path: root/Documentation/cgroups
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authorLinus Torvalds <torvalds@linux-foundation.org>2014-06-09 15:03:33 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2014-06-09 15:03:33 -0700
commit14208b0ec56919f5333dd654b1a7d10765d0ad05 (patch)
tree474b46c351efced45925d15dc2e0049c49784716 /Documentation/cgroups
parent6ea4fa70e4af0da8b133b246458fb789d8cb3985 (diff)
parentc731ae1d0f02a300697a8b1564780ad28a6c2013 (diff)
Merge branch 'for-3.16' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup
Pull cgroup updates from Tejun Heo: "A lot of activities on cgroup side. Heavy restructuring including locking simplification took place to improve the code base and enable implementation of the unified hierarchy, which currently exists behind a __DEVEL__ mount option. The core support is mostly complete but individual controllers need further work. To explain the design and rationales of the the unified hierarchy Documentation/cgroups/unified-hierarchy.txt is added. Another notable change is css (cgroup_subsys_state - what each controller uses to identify and interact with a cgroup) iteration update. This is part of continuing updates on css object lifetime and visibility. cgroup started with reference count draining on removal way back and is now reaching a point where csses behave and are iterated like normal refcnted objects albeit with some complexities to allow distinguishing the state where they're being deleted. The css iteration update isn't taken advantage of yet but is planned to be used to simplify memcg significantly" * 'for-3.16' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup: (77 commits) cgroup: disallow disabled controllers on the default hierarchy cgroup: don't destroy the default root cgroup: disallow debug controller on the default hierarchy cgroup: clean up MAINTAINERS entries cgroup: implement css_tryget() device_cgroup: use css_has_online_children() instead of has_children() cgroup: convert cgroup_has_live_children() into css_has_online_children() cgroup: use CSS_ONLINE instead of CGRP_DEAD cgroup: iterate cgroup_subsys_states directly cgroup: introduce CSS_RELEASED and reduce css iteration fallback window cgroup: move cgroup->serial_nr into cgroup_subsys_state cgroup: link all cgroup_subsys_states in their sibling lists cgroup: move cgroup->sibling and ->children into cgroup_subsys_state cgroup: remove cgroup->parent device_cgroup: remove direct access to cgroup->children memcg: update memcg_has_children() to use css_next_child() memcg: remove tasks/children test from mem_cgroup_force_empty() cgroup: remove css_parent() cgroup: skip refcnting on normal root csses and cgrp_dfl_root self css cgroup: use cgroup->self.refcnt for cgroup refcnting ...
Diffstat (limited to 'Documentation/cgroups')
2 files changed, 360 insertions, 5 deletions
diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index b3429aec444c..02ab997a1ed2 100644
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -458,15 +458,11 @@ About use_hierarchy, see Section 6.
5.1 force_empty
memory.force_empty interface is provided to make cgroup's memory usage empty.
- You can use this interface only when the cgroup has no tasks.
When writing anything to this
# echo 0 > memory.force_empty
- Almost all pages tracked by this memory cgroup will be unmapped and freed.
- Some pages cannot be freed because they are locked or in-use. Such pages are
- moved to parent (if use_hierarchy==1) or root (if use_hierarchy==0) and this
- cgroup will be empty.
+ the cgroup will be reclaimed and as many pages reclaimed as possible.
The typical use case for this interface is before calling rmdir().
Because rmdir() moves all pages to parent, some out-of-use page caches can be
diff --git a/Documentation/cgroups/unified-hierarchy.txt b/Documentation/cgroups/unified-hierarchy.txt
new file mode 100644
index 000000000000..324b182e6000
--- /dev/null
+++ b/Documentation/cgroups/unified-hierarchy.txt
@@ -0,0 +1,359 @@
+Cgroup unified hierarchy
+April, 2014 Tejun Heo <tj@kernel.org>
+This document describes the changes made by unified hierarchy and
+their rationales. It will eventually be merged into the main cgroup
+1. Background
+2. Basic Operation
+ 2-1. Mounting
+ 2-2. cgroup.subtree_control
+ 2-3. cgroup.controllers
+3. Structural Constraints
+ 3-1. Top-down
+ 3-2. No internal tasks
+4. Other Changes
+ 4-1. [Un]populated Notification
+ 4-2. Other Core Changes
+ 4-3. Per-Controller Changes
+ 4-3-1. blkio
+ 4-3-2. cpuset
+ 4-3-3. memory
+5. Planned Changes
+ 5-1. CAP for resource control
+1. Background
+cgroup allows an arbitrary number of hierarchies and each hierarchy
+can host any number of controllers. While this seems to provide a
+high level of flexibility, it isn't quite useful in practice.
+For example, as there is only one instance of each controller, utility
+type controllers such as freezer which can be useful in all
+hierarchies can only be used in one. The issue is exacerbated by the
+fact that controllers can't be moved around once hierarchies are
+populated. Another issue is that all controllers bound to a hierarchy
+are forced to have exactly the same view of the hierarchy. It isn't
+possible to vary the granularity depending on the specific controller.
+In practice, these issues heavily limit which controllers can be put
+on the same hierarchy and most configurations resort to putting each
+controller on its own hierarchy. Only closely related ones, such as
+the cpu and cpuacct controllers, make sense to put on the same
+hierarchy. This often means that userland ends up managing multiple
+similar hierarchies repeating the same steps on each hierarchy
+whenever a hierarchy management operation is necessary.
+Unfortunately, support for multiple hierarchies comes at a steep cost.
+Internal implementation in cgroup core proper is dazzlingly
+complicated but more importantly the support for multiple hierarchies
+restricts how cgroup is used in general and what controllers can do.
+There's no limit on how many hierarchies there may be, which means
+that a task's cgroup membership can't be described in finite length.
+The key may contain any varying number of entries and is unlimited in
+length, which makes it highly awkward to handle and leads to addition
+of controllers which exist only to identify membership, which in turn
+exacerbates the original problem.
+Also, as a controller can't have any expectation regarding what shape
+of hierarchies other controllers would be on, each controller has to
+assume that all other controllers are operating on completely
+orthogonal hierarchies. This makes it impossible, or at least very
+cumbersome, for controllers to cooperate with each other.
+In most use cases, putting controllers on hierarchies which are
+completely orthogonal to each other isn't necessary. What usually is
+called for is the ability to have differing levels of granularity
+depending on the specific controller. In other words, hierarchy may
+be collapsed from leaf towards root when viewed from specific
+controllers. For example, a given configuration might not care about
+how memory is distributed beyond a certain level while still wanting
+to control how CPU cycles are distributed.
+Unified hierarchy is the next version of cgroup interface. It aims to
+address the aforementioned issues by having more structure while
+retaining enough flexibility for most use cases. Various other
+general and controller-specific interface issues are also addressed in
+the process.
+2. Basic Operation
+2-1. Mounting
+Currently, unified hierarchy can be mounted with the following mount
+command. Note that this is still under development and scheduled to
+change soon.
+ mount -t cgroup -o __DEVEL__sane_behavior cgroup $MOUNT_POINT
+All controllers which are not bound to other hierarchies are
+automatically bound to unified hierarchy and show up at the root of
+it. Controllers which are enabled only in the root of unified
+hierarchy can be bound to other hierarchies at any time. This allows
+mixing unified hierarchy with the traditional multiple hierarchies in
+a fully backward compatible way.
+2-2. cgroup.subtree_control
+All cgroups on unified hierarchy have a "cgroup.subtree_control" file
+which governs which controllers are enabled on the children of the
+cgroup. Let's assume a hierarchy like the following.
+ root - A - B - C
+ \ D
+root's "cgroup.subtree_control" file determines which controllers are
+enabled on A. A's on B. B's on C and D. This coincides with the
+fact that controllers on the immediate sub-level are used to
+distribute the resources of the parent. In fact, it's natural to
+assume that resource control knobs of a child belong to its parent.
+Enabling a controller in a "cgroup.subtree_control" file declares that
+distribution of the respective resources of the cgroup will be
+controlled. Note that this means that controller enable states are
+shared among siblings.
+When read, the file contains a space-separated list of currently
+enabled controllers. A write to the file should contain a
+space-separated list of controllers with '+' or '-' prefixed (without
+the quotes). Controllers prefixed with '+' are enabled and '-'
+disabled. If a controller is listed multiple times, the last entry
+wins. The specific operations are executed atomically - either all
+succeed or fail.
+2-3. cgroup.controllers
+Read-only "cgroup.controllers" file contains a space-separated list of
+controllers which can be enabled in the cgroup's
+"cgroup.subtree_control" file.
+In the root cgroup, this lists controllers which are not bound to
+other hierarchies and the content changes as controllers are bound to
+and unbound from other hierarchies.
+In non-root cgroups, the content of this file equals that of the
+parent's "cgroup.subtree_control" file as only controllers enabled
+from the parent can be used in its children.
+3. Structural Constraints
+3-1. Top-down
+As it doesn't make sense to nest control of an uncontrolled resource,
+all non-root "cgroup.subtree_control" files can only contain
+controllers which are enabled in the parent's "cgroup.subtree_control"
+file. A controller can be enabled only if the parent has the
+controller enabled and a controller can't be disabled if one or more
+children have it enabled.
+3-2. No internal tasks
+One long-standing issue that cgroup faces is the competition between
+tasks belonging to the parent cgroup and its children cgroups. This
+is inherently nasty as two different types of entities compete and
+there is no agreed-upon obvious way to handle it. Different
+controllers are doing different things.
+The cpu controller considers tasks and cgroups as equivalents and maps
+nice levels to cgroup weights. This works for some cases but falls
+flat when children should be allocated specific ratios of CPU cycles
+and the number of internal tasks fluctuates - the ratios constantly
+change as the number of competing entities fluctuates. There also are
+other issues. The mapping from nice level to weight isn't obvious or
+universal, and there are various other knobs which simply aren't
+available for tasks.
+The blkio controller implicitly creates a hidden leaf node for each
+cgroup to host the tasks. The hidden leaf has its own copies of all
+the knobs with "leaf_" prefixed. While this allows equivalent control
+over internal tasks, it's with serious drawbacks. It always adds an
+extra layer of nesting which may not be necessary, makes the interface
+messy and significantly complicates the implementation.
+The memory controller currently doesn't have a way to control what
+happens between internal tasks and child cgroups and the behavior is
+not clearly defined. There have been attempts to add ad-hoc behaviors
+and knobs to tailor the behavior to specific workloads. Continuing
+this direction will lead to problems which will be extremely difficult
+to resolve in the long term.
+Multiple controllers struggle with internal tasks and came up with
+different ways to deal with it; unfortunately, all the approaches in
+use now are severely flawed and, furthermore, the widely different
+behaviors make cgroup as whole highly inconsistent.
+It is clear that this is something which needs to be addressed from
+cgroup core proper in a uniform way so that controllers don't need to
+worry about it and cgroup as a whole shows a consistent and logical
+behavior. To achieve that, unified hierarchy enforces the following
+structural constraint:
+ Except for the root, only cgroups which don't contain any task may
+ have controllers enabled in their "cgroup.subtree_control" files.
+Combined with other properties, this guarantees that, when a
+controller is looking at the part of the hierarchy which has it
+enabled, tasks are always only on the leaves. This rules out
+situations where child cgroups compete against internal tasks of the
+There are two things to note. Firstly, the root cgroup is exempt from
+the restriction. Root contains tasks and anonymous resource
+consumption which can't be associated with any other cgroup and
+requires special treatment from most controllers. How resource
+consumption in the root cgroup is governed is up to each controller.
+Secondly, the restriction doesn't take effect if there is no enabled
+controller in the cgroup's "cgroup.subtree_control" file. This is
+important as otherwise it wouldn't be possible to create children of a
+populated cgroup. To control resource distribution of a cgroup, the
+cgroup must create children and transfer all its tasks to the children
+before enabling controllers in its "cgroup.subtree_control" file.
+4. Other Changes
+4-1. [Un]populated Notification
+cgroup users often need a way to determine when a cgroup's
+subhierarchy becomes empty so that it can be cleaned up. cgroup
+currently provides release_agent for it; unfortunately, this mechanism
+is riddled with issues.
+- It delivers events by forking and execing a userland binary
+ specified as the release_agent. This is a long deprecated method of
+ notification delivery. It's extremely heavy, slow and cumbersome to
+ integrate with larger infrastructure.
+- There is single monitoring point at the root. There's no way to
+ delegate management of a subtree.
+- The event isn't recursive. It triggers when a cgroup doesn't have
+ any tasks or child cgroups. Events for internal nodes trigger only
+ after all children are removed. This again makes it impossible to
+ delegate management of a subtree.
+- Events are filtered from the kernel side. A "notify_on_release"
+ file is used to subscribe to or suppress release events. This is
+ unnecessarily complicated and probably done this way because event
+ delivery itself was expensive.
+Unified hierarchy implements an interface file "cgroup.populated"
+which can be used to monitor whether the cgroup's subhierarchy has
+tasks in it or not. Its value is 0 if there is no task in the cgroup
+and its descendants; otherwise, 1. poll and [id]notify events are
+triggered when the value changes.
+This is significantly lighter and simpler and trivially allows
+delegating management of subhierarchy - subhierarchy monitoring can
+block further propagation simply by putting itself or another process
+in the subhierarchy and monitor events that it's interested in from
+there without interfering with monitoring higher in the tree.
+In unified hierarchy, the release_agent mechanism is no longer
+supported and the interface files "release_agent" and
+"notify_on_release" do not exist.
+4-2. Other Core Changes
+- None of the mount options is allowed.
+- remount is disallowed.
+- rename(2) is disallowed.
+- The "tasks" file is removed. Everything should at process
+ granularity. Use the "cgroup.procs" file instead.
+- The "cgroup.procs" file is not sorted. pids will be unique unless
+ they got recycled in-between reads.
+- The "cgroup.clone_children" file is removed.
+4-3. Per-Controller Changes
+4-3-1. blkio
+- blk-throttle becomes properly hierarchical.
+4-3-2. cpuset
+- Tasks are kept in empty cpusets after hotplug and take on the masks
+ of the nearest non-empty ancestor, instead of being moved to it.
+- A task can be moved into an empty cpuset, and again it takes on the
+ masks of the nearest non-empty ancestor.
+4-3-3. memory
+- use_hierarchy is on by default and the cgroup file for the flag is
+ not created.
+5. Planned Changes
+5-1. CAP for resource control
+Unified hierarchy will require one of the capabilities(7), which is
+yet to be decided, for all resource control related knobs. Process
+organization operations - creation of sub-cgroups and migration of
+processes in sub-hierarchies may be delegated by changing the
+ownership and/or permissions on the cgroup directory and
+"cgroup.procs" interface file; however, all operations which affect
+resource control - writes to a "cgroup.subtree_control" file or any
+controller-specific knobs - will require an explicit CAP privilege.
+This, in part, is to prevent the cgroup interface from being
+inadvertently promoted to programmable API used by non-privileged
+binaries. cgroup exposes various aspects of the system in ways which
+aren't properly abstracted for direct consumption by regular programs.
+This is an administration interface much closer to sysctl knobs than
+system calls. Even the basic access model, being filesystem path
+based, isn't suitable for direct consumption. There's no way to
+access "my cgroup" in a race-free way or make multiple operations
+atomic against migration to another cgroup.
+Another aspect is that, for better or for worse, the cgroup interface
+goes through far less scrutiny than regular interfaces for
+unprivileged userland. The upside is that cgroup is able to expose
+useful features which may not be suitable for general consumption in a
+reasonable time frame. It provides a relatively short path between
+internal details and userland-visible interface. Of course, this
+shortcut comes with high risk. We go through what we go through for
+general kernel APIs for good reasons. It may end up leaking internal
+details in a way which can exert significant pain by locking the
+kernel into a contract that can't be maintained in a reasonable
+Also, due to the specific nature, cgroup and its controllers don't
+tend to attract attention from a wide scope of developers. cgroup's
+short history is already fraught with severely mis-designed
+interfaces, unnecessary commitments to and exposing of internal
+details, broken and dangerous implementations of various features.
+Keeping cgroup as an administration interface is both advantageous for
+its role and imperative given its nature. Some of the cgroup features
+may make sense for unprivileged access. If deemed justified, those
+must be further abstracted and implemented as a different interface,
+be it a system call or process-private filesystem, and survive through
+the scrutiny that any interface for general consumption is required to
+go through.
+Requiring CAP is not a complete solution but should serve as a
+significant deterrent against spraying cgroup usages in non-privileged

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