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Unified hierarchy will be the new version of cgroup interface. This patch adds Documentation/cgroups/unified-hierarchy.txt which describes the design and rationales of unified hierarchy. v2: Grammatical updates as per Randy Dunlap's review. Signed-off-by: Tejun Heo <tj@kernel.org> Cc: Randy Dunlap <rdunlap@infradead.org>
360 lines
15 KiB
Plaintext
360 lines
15 KiB
Plaintext
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Cgroup unified hierarchy
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April, 2014 Tejun Heo <tj@kernel.org>
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This document describes the changes made by unified hierarchy and
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their rationales. It will eventually be merged into the main cgroup
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documentation.
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CONTENTS
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1. Background
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2. Basic Operation
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2-1. Mounting
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2-2. cgroup.subtree_control
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2-3. cgroup.controllers
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3. Structural Constraints
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3-1. Top-down
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3-2. No internal tasks
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4. Other Changes
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4-1. [Un]populated Notification
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4-2. Other Core Changes
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4-3. Per-Controller Changes
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4-3-1. blkio
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4-3-2. cpuset
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4-3-3. memory
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5. Planned Changes
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5-1. CAP for resource control
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1. Background
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cgroup allows an arbitrary number of hierarchies and each hierarchy
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can host any number of controllers. While this seems to provide a
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high level of flexibility, it isn't quite useful in practice.
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For example, as there is only one instance of each controller, utility
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type controllers such as freezer which can be useful in all
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hierarchies can only be used in one. The issue is exacerbated by the
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fact that controllers can't be moved around once hierarchies are
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populated. Another issue is that all controllers bound to a hierarchy
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are forced to have exactly the same view of the hierarchy. It isn't
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possible to vary the granularity depending on the specific controller.
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In practice, these issues heavily limit which controllers can be put
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on the same hierarchy and most configurations resort to putting each
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controller on its own hierarchy. Only closely related ones, such as
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the cpu and cpuacct controllers, make sense to put on the same
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hierarchy. This often means that userland ends up managing multiple
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similar hierarchies repeating the same steps on each hierarchy
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whenever a hierarchy management operation is necessary.
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Unfortunately, support for multiple hierarchies comes at a steep cost.
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Internal implementation in cgroup core proper is dazzlingly
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complicated but more importantly the support for multiple hierarchies
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restricts how cgroup is used in general and what controllers can do.
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There's no limit on how many hierarchies there may be, which means
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that a task's cgroup membership can't be described in finite length.
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The key may contain any varying number of entries and is unlimited in
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length, which makes it highly awkward to handle and leads to addition
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of controllers which exist only to identify membership, which in turn
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exacerbates the original problem.
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Also, as a controller can't have any expectation regarding what shape
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of hierarchies other controllers would be on, each controller has to
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assume that all other controllers are operating on completely
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orthogonal hierarchies. This makes it impossible, or at least very
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cumbersome, for controllers to cooperate with each other.
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In most use cases, putting controllers on hierarchies which are
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completely orthogonal to each other isn't necessary. What usually is
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called for is the ability to have differing levels of granularity
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depending on the specific controller. In other words, hierarchy may
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be collapsed from leaf towards root when viewed from specific
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controllers. For example, a given configuration might not care about
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how memory is distributed beyond a certain level while still wanting
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to control how CPU cycles are distributed.
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Unified hierarchy is the next version of cgroup interface. It aims to
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address the aforementioned issues by having more structure while
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retaining enough flexibility for most use cases. Various other
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general and controller-specific interface issues are also addressed in
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the process.
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2. Basic Operation
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2-1. Mounting
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Currently, unified hierarchy can be mounted with the following mount
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command. Note that this is still under development and scheduled to
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change soon.
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mount -t cgroup -o __DEVEL__sane_behavior cgroup $MOUNT_POINT
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All controllers which are not bound to other hierarchies are
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automatically bound to unified hierarchy and show up at the root of
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it. Controllers which are enabled only in the root of unified
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hierarchy can be bound to other hierarchies at any time. This allows
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mixing unified hierarchy with the traditional multiple hierarchies in
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a fully backward compatible way.
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2-2. cgroup.subtree_control
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All cgroups on unified hierarchy have a "cgroup.subtree_control" file
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which governs which controllers are enabled on the children of the
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cgroup. Let's assume a hierarchy like the following.
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root - A - B - C
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\ D
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root's "cgroup.subtree_control" file determines which controllers are
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enabled on A. A's on B. B's on C and D. This coincides with the
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fact that controllers on the immediate sub-level are used to
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distribute the resources of the parent. In fact, it's natural to
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assume that resource control knobs of a child belong to its parent.
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Enabling a controller in a "cgroup.subtree_control" file declares that
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distribution of the respective resources of the cgroup will be
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controlled. Note that this means that controller enable states are
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shared among siblings.
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When read, the file contains a space-separated list of currently
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enabled controllers. A write to the file should contain a
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space-separated list of controllers with '+' or '-' prefixed (without
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the quotes). Controllers prefixed with '+' are enabled and '-'
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disabled. If a controller is listed multiple times, the last entry
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wins. The specific operations are executed atomically - either all
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succeed or fail.
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2-3. cgroup.controllers
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Read-only "cgroup.controllers" file contains a space-separated list of
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controllers which can be enabled in the cgroup's
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"cgroup.subtree_control" file.
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In the root cgroup, this lists controllers which are not bound to
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other hierarchies and the content changes as controllers are bound to
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and unbound from other hierarchies.
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In non-root cgroups, the content of this file equals that of the
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parent's "cgroup.subtree_control" file as only controllers enabled
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from the parent can be used in its children.
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3. Structural Constraints
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3-1. Top-down
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As it doesn't make sense to nest control of an uncontrolled resource,
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all non-root "cgroup.subtree_control" files can only contain
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controllers which are enabled in the parent's "cgroup.subtree_control"
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file. A controller can be enabled only if the parent has the
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controller enabled and a controller can't be disabled if one or more
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children have it enabled.
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3-2. No internal tasks
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One long-standing issue that cgroup faces is the competition between
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tasks belonging to the parent cgroup and its children cgroups. This
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is inherently nasty as two different types of entities compete and
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there is no agreed-upon obvious way to handle it. Different
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controllers are doing different things.
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The cpu controller considers tasks and cgroups as equivalents and maps
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nice levels to cgroup weights. This works for some cases but falls
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flat when children should be allocated specific ratios of CPU cycles
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and the number of internal tasks fluctuates - the ratios constantly
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change as the number of competing entities fluctuates. There also are
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other issues. The mapping from nice level to weight isn't obvious or
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universal, and there are various other knobs which simply aren't
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available for tasks.
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The blkio controller implicitly creates a hidden leaf node for each
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cgroup to host the tasks. The hidden leaf has its own copies of all
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the knobs with "leaf_" prefixed. While this allows equivalent control
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over internal tasks, it's with serious drawbacks. It always adds an
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extra layer of nesting which may not be necessary, makes the interface
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messy and significantly complicates the implementation.
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The memory controller currently doesn't have a way to control what
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happens between internal tasks and child cgroups and the behavior is
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not clearly defined. There have been attempts to add ad-hoc behaviors
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and knobs to tailor the behavior to specific workloads. Continuing
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this direction will lead to problems which will be extremely difficult
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to resolve in the long term.
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Multiple controllers struggle with internal tasks and came up with
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different ways to deal with it; unfortunately, all the approaches in
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use now are severely flawed and, furthermore, the widely different
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behaviors make cgroup as whole highly inconsistent.
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It is clear that this is something which needs to be addressed from
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cgroup core proper in a uniform way so that controllers don't need to
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worry about it and cgroup as a whole shows a consistent and logical
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behavior. To achieve that, unified hierarchy enforces the following
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structural constraint:
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Except for the root, only cgroups which don't contain any task may
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have controllers enabled in their "cgroup.subtree_control" files.
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Combined with other properties, this guarantees that, when a
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controller is looking at the part of the hierarchy which has it
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enabled, tasks are always only on the leaves. This rules out
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situations where child cgroups compete against internal tasks of the
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parent.
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There are two things to note. Firstly, the root cgroup is exempt from
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the restriction. Root contains tasks and anonymous resource
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consumption which can't be associated with any other cgroup and
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requires special treatment from most controllers. How resource
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consumption in the root cgroup is governed is up to each controller.
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Secondly, the restriction doesn't take effect if there is no enabled
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controller in the cgroup's "cgroup.subtree_control" file. This is
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important as otherwise it wouldn't be possible to create children of a
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populated cgroup. To control resource distribution of a cgroup, the
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cgroup must create children and transfer all its tasks to the children
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before enabling controllers in its "cgroup.subtree_control" file.
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4. Other Changes
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4-1. [Un]populated Notification
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cgroup users often need a way to determine when a cgroup's
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subhierarchy becomes empty so that it can be cleaned up. cgroup
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currently provides release_agent for it; unfortunately, this mechanism
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is riddled with issues.
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- It delivers events by forking and execing a userland binary
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specified as the release_agent. This is a long deprecated method of
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notification delivery. It's extremely heavy, slow and cumbersome to
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integrate with larger infrastructure.
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- There is single monitoring point at the root. There's no way to
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delegate management of a subtree.
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- The event isn't recursive. It triggers when a cgroup doesn't have
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any tasks or child cgroups. Events for internal nodes trigger only
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after all children are removed. This again makes it impossible to
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delegate management of a subtree.
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- Events are filtered from the kernel side. A "notify_on_release"
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file is used to subscribe to or suppress release events. This is
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unnecessarily complicated and probably done this way because event
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delivery itself was expensive.
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Unified hierarchy implements an interface file "cgroup.populated"
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which can be used to monitor whether the cgroup's subhierarchy has
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tasks in it or not. Its value is 0 if there is no task in the cgroup
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and its descendants; otherwise, 1. poll and [id]notify events are
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triggered when the value changes.
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This is significantly lighter and simpler and trivially allows
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delegating management of subhierarchy - subhierarchy monitoring can
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block further propagation simply by putting itself or another process
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in the subhierarchy and monitor events that it's interested in from
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there without interfering with monitoring higher in the tree.
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In unified hierarchy, the release_agent mechanism is no longer
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supported and the interface files "release_agent" and
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"notify_on_release" do not exist.
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4-2. Other Core Changes
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- None of the mount options is allowed.
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- remount is disallowed.
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- rename(2) is disallowed.
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- The "tasks" file is removed. Everything should at process
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granularity. Use the "cgroup.procs" file instead.
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- The "cgroup.procs" file is not sorted. pids will be unique unless
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they got recycled in-between reads.
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- The "cgroup.clone_children" file is removed.
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4-3. Per-Controller Changes
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4-3-1. blkio
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- blk-throttle becomes properly hierarchical.
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4-3-2. cpuset
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- Tasks are kept in empty cpusets after hotplug and take on the masks
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of the nearest non-empty ancestor, instead of being moved to it.
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- A task can be moved into an empty cpuset, and again it takes on the
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masks of the nearest non-empty ancestor.
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4-3-3. memory
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- use_hierarchy is on by default and the cgroup file for the flag is
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not created.
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5. Planned Changes
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5-1. CAP for resource control
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Unified hierarchy will require one of the capabilities(7), which is
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yet to be decided, for all resource control related knobs. Process
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organization operations - creation of sub-cgroups and migration of
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processes in sub-hierarchies may be delegated by changing the
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ownership and/or permissions on the cgroup directory and
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"cgroup.procs" interface file; however, all operations which affect
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resource control - writes to a "cgroup.subtree_control" file or any
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controller-specific knobs - will require an explicit CAP privilege.
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This, in part, is to prevent the cgroup interface from being
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inadvertently promoted to programmable API used by non-privileged
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binaries. cgroup exposes various aspects of the system in ways which
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aren't properly abstracted for direct consumption by regular programs.
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This is an administration interface much closer to sysctl knobs than
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system calls. Even the basic access model, being filesystem path
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based, isn't suitable for direct consumption. There's no way to
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access "my cgroup" in a race-free way or make multiple operations
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atomic against migration to another cgroup.
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Another aspect is that, for better or for worse, the cgroup interface
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goes through far less scrutiny than regular interfaces for
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unprivileged userland. The upside is that cgroup is able to expose
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useful features which may not be suitable for general consumption in a
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reasonable time frame. It provides a relatively short path between
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internal details and userland-visible interface. Of course, this
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shortcut comes with high risk. We go through what we go through for
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general kernel APIs for good reasons. It may end up leaking internal
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details in a way which can exert significant pain by locking the
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kernel into a contract that can't be maintained in a reasonable
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manner.
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Also, due to the specific nature, cgroup and its controllers don't
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tend to attract attention from a wide scope of developers. cgroup's
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short history is already fraught with severely mis-designed
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interfaces, unnecessary commitments to and exposing of internal
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details, broken and dangerous implementations of various features.
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Keeping cgroup as an administration interface is both advantageous for
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its role and imperative given its nature. Some of the cgroup features
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may make sense for unprivileged access. If deemed justified, those
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must be further abstracted and implemented as a different interface,
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be it a system call or process-private filesystem, and survive through
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the scrutiny that any interface for general consumption is required to
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go through.
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Requiring CAP is not a complete solution but should serve as a
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significant deterrent against spraying cgroup usages in non-privileged
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programs.
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