and is not indicative of any problem. 🚫 A process can't be made a member of a dying cgroup, and a dying cgroup can't be brought back to life. 📏 Limiting the number of descendant cgroups 📐 Each cgroup in the v2 hierarchy contains the following files, which can be used to view and set limits on the number of descendant cgroups under that cgroup: cgroup.max.depth (since Linux 4.14) 📏 Defines a limit on the depth of nesting of descendant cgroups. A value of 0 means that no descendant cgroups can be created. An attempt to create a descendant whose nesting level exceeds the limit fails (mkdir(2) fails with EAGAIN). Writing the string "max" means no limit is imposed. The default value is "max". cgroup.max.descendants (since Linux 4.14) 🔢 Defines a limit on the number of live descendant cgroups that this cgroup may have. An attempt to create more descendants than allowed by the limit fails (mkdir(2) fails with EAGAIN). Writing the string "max" means no limit is imposed. The default value is "max". 🤝 CGROUPS DELEGATION: DELEGATING A HIERARCHY TO A LESS PRIVILEGED USER 🤝 In the context of cgroups, delegation means passing management of some subtree of the cgroup hierarchy to a nonprivileged user. Cgroups v1 provides support for delegation based on file permissions in the cgroup hierarchy but with less strict containment rules than v2 (as noted below). Cgroups v2 supports delegation with containment by explicit design. The focus of the discussion in this section is on delegation in cgroups v2, with some differences for cgroups v1 noted along the way. 📖 Some terminology is required in order to describe delegation. A delegater is a privileged user (i.e., root) who owns a parent cgroup. A delegatee is a nonprivileged user who will be granted the permissions needed to manage some subhierarchy under that parent cgroup, known as the delegated subtree. 🔧 To perform delegation, the delegater makes certain directories and files writable by the delegatee, typically by changing the ownership of the objects to be the user ID of the delegatee. Assuming that we want to delegate the hierarchy rooted at (say) /dlgt_grp and that there are not yet any child cgroups under that cgroup, the ownership of the following is changed to the user ID of the delegatee: /dlgt_grp 🌳 Changing the ownership of the root of the subtree means that any new cgroups created under the subtree (and the files they contain) will also be owned by the delegatee. /dlgt_grp/cgroup.procs ➡️ Changing the ownership of this file means that the delegatee can move processes into the root of the delegated subtree. /dlgt_grp/cgroup.subtree_control (cgroups v2 only) 🎛️ Changing the ownership of this file means that the delegatee can enable controllers (that are present in /dlgt_grp/cgroup.controllers) in order to further redistribute resources at lower levels in the subtree. (As an alternative, the delegater might instead add selected controllers to this file.) /dlgt_grp/cgroup.threads (cgroups v2 only) 🧵 Changing the ownership of this file is necessary if a threaded subtree is being delegated (see the description of "thread mode" below). This permits the delegatee to write thread IDs to the file. (The ownership of this file can also be changed when delegating a domain subtree, but currently this serves no purpose, since it is not possible to move a thread between domain cgroups by writing its thread ID to the cgroup.threads file.) 🔄 In cgroups v1, the corresponding file that should instead be delegated is the tasks file. 🚫 The delegater should not change the ownership of any of the controller interface files (e.g., pids.max, memory.high) in dlgt_grp. Those files are used from the next level above the delegated subtree in order to distribute resources into the subtree, and the delegatee should not have permission to change the resources that are distributed into the delegated subtree. 📚 See also the discussion of the /sys/kernel/cgroup/delegate file in NOTES for information about further delegatable files in cgroups v2. ✅ After the aforementioned steps have been performed, the delegatee can create child cgroups within the delegated subtree (the cgroup subdirectories and the files they contain will be owned by the delegatee) and move processes between cgroups in the subtree. If some controllers are present in dlgt_grp/cgroup.subtree_control, or the ownership of that file was passed to the delegatee, the delegatee can also control the further redistribution of the corresponding resources into the delegated subtree. 🛡️ Cgroups v2 delegation: nsdelegate and cgroup namespaces 🌐 Starting with Linux 4.13, there is a second way to perform cgroup delegation in the cgroups v2 hierarchy. This is done by mounting or remounting the cgroup v2 filesystem with the nsdelegate mount option. For example, if the cgroup v2 filesystem has already been mounted, we can remount it with the nsdelegate option as follows: mount -t cgroup2 -o remount,nsdelegate \ none /sys/fs/cgroup/unified 🔒 The effect of this mount option is to cause cgroup namespaces to automatically become delegation boundaries. More specifically, the following restrictions apply for processes inside the cgroup namespace: 🚫 Writes to controller interface files in the root directory of the namespace will fail with the error EPERM. Processes inside the cgroup namespace can still write to delegatable files in the root directory of the cgroup namespace such as cgroup.procs and cgroup.subtree_control, and can create subhierarchy underneath the root directory. 🚧 Attempts to migrate processes across the namespace boundary are denied (with the error ENOENT). Processes inside the cgroup namespace can still (subject to the containment rules described below) move processes between cgroups within the subhierarchy under the namespace root. 💡 The ability to define cgroup namespaces as delegation boundaries makes cgroup namespaces more useful. To understand why, suppose that we already have one cgroup hierarchy that has been delegated to a nonprivileged user, cecilia, using the older delegation technique described above. Suppose further that cecilia wanted to further delegate a subhierarchy under the existing delegated hierarchy. (For example, the delegated hierarchy might be associated with an unprivileged container run by cecilia.) Even if a cgroup namespace was employed, because both hierarchies are owned by the unprivileged user cecilia, the following illegitimate actions could be performed: ⚙️ A process in the inferior hierarchy could change the resource controller settings in the root directory of that hierarchy. (These resource controller settings are intended to allow control to be exercised from the parent cgroup; a process inside the child cgroup should not be allowed to modify them.) 🔀 A process inside the inferior hierarchy could move processes into and out of the inferior hierarchy if the cgroups in the superior hierarchy were somehow visible. 🛡️ Employing the nsdelegate mount option prevents both of these possibilities. 🌌 The nsdelegate mount option only has an effect when performed in the initial mount namespace; in other mount namespaces, the option is silently ignored. 💡 Note: On some systems, systemd(1) automatically mounts the cgroup v2 filesystem. In order to experiment with the nsdelegate operation, it may be useful to boot the kernel with the following command-line options: cgroup_no_v1=all systemd.legacy_systemd_cgroup_controller 🚫 These options cause the kernel to boot with the cgroups v1 controllers disabled (meaning that the controllers are available in the v2 hierarchy), and tells systemd(1) not to mount and use the cgroup v2 hierarchy, so that the v2 hierarchy can be manually mounted with the desired options after boot-up. 🔒 Cgroup delegation containment rules 🛡️ Some delegation containment rules ensure that the delegatee can move processes between cgroups within the delegated subtree, but can't move processes from outside the delegated subtree into the subtree or vice versa. A nonprivileged process (i.e., the delegatee) can write the PID of a "target" process into a cgroup.procs file only if all of the following are true: ✍️ The writer has write permission on the cgroup.procs file in the destination cgroup. ✍️ The writer has write permission on the cgroup.procs file in the nearest common ancestor of the source and destination cgroups. Note that in some cases, the nearest common ancestor may be the source or destination cgroup itself. This requirement is not enforced for cgroups v1 hierarchies, with the consequence that containment in v1 is less strict than in v2. (For example, in cgroups v1 the user that owns two distinct delegated subhierarchies can move a process between the hierarchies.) 🔐 If the cgroup v2 filesystem was mounted with the nsdelegate option, the writer must be able to see the source and destination cgroups from its cgroup namespace. 👤 In cgroups v1: the effective UID of the writer (i.e., the delegatee) matches the real user ID or the saved set-user-ID of the target process. Before Linux 4.11, this requirement also applied in cgroups v2. (This was a historical requirement inherited from cgroups v1 that was later deemed unnecessary, since the other rules suffice for containment in cgroups v2.) ⚠️ Note: one consequence of these delegation containment rules is that the unprivileged delegatee can't place the first process into the delegated subtree; instead, the delegater must place the first process (a process owned by the delegatee) into the delegated subtree. 🧵 CGROUPS VERSION 2 THREAD MODE 🧬 Among the restrictions imposed by cgroups v2 that were not present in cgroups v1 are the following: 🚫 No thread-granularity control: all of the threads of a process must be in the same cgroup. 🚫 No internal processes: a cgroup can't both have member processes and exercise controllers on child cgroups. 💡 Both of these restrictions were added because the lack of these restrictions had caused problems in cgroups v1. In particular, the cgroups v1 ability to allow thread-level granularity for cgroup membership made no sense for some controllers. (A notable example was the memory controller: since threads share an address space, it made no sense to split threads across different memory cgroups.) ✨ Notwithstanding the initial design decision in cgroups v2, there were use cases for certain controllers, notably the cpu controller, for which thread-level granularity of control was meaningful and useful. To accommodate such use cases, Linux 4.14 added thread mode for cgroups v2. 🧵 Thread mode allows the following: 🌳 The creation of threaded subtrees in which the threads of a process may be spread across cgroups inside the tree. (A threaded subtree may contain multiple multithreaded processes.) 🎛️ The concept of threaded controllers, which can distribute resources across the cgroups in a threaded subtree. 🍃 A relaxation of the "no internal processes rule", so that, within a threaded subtree, a cgroup can both contain member threads and exercise resource control over child cgroups. 📂 With the addition of thread mode, each nonroot cgroup now contains a new file, cgroup.type, that exposes, and in some circumstances can be used to change, the "type" of a cgroup. This file contains one of the following type values: domain 🌐 A normal v2 cgroup that provides process-granularity control. If a process is a member of this cgroup, then all threads of the process are (by definition) in the same cgroup. This is the default cgroup type, and provides the same behavior that was provided for cgroups in the initial cgroups v2 implementation. threaded 🧵 This cgroup is a member of a threaded subtree. Threads can be added to this cgroup, and controllers can be enabled for the cgroup. domain threaded 🔗 A domain cgroup that serves as the root of a threaded subtree. This cgroup type is also known as "threaded root". domain invalid 🚧 A cgroup inside a threaded subtree that is in an "invalid" state. Processes can't be added to the cgroup, and controllers can't be enabled for the cgroup. The only thing that can be done with this cgroup (other than deleting it) is to convert it to a threaded cgroup by writing the string "threaded" to the cgroup.type file. The rationale for the existence of this "interim" type during the creation of a threaded subtree (rather than the kernel simply immediately converting all cgroups under the threaded root to the type threaded) is to allow for possible future extensions to the thread mode model. 🧬 Threaded versus domain controllers 🔀 With the addition of thread mode, cgroups v2 now distinguishes two types of resource controllers: 🧵 Threaded controllers: these controllers support thread-granularity for resource control and can be enabled inside threaded subtrees, with the result that the corresponding controller-interface files appear inside the cgroups in the threaded subtree. As at Linux 4.19, the following controllers are threaded: cpu, perf_event, and pids. 🏠 Domain controllers: these controllers support only process granularity for resource control. From the perspective of a domain controller, all threads of a process are always in the same cgroup. Domain controllers can't be enabled inside a threaded subtree. 🌱 Creating a threaded subtree 🛤️ There are two pathways that lead to the creation of a threaded subtree. The first pathway proceeds as follows: ✍️ We write the string "threaded" to the cgroup.type file of a cgroup y/z that currently has the type domain. This has the following effects: The type of the cgroup y/z becomes threaded. The type of the parent cgroup, y, becomes domain threaded. The parent cgroup is the root of a threaded subtree (also known as the "threaded root"). All other cgroups under y that were not already of type threaded are converted to type domain invalid. Any subsequently created cgroups under y will also have the type domain invalid. ✍️ We write the string "threaded" to each of the domain invalid cgroups under y, in order to convert them to the type threaded. As a consequence of this step, all cgroups under the threaded root now have the type threaded and the threaded subtree is now fully usable. The requirement to write "threaded" to each of these cgroups is somewhat cumbersome, but allows for possible future extensions to the thread-mode model. 🛤️ The second way of creating a threaded subtree is as follows: In an existing cgroup, z, that currently has the type domain, we (1) enable one or more threaded controllers and (2) make a process a member of z. (These two steps can be done in either order.) This has the following consequences: The type of z becomes domain threaded. All of the descendant cgroups of x that were not already of type threaded are converted to type domain invalid. As before, we make the threaded subtree usable by writing the string "threaded" to each of the domain invalid cgroups under y, in order to convert them to the type threaded. 🔒 One of the consequences of the above pathways to creating a threaded subtree is that the threaded root cgroup can be a parent only to threaded (and domain invalid) cgroups. The threaded root cgroup can't be a parent of a domain cgroup, and a threaded cgroup can't have a sibling that is a domain cgroup. 🎯 Using a threaded subtree 🎛️ Within a threaded subtree, threaded controllers can be enabled in each subgroup whose type has been changed to threaded; upon doing so, the corresponding controller interface files appear in the children of that cgroup. ➡️ A process can be moved into a threaded subtree by writing its PID to the cgroup.procs file in one of the cgroups inside the tree. This has the effect of making all of the threads in the process members of the corresponding cgroup and makes the process a member of the threaded subtree. The threads of the process can then be spread across the threaded subtree by writing their thread IDs (see gettid(2)) to the cgroup.threads files in different cgroups inside the subtree. The threads of a process must all reside in the same threaded subtree. 🔒 As with writing to cgroup.procs, some containment rules apply when writing to the cgroup.threads file: ✍️ The writer must have write permission on the cgroup.threads file in the destination cgroup. ✍️ The writer must have write permission on the cgroup.procs file in the common ancestor of the source and destination cgroups. (In some cases, the common ancestor may be the source or destination cgroup itself.) 🌳 The source and destination cgroups must be in the same threaded subtree. (Outside a threaded subtree, an attempt to move a thread by writing its thread ID to the cgroup.threads file in a different domain cgroup fails with the error EOPNOTSUPP.) 📋 The cgroup.threads file is present in each cgroup (including domain cgroups) and can be read in order to discover the set of threads that is present in the cgroup. The set of thread IDs obtained when reading this file is not guaranteed to be ordered or free of duplicates. 👥 The cgroup.procs file in the threaded root shows the PIDs of all processes that are members of the threaded subtree. The cgroup.procs files in the other cgroups in the subtree are not readable. 🚫 Domain controllers can't be enabled in a threaded subtree; no controller-interface files appear inside the cgroups underneath the threaded root. From the point of view of a domain controller, threaded subtrees are invisible: a multithreaded process inside a threaded subtree appears to a domain controller as a process that resides in the threaded root cgroup. 🍃 Within a threaded subtree, the "no internal processes" rule does not apply: a cgroup can both contain member processes (or threads) and exercise controllers on child cgroups. 📜 Rules for writing to cgroup.type and creating threaded subtrees 📏 A number of rules apply when writing to the cgroup.type file: ✍️ Only the string "threaded" may be written. In other words, the only explicit transition that is possible is to convert a domain cgroup to type threaded. 🔄 The effect of writing "threaded" depends on the current value in cgroup.type: domain or domain threaded: start the creation of a threaded subtree (whose root is the parent of this cgroup) via the first of the pathways described above; domain invalid: convert this cgroup (which is inside a threaded subtree) to a usable (i.e., threaded) state; threaded: no effect (a "no-op"). 🚫 We can't write to a cgroup.type file if the parent's type is domain invalid. In other words, the cgroups of a threaded subtree must be converted to the threaded state in a top-down manner. 🛡️ There are also some constraints that must be satisfied in order to create a threaded subtree rooted at the cgroup x: 🚫 There can be no member processes in the descendant cgroups of x. (The cgroup x can itself have member processes.) 🚫 No domain controllers may be enabled in x's cgroup.subtree_control file. ❌ If any of the above constraints is violated, then an attempt to write "threaded" to a cgroup.type file fails with the error ENOTSUP. 🔗 The "domain threaded" cgroup type 🔄 According to the pathways described above, the type of a cgroup can change to domain threaded in either of the following cases: ✍️ The string "threaded" is written to a child cgroup. 🎛️ A threaded controller is enabled inside the cgroup and a process is made a member of the cgroup. ↩️ A domain threaded cgroup, x, can revert to the type domain if the above conditions no longer hold true — that is, if all threaded child cgroups of x are removed and either x no longer has threaded controllers enabled or no longer has member processes. 🔁 When a domain threaded cgroup x reverts to the type domain: ↩️ All domain invalid descendants of x that are not in lower-level threaded subtrees revert to the type domain. 🔗 The root cgroups in any lower-level threaded subtrees revert to the type domain threaded. ⭐ Exceptions for the root cgroup 🌟 The root cgroup of the v2 hierarchy is treated exceptionally: it can be the parent of both domain and threaded cgroups. If the string "threaded" is written to the cgroup.type file of one of the children of the root cgroup, then: 🧵 The type of that cgroup becomes threaded. 🚧 The type of any descendants of that cgroup that are not part of lower-level threaded subtrees changes to domain invalid. 💡 Note that in this case, there is no cgroup whose type becomes domain threaded. (Notionally, the root cgroup can be considered as the threaded root for the cgroup whose type was changed to threaded.) 🎯 The aim of this exceptional treatment for the root cgroup is to allow a threaded cgroup that employs the cpu controller to be placed as high as possible in the hierarchy, so as to minimize the (small) cost of traversing the cgroup hierarchy. ⏱️ The cgroups v2 "cpu" controller and realtime threads ⚠️ As at Linux 4.19, the cgroups v2 cpu controller does not support control of realtime threads (specifically threads scheduled under any of the policies SCHED_FIFO, SCHED_RR, and SCHED_DEADLINE; see sched(7)). Therefore, the cpu controller can be enabled in the root cgroup only if all realtime threads are in the root cgroup. (If there are realtime threads in nonroot cgroups, then a write(2) of the string "+cpu" to the cgroup.subtree_control file fails with the error EINVAL.) 🐉 On some systems, systemd(1) places certain realtime threads in nonroot cgroups in the v2 hierarchy. On such systems, these threads must first be moved to the root cgroup before the cpu controller can be enabled. ❌ ERRORS 🚪 The following errors can occur for mount(2): EBUSY 🔄 An attempt to mount a cgroup version 1 filesystem specified neither the name= option (to mount a named hierarchy) nor a controller name (or all). 📝 NOTES 🌱 A child process created via fork(2) inherits its parent's cgroup memberships. A process's cgroup memberships are preserved across execve(2). 🆕 The clone3(2) CLONE_INTO_CGROUP flag can be used to create a child process that begins its life in a different version 2 cgroup from the parent process. 📂 /proc files 📄 /proc/cgroups (since Linux 2.6.24) — contains information about the controllers that are compiled into the kernel. An example (reformatted for readability): #subsys_name hierarchy num_cgroups enabled cpuset 4 1 1 cpu 8 1 1 cpuacct 8 1 1 blkio 6 1 1 memory 3 1 1 devices 10 84 1 freezer 7 1 1 net_cls 9 1 1 perf_event 5 1 1 net_prio 9 1 1 hugetlb 0 1 0 pids 2 1 1 📑 The fields are, from left to right: 🏷️ The name of the controller. 🔢 The unique ID of the cgroup hierarchy on which this controller is mounted. If multiple cgroups v1 controllers are bound to the same hierarchy, then each will show the same hierarchy ID in this field. The value will be 0 if: the controller is not mounted on a cgroups v1 hierarchy; the controller is bound to the cgroups v2 single unified hierarchy; or the controller is disabled (see below). 📊 The number of control groups in this hierarchy using this controller. 🚦 Contains the value 1 if this controller is enabled, or 0 if it has been disabled (via the cgroup_disable kernel command-line boot parameter). 📄 /proc/[pid]/cgroup (since Linux 2.6.24) — describes control groups to which the process with the corresponding PID belongs. The displayed information differs for cgroups version 1 and version 2 hierarchies. For each cgroup hierarchy of which the process is a member, there is one entry containing three colon-separated fields: hierarchy-ID:controller-list:cgroup-path 📝 For example: 5:cpuacct,cpu,cpuset:/daemons 📑 The colon-separated fields are, from left to right: 🆔 For cgroups version 1 hierarchies, a unique hierarchy ID number that can be matched to a hierarchy ID in /proc/cgroups. For the cgroups version 2 hierarchy, this field contains the value 0. 🎮 For cgroups version 1 hierarchies, a comma-separated list of the controllers bound to the hierarchy. For the cgroups version 2 hierarchy, this field is empty. 📂 The pathname of the control group in the hierarchy to which the process belongs. This pathname is relative to the mount point of the hierarchy. 🗂️ /sys/kernel/cgroup files 📄 /sys/kernel/cgroup/delegate (since Linux 4.15) — exports a list of the cgroups v2 files (one per line) that are delegatable (i.e., whose ownership should be changed to the user ID of the delegatee). In the future, the set of delegatable files may change or grow, and this file provides a way for the kernel to inform user-space applications of which files must be delegated. As at Linux 4.15, one sees the following when inspecting this file: $ cat /sys/kernel/cgroup/delegate cgroup.procs cgroup.subtree_control cgroup.threads 📄 /sys/kernel/cgroup/features (since Linux 4.15) — over time, the set of cgroups v2 features provided by the kernel may change or grow, or some features may not be enabled by default. This file provides a way for user-space applications to discover what features the running kernel supports and has enabled. Features are listed one per line: $ cat /sys/kernel/cgroup/features nsdelegate memory_localevents 📑 The entries that can appear in this file are: memory_localevents (since Linux 5.2) 🧮 The kernel supports the memory_localevents mount option. nsdelegate (since Linux 4.15) 🔐 The kernel supports the nsdelegate mount option. 🔗 SEE ALSO prlimit(1), systemd(1), systemd-cgls(1), systemd-cgtop(1), clone(2), ioprio_set(2), perf_event_open(2), setrlimit(2), cgroup_namespaces(7), cpuset(7), namespaces(7), sched(7), user_namespaces(7) 📚 The kernel source file Documentation/admin-guide/cgroup-v2.rst. 📘 COLOPHON 📄 This page is part of release 5.10 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/. 🐧 Linux 2020-08-13 CGROUPS(7)