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Message-Id: <20080819232235.369649653@us.ibm.com> Date: Tue, 19 Aug 2008 16:22:38 -0700 From: Matt Helsley <matthltc@...ibm.com> To: Andrew Morton <akpm@...ux-foundation.org> Cc: "Rafael J. Wysocki" <rjw@...k.pl>, Paul Menage <menage@...gle.com>, Li Zefan <lizf@...fujitsu.com>, Linux-Kernel <linux-kernel@...r.kernel.org>, Linux Containers <containers@...ts.linux-foundation.org>, linux-pm@...ts.linux-foundation.org Subject: [PATCH 6/6] Container Freezer: Document the cgroup freezer subsystem. Describe why we need the freezer subsystem and how to use it in a documentation file. Since the cgroups.txt file is focused on the subsystem-agnostic portions of cgroups make a directory and move the old cgroups.txt file at the same time. Signed-off-by: Matt Helsley <matthltc@...ibm.com> Cc: Andrew Morton <akpm@...ux-foundation.org> Cc: Paul Menage <menage@...gle.com> Cc: containers@...ts.linux-foundation.org --- Documentation/cgroups.txt | 548 ---------------------------- Documentation/cgroups/cgroups.txt | 548 ++++++++++++++++++++++++++++ Documentation/cgroups/freezer-subsystem.txt | 99 +++++ Documentation/cpusets.txt | 2 4 files changed, 648 insertions(+), 549 deletions(-) Index: linux-2.6.27-rc1-mm1/Documentation/cgroups.txt =================================================================== --- linux-2.6.27-rc1-mm1.orig/Documentation/cgroups.txt +++ /dev/null @@ -1,548 +0,0 @@ - CGROUPS - ------- - -Written by Paul Menage <menage@...gle.com> based on Documentation/cpusets.txt - -Original copyright statements from cpusets.txt: -Portions Copyright (C) 2004 BULL SA. -Portions Copyright (c) 2004-2006 Silicon Graphics, Inc. -Modified by Paul Jackson <pj@....com> -Modified by Christoph Lameter <clameter@....com> - -CONTENTS: -========= - -1. Control Groups - 1.1 What are cgroups ? - 1.2 Why are cgroups needed ? - 1.3 How are cgroups implemented ? - 1.4 What does notify_on_release do ? - 1.5 How do I use cgroups ? -2. Usage Examples and Syntax - 2.1 Basic Usage - 2.2 Attaching processes -3. Kernel API - 3.1 Overview - 3.2 Synchronization - 3.3 Subsystem API -4. Questions - -1. Control Groups -================= - -1.1 What are cgroups ? ----------------------- - -Control Groups provide a mechanism for aggregating/partitioning sets of -tasks, and all their future children, into hierarchical groups with -specialized behaviour. - -Definitions: - -A *cgroup* associates a set of tasks with a set of parameters for one -or more subsystems. - -A *subsystem* is a module that makes use of the task grouping -facilities provided by cgroups to treat groups of tasks in -particular ways. A subsystem is typically a "resource controller" that -schedules a resource or applies per-cgroup limits, but it may be -anything that wants to act on a group of processes, e.g. a -virtualization subsystem. - -A *hierarchy* is a set of cgroups arranged in a tree, such that -every task in the system is in exactly one of the cgroups in the -hierarchy, and a set of subsystems; each subsystem has system-specific -state attached to each cgroup in the hierarchy. Each hierarchy has -an instance of the cgroup virtual filesystem associated with it. - -At any one time there may be multiple active hierachies of task -cgroups. Each hierarchy is a partition of all tasks in the system. - -User level code may create and destroy cgroups by name in an -instance of the cgroup virtual file system, specify and query to -which cgroup a task is assigned, and list the task pids assigned to -a cgroup. Those creations and assignments only affect the hierarchy -associated with that instance of the cgroup file system. - -On their own, the only use for cgroups is for simple job -tracking. The intention is that other subsystems hook into the generic -cgroup support to provide new attributes for cgroups, such as -accounting/limiting the resources which processes in a cgroup can -access. For example, cpusets (see Documentation/cpusets.txt) allows -you to associate a set of CPUs and a set of memory nodes with the -tasks in each cgroup. - -1.2 Why are cgroups needed ? ----------------------------- - -There are multiple efforts to provide process aggregations in the -Linux kernel, mainly for resource tracking purposes. Such efforts -include cpusets, CKRM/ResGroups, UserBeanCounters, and virtual server -namespaces. These all require the basic notion of a -grouping/partitioning of processes, with newly forked processes ending -in the same group (cgroup) as their parent process. - -The kernel cgroup patch provides the minimum essential kernel -mechanisms required to efficiently implement such groups. It has -minimal impact on the system fast paths, and provides hooks for -specific subsystems such as cpusets to provide additional behaviour as -desired. - -Multiple hierarchy support is provided to allow for situations where -the division of tasks into cgroups is distinctly different for -different subsystems - having parallel hierarchies allows each -hierarchy to be a natural division of tasks, without having to handle -complex combinations of tasks that would be present if several -unrelated subsystems needed to be forced into the same tree of -cgroups. - -At one extreme, each resource controller or subsystem could be in a -separate hierarchy; at the other extreme, all subsystems -would be attached to the same hierarchy. - -As an example of a scenario (originally proposed by vatsa@...ibm.com) -that can benefit from multiple hierarchies, consider a large -university server with various users - students, professors, system -tasks etc. The resource planning for this server could be along the -following lines: - - CPU : Top cpuset - / \ - CPUSet1 CPUSet2 - | | - (Profs) (Students) - - In addition (system tasks) are attached to topcpuset (so - that they can run anywhere) with a limit of 20% - - Memory : Professors (50%), students (30%), system (20%) - - Disk : Prof (50%), students (30%), system (20%) - - Network : WWW browsing (20%), Network File System (60%), others (20%) - / \ - Prof (15%) students (5%) - -Browsers like firefox/lynx go into the WWW network class, while (k)nfsd go -into NFS network class. - -At the same time firefox/lynx will share an appropriate CPU/Memory class -depending on who launched it (prof/student). - -With the ability to classify tasks differently for different resources -(by putting those resource subsystems in different hierarchies) then -the admin can easily set up a script which receives exec notifications -and depending on who is launching the browser he can - - # echo browser_pid > /mnt/<restype>/<userclass>/tasks - -With only a single hierarchy, he now would potentially have to create -a separate cgroup for every browser launched and associate it with -approp network and other resource class. This may lead to -proliferation of such cgroups. - -Also lets say that the administrator would like to give enhanced network -access temporarily to a student's browser (since it is night and the user -wants to do online gaming :)) OR give one of the students simulation -apps enhanced CPU power, - -With ability to write pids directly to resource classes, it's just a -matter of : - - # echo pid > /mnt/network/<new_class>/tasks - (after some time) - # echo pid > /mnt/network/<orig_class>/tasks - -Without this ability, he would have to split the cgroup into -multiple separate ones and then associate the new cgroups with the -new resource classes. - - - -1.3 How are cgroups implemented ? ---------------------------------- - -Control Groups extends the kernel as follows: - - - Each task in the system has a reference-counted pointer to a - css_set. - - - A css_set contains a set of reference-counted pointers to - cgroup_subsys_state objects, one for each cgroup subsystem - registered in the system. There is no direct link from a task to - the cgroup of which it's a member in each hierarchy, but this - can be determined by following pointers through the - cgroup_subsys_state objects. This is because accessing the - subsystem state is something that's expected to happen frequently - and in performance-critical code, whereas operations that require a - task's actual cgroup assignments (in particular, moving between - cgroups) are less common. A linked list runs through the cg_list - field of each task_struct using the css_set, anchored at - css_set->tasks. - - - A cgroup hierarchy filesystem can be mounted for browsing and - manipulation from user space. - - - You can list all the tasks (by pid) attached to any cgroup. - -The implementation of cgroups requires a few, simple hooks -into the rest of the kernel, none in performance critical paths: - - - in init/main.c, to initialize the root cgroups and initial - css_set at system boot. - - - in fork and exit, to attach and detach a task from its css_set. - -In addition a new file system, of type "cgroup" may be mounted, to -enable browsing and modifying the cgroups presently known to the -kernel. When mounting a cgroup hierarchy, you may specify a -comma-separated list of subsystems to mount as the filesystem mount -options. By default, mounting the cgroup filesystem attempts to -mount a hierarchy containing all registered subsystems. - -If an active hierarchy with exactly the same set of subsystems already -exists, it will be reused for the new mount. If no existing hierarchy -matches, and any of the requested subsystems are in use in an existing -hierarchy, the mount will fail with -EBUSY. Otherwise, a new hierarchy -is activated, associated with the requested subsystems. - -It's not currently possible to bind a new subsystem to an active -cgroup hierarchy, or to unbind a subsystem from an active cgroup -hierarchy. This may be possible in future, but is fraught with nasty -error-recovery issues. - -When a cgroup filesystem is unmounted, if there are any -child cgroups created below the top-level cgroup, that hierarchy -will remain active even though unmounted; if there are no -child cgroups then the hierarchy will be deactivated. - -No new system calls are added for cgroups - all support for -querying and modifying cgroups is via this cgroup file system. - -Each task under /proc has an added file named 'cgroup' displaying, -for each active hierarchy, the subsystem names and the cgroup name -as the path relative to the root of the cgroup file system. - -Each cgroup is represented by a directory in the cgroup file system -containing the following files describing that cgroup: - - - tasks: list of tasks (by pid) attached to that cgroup - - releasable flag: cgroup currently removeable? - - notify_on_release flag: run the release agent on exit? - - release_agent: the path to use for release notifications (this file - exists in the top cgroup only) - -Other subsystems such as cpusets may add additional files in each -cgroup dir. - -New cgroups are created using the mkdir system call or shell -command. The properties of a cgroup, such as its flags, are -modified by writing to the appropriate file in that cgroups -directory, as listed above. - -The named hierarchical structure of nested cgroups allows partitioning -a large system into nested, dynamically changeable, "soft-partitions". - -The attachment of each task, automatically inherited at fork by any -children of that task, to a cgroup allows organizing the work load -on a system into related sets of tasks. A task may be re-attached to -any other cgroup, if allowed by the permissions on the necessary -cgroup file system directories. - -When a task is moved from one cgroup to another, it gets a new -css_set pointer - if there's an already existing css_set with the -desired collection of cgroups then that group is reused, else a new -css_set is allocated. Note that the current implementation uses a -linear search to locate an appropriate existing css_set, so isn't -very efficient. A future version will use a hash table for better -performance. - -To allow access from a cgroup to the css_sets (and hence tasks) -that comprise it, a set of cg_cgroup_link objects form a lattice; -each cg_cgroup_link is linked into a list of cg_cgroup_links for -a single cgroup on its cgrp_link_list field, and a list of -cg_cgroup_links for a single css_set on its cg_link_list. - -Thus the set of tasks in a cgroup can be listed by iterating over -each css_set that references the cgroup, and sub-iterating over -each css_set's task set. - -The use of a Linux virtual file system (vfs) to represent the -cgroup hierarchy provides for a familiar permission and name space -for cgroups, with a minimum of additional kernel code. - -1.4 What does notify_on_release do ? ------------------------------------- - -If the notify_on_release flag is enabled (1) in a cgroup, then -whenever the last task in the cgroup leaves (exits or attaches to -some other cgroup) and the last child cgroup of that cgroup -is removed, then the kernel runs the command specified by the contents -of the "release_agent" file in that hierarchy's root directory, -supplying the pathname (relative to the mount point of the cgroup -file system) of the abandoned cgroup. This enables automatic -removal of abandoned cgroups. The default value of -notify_on_release in the root cgroup at system boot is disabled -(0). The default value of other cgroups at creation is the current -value of their parents notify_on_release setting. The default value of -a cgroup hierarchy's release_agent path is empty. - -1.5 How do I use cgroups ? --------------------------- - -To start a new job that is to be contained within a cgroup, using -the "cpuset" cgroup subsystem, the steps are something like: - - 1) mkdir /dev/cgroup - 2) mount -t cgroup -ocpuset cpuset /dev/cgroup - 3) Create the new cgroup by doing mkdir's and write's (or echo's) in - the /dev/cgroup virtual file system. - 4) Start a task that will be the "founding father" of the new job. - 5) Attach that task to the new cgroup by writing its pid to the - /dev/cgroup tasks file for that cgroup. - 6) fork, exec or clone the job tasks from this founding father task. - -For example, the following sequence of commands will setup a cgroup -named "Charlie", containing just CPUs 2 and 3, and Memory Node 1, -and then start a subshell 'sh' in that cgroup: - - mount -t cgroup cpuset -ocpuset /dev/cgroup - cd /dev/cgroup - mkdir Charlie - cd Charlie - /bin/echo 2-3 > cpuset.cpus - /bin/echo 1 > cpuset.mems - /bin/echo $$ > tasks - sh - # The subshell 'sh' is now running in cgroup Charlie - # The next line should display '/Charlie' - cat /proc/self/cgroup - -2. Usage Examples and Syntax -============================ - -2.1 Basic Usage ---------------- - -Creating, modifying, using the cgroups can be done through the cgroup -virtual filesystem. - -To mount a cgroup hierarchy will all available subsystems, type: -# mount -t cgroup xxx /dev/cgroup - -The "xxx" is not interpreted by the cgroup code, but will appear in -/proc/mounts so may be any useful identifying string that you like. - -To mount a cgroup hierarchy with just the cpuset and numtasks -subsystems, type: -# mount -t cgroup -o cpuset,numtasks hier1 /dev/cgroup - -To change the set of subsystems bound to a mounted hierarchy, just -remount with different options: - -# mount -o remount,cpuset,ns /dev/cgroup - -Note that changing the set of subsystems is currently only supported -when the hierarchy consists of a single (root) cgroup. Supporting -the ability to arbitrarily bind/unbind subsystems from an existing -cgroup hierarchy is intended to be implemented in the future. - -Then under /dev/cgroup you can find a tree that corresponds to the -tree of the cgroups in the system. For instance, /dev/cgroup -is the cgroup that holds the whole system. - -If you want to create a new cgroup under /dev/cgroup: -# cd /dev/cgroup -# mkdir my_cgroup - -Now you want to do something with this cgroup. -# cd my_cgroup - -In this directory you can find several files: -# ls -notify_on_release releasable tasks -(plus whatever files added by the attached subsystems) - -Now attach your shell to this cgroup: -# /bin/echo $$ > tasks - -You can also create cgroups inside your cgroup by using mkdir in this -directory. -# mkdir my_sub_cs - -To remove a cgroup, just use rmdir: -# rmdir my_sub_cs - -This will fail if the cgroup is in use (has cgroups inside, or -has processes attached, or is held alive by other subsystem-specific -reference). - -2.2 Attaching processes ------------------------ - -# /bin/echo PID > tasks - -Note that it is PID, not PIDs. You can only attach ONE task at a time. -If you have several tasks to attach, you have to do it one after another: - -# /bin/echo PID1 > tasks -# /bin/echo PID2 > tasks - ... -# /bin/echo PIDn > tasks - -You can attach the current shell task by echoing 0: - -# echo 0 > tasks - -3. Kernel API -============= - -3.1 Overview ------------- - -Each kernel subsystem that wants to hook into the generic cgroup -system needs to create a cgroup_subsys object. This contains -various methods, which are callbacks from the cgroup system, along -with a subsystem id which will be assigned by the cgroup system. - -Other fields in the cgroup_subsys object include: - -- subsys_id: a unique array index for the subsystem, indicating which - entry in cgroup->subsys[] this subsystem should be managing. - -- name: should be initialized to a unique subsystem name. Should be - no longer than MAX_CGROUP_TYPE_NAMELEN. - -- early_init: indicate if the subsystem needs early initialization - at system boot. - -Each cgroup object created by the system has an array of pointers, -indexed by subsystem id; this pointer is entirely managed by the -subsystem; the generic cgroup code will never touch this pointer. - -3.2 Synchronization -------------------- - -There is a global mutex, cgroup_mutex, used by the cgroup -system. This should be taken by anything that wants to modify a -cgroup. It may also be taken to prevent cgroups from being -modified, but more specific locks may be more appropriate in that -situation. - -See kernel/cgroup.c for more details. - -Subsystems can take/release the cgroup_mutex via the functions -cgroup_lock()/cgroup_unlock(). - -Accessing a task's cgroup pointer may be done in the following ways: -- while holding cgroup_mutex -- while holding the task's alloc_lock (via task_lock()) -- inside an rcu_read_lock() section via rcu_dereference() - -3.3 Subsystem API ------------------ - -Each subsystem should: - -- add an entry in linux/cgroup_subsys.h -- define a cgroup_subsys object called <name>_subsys - -Each subsystem may export the following methods. The only mandatory -methods are create/destroy. Any others that are null are presumed to -be successful no-ops. - -struct cgroup_subsys_state *create(struct cgroup_subsys *ss, - struct cgroup *cgrp) -(cgroup_mutex held by caller) - -Called to create a subsystem state object for a cgroup. The -subsystem should allocate its subsystem state object for the passed -cgroup, returning a pointer to the new object on success or a -negative error code. On success, the subsystem pointer should point to -a structure of type cgroup_subsys_state (typically embedded in a -larger subsystem-specific object), which will be initialized by the -cgroup system. Note that this will be called at initialization to -create the root subsystem state for this subsystem; this case can be -identified by the passed cgroup object having a NULL parent (since -it's the root of the hierarchy) and may be an appropriate place for -initialization code. - -void destroy(struct cgroup_subsys *ss, struct cgroup *cgrp) -(cgroup_mutex held by caller) - -The cgroup system is about to destroy the passed cgroup; the subsystem -should do any necessary cleanup and free its subsystem state -object. By the time this method is called, the cgroup has already been -unlinked from the file system and from the child list of its parent; -cgroup->parent is still valid. (Note - can also be called for a -newly-created cgroup if an error occurs after this subsystem's -create() method has been called for the new cgroup). - -void pre_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp); -(cgroup_mutex held by caller) - -Called before checking the reference count on each subsystem. This may -be useful for subsystems which have some extra references even if -there are not tasks in the cgroup. - -int can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp, - struct task_struct *task) -(cgroup_mutex held by caller) - -Called prior to moving a task into a cgroup; if the subsystem -returns an error, this will abort the attach operation. If a NULL -task is passed, then a successful result indicates that *any* -unspecified task can be moved into the cgroup. Note that this isn't -called on a fork. If this method returns 0 (success) then this should -remain valid while the caller holds cgroup_mutex. - -void attach(struct cgroup_subsys *ss, struct cgroup *cgrp, - struct cgroup *old_cgrp, struct task_struct *task) - -Called after the task has been attached to the cgroup, to allow any -post-attachment activity that requires memory allocations or blocking. - -void fork(struct cgroup_subsy *ss, struct task_struct *task) - -Called when a task is forked into a cgroup. - -void exit(struct cgroup_subsys *ss, struct task_struct *task) - -Called during task exit. - -int populate(struct cgroup_subsys *ss, struct cgroup *cgrp) - -Called after creation of a cgroup to allow a subsystem to populate -the cgroup directory with file entries. The subsystem should make -calls to cgroup_add_file() with objects of type cftype (see -include/linux/cgroup.h for details). Note that although this -method can return an error code, the error code is currently not -always handled well. - -void post_clone(struct cgroup_subsys *ss, struct cgroup *cgrp) - -Called at the end of cgroup_clone() to do any paramater -initialization which might be required before a task could attach. For -example in cpusets, no task may attach before 'cpus' and 'mems' are set -up. - -void bind(struct cgroup_subsys *ss, struct cgroup *root) -(cgroup_mutex held by caller) - -Called when a cgroup subsystem is rebound to a different hierarchy -and root cgroup. Currently this will only involve movement between -the default hierarchy (which never has sub-cgroups) and a hierarchy -that is being created/destroyed (and hence has no sub-cgroups). - -4. Questions -============ - -Q: what's up with this '/bin/echo' ? -A: bash's builtin 'echo' command does not check calls to write() against - errors. If you use it in the cgroup file system, you won't be - able to tell whether a command succeeded or failed. - -Q: When I attach processes, only the first of the line gets really attached ! -A: We can only return one error code per call to write(). So you should also - put only ONE pid. - Index: linux-2.6.27-rc1-mm1/Documentation/cgroups/cgroups.txt =================================================================== --- /dev/null +++ linux-2.6.27-rc1-mm1/Documentation/cgroups/cgroups.txt @@ -0,0 +1,548 @@ + CGROUPS + ------- + +Written by Paul Menage <menage@...gle.com> based on Documentation/cpusets.txt + +Original copyright statements from cpusets.txt: +Portions Copyright (C) 2004 BULL SA. +Portions Copyright (c) 2004-2006 Silicon Graphics, Inc. +Modified by Paul Jackson <pj@....com> +Modified by Christoph Lameter <clameter@....com> + +CONTENTS: +========= + +1. Control Groups + 1.1 What are cgroups ? + 1.2 Why are cgroups needed ? + 1.3 How are cgroups implemented ? + 1.4 What does notify_on_release do ? + 1.5 How do I use cgroups ? +2. Usage Examples and Syntax + 2.1 Basic Usage + 2.2 Attaching processes +3. Kernel API + 3.1 Overview + 3.2 Synchronization + 3.3 Subsystem API +4. Questions + +1. Control Groups +================= + +1.1 What are cgroups ? +---------------------- + +Control Groups provide a mechanism for aggregating/partitioning sets of +tasks, and all their future children, into hierarchical groups with +specialized behaviour. + +Definitions: + +A *cgroup* associates a set of tasks with a set of parameters for one +or more subsystems. + +A *subsystem* is a module that makes use of the task grouping +facilities provided by cgroups to treat groups of tasks in +particular ways. A subsystem is typically a "resource controller" that +schedules a resource or applies per-cgroup limits, but it may be +anything that wants to act on a group of processes, e.g. a +virtualization subsystem. + +A *hierarchy* is a set of cgroups arranged in a tree, such that +every task in the system is in exactly one of the cgroups in the +hierarchy, and a set of subsystems; each subsystem has system-specific +state attached to each cgroup in the hierarchy. Each hierarchy has +an instance of the cgroup virtual filesystem associated with it. + +At any one time there may be multiple active hierachies of task +cgroups. Each hierarchy is a partition of all tasks in the system. + +User level code may create and destroy cgroups by name in an +instance of the cgroup virtual file system, specify and query to +which cgroup a task is assigned, and list the task pids assigned to +a cgroup. Those creations and assignments only affect the hierarchy +associated with that instance of the cgroup file system. + +On their own, the only use for cgroups is for simple job +tracking. The intention is that other subsystems hook into the generic +cgroup support to provide new attributes for cgroups, such as +accounting/limiting the resources which processes in a cgroup can +access. For example, cpusets (see Documentation/cpusets.txt) allows +you to associate a set of CPUs and a set of memory nodes with the +tasks in each cgroup. + +1.2 Why are cgroups needed ? +---------------------------- + +There are multiple efforts to provide process aggregations in the +Linux kernel, mainly for resource tracking purposes. Such efforts +include cpusets, CKRM/ResGroups, UserBeanCounters, and virtual server +namespaces. These all require the basic notion of a +grouping/partitioning of processes, with newly forked processes ending +in the same group (cgroup) as their parent process. + +The kernel cgroup patch provides the minimum essential kernel +mechanisms required to efficiently implement such groups. It has +minimal impact on the system fast paths, and provides hooks for +specific subsystems such as cpusets to provide additional behaviour as +desired. + +Multiple hierarchy support is provided to allow for situations where +the division of tasks into cgroups is distinctly different for +different subsystems - having parallel hierarchies allows each +hierarchy to be a natural division of tasks, without having to handle +complex combinations of tasks that would be present if several +unrelated subsystems needed to be forced into the same tree of +cgroups. + +At one extreme, each resource controller or subsystem could be in a +separate hierarchy; at the other extreme, all subsystems +would be attached to the same hierarchy. + +As an example of a scenario (originally proposed by vatsa@...ibm.com) +that can benefit from multiple hierarchies, consider a large +university server with various users - students, professors, system +tasks etc. The resource planning for this server could be along the +following lines: + + CPU : Top cpuset + / \ + CPUSet1 CPUSet2 + | | + (Profs) (Students) + + In addition (system tasks) are attached to topcpuset (so + that they can run anywhere) with a limit of 20% + + Memory : Professors (50%), students (30%), system (20%) + + Disk : Prof (50%), students (30%), system (20%) + + Network : WWW browsing (20%), Network File System (60%), others (20%) + / \ + Prof (15%) students (5%) + +Browsers like firefox/lynx go into the WWW network class, while (k)nfsd go +into NFS network class. + +At the same time firefox/lynx will share an appropriate CPU/Memory class +depending on who launched it (prof/student). + +With the ability to classify tasks differently for different resources +(by putting those resource subsystems in different hierarchies) then +the admin can easily set up a script which receives exec notifications +and depending on who is launching the browser he can + + # echo browser_pid > /mnt/<restype>/<userclass>/tasks + +With only a single hierarchy, he now would potentially have to create +a separate cgroup for every browser launched and associate it with +approp network and other resource class. This may lead to +proliferation of such cgroups. + +Also lets say that the administrator would like to give enhanced network +access temporarily to a student's browser (since it is night and the user +wants to do online gaming :)) OR give one of the students simulation +apps enhanced CPU power, + +With ability to write pids directly to resource classes, it's just a +matter of : + + # echo pid > /mnt/network/<new_class>/tasks + (after some time) + # echo pid > /mnt/network/<orig_class>/tasks + +Without this ability, he would have to split the cgroup into +multiple separate ones and then associate the new cgroups with the +new resource classes. + + + +1.3 How are cgroups implemented ? +--------------------------------- + +Control Groups extends the kernel as follows: + + - Each task in the system has a reference-counted pointer to a + css_set. + + - A css_set contains a set of reference-counted pointers to + cgroup_subsys_state objects, one for each cgroup subsystem + registered in the system. There is no direct link from a task to + the cgroup of which it's a member in each hierarchy, but this + can be determined by following pointers through the + cgroup_subsys_state objects. This is because accessing the + subsystem state is something that's expected to happen frequently + and in performance-critical code, whereas operations that require a + task's actual cgroup assignments (in particular, moving between + cgroups) are less common. A linked list runs through the cg_list + field of each task_struct using the css_set, anchored at + css_set->tasks. + + - A cgroup hierarchy filesystem can be mounted for browsing and + manipulation from user space. + + - You can list all the tasks (by pid) attached to any cgroup. + +The implementation of cgroups requires a few, simple hooks +into the rest of the kernel, none in performance critical paths: + + - in init/main.c, to initialize the root cgroups and initial + css_set at system boot. + + - in fork and exit, to attach and detach a task from its css_set. + +In addition a new file system, of type "cgroup" may be mounted, to +enable browsing and modifying the cgroups presently known to the +kernel. When mounting a cgroup hierarchy, you may specify a +comma-separated list of subsystems to mount as the filesystem mount +options. By default, mounting the cgroup filesystem attempts to +mount a hierarchy containing all registered subsystems. + +If an active hierarchy with exactly the same set of subsystems already +exists, it will be reused for the new mount. If no existing hierarchy +matches, and any of the requested subsystems are in use in an existing +hierarchy, the mount will fail with -EBUSY. Otherwise, a new hierarchy +is activated, associated with the requested subsystems. + +It's not currently possible to bind a new subsystem to an active +cgroup hierarchy, or to unbind a subsystem from an active cgroup +hierarchy. This may be possible in future, but is fraught with nasty +error-recovery issues. + +When a cgroup filesystem is unmounted, if there are any +child cgroups created below the top-level cgroup, that hierarchy +will remain active even though unmounted; if there are no +child cgroups then the hierarchy will be deactivated. + +No new system calls are added for cgroups - all support for +querying and modifying cgroups is via this cgroup file system. + +Each task under /proc has an added file named 'cgroup' displaying, +for each active hierarchy, the subsystem names and the cgroup name +as the path relative to the root of the cgroup file system. + +Each cgroup is represented by a directory in the cgroup file system +containing the following files describing that cgroup: + + - tasks: list of tasks (by pid) attached to that cgroup + - releasable flag: cgroup currently removeable? + - notify_on_release flag: run the release agent on exit? + - release_agent: the path to use for release notifications (this file + exists in the top cgroup only) + +Other subsystems such as cpusets may add additional files in each +cgroup dir. + +New cgroups are created using the mkdir system call or shell +command. The properties of a cgroup, such as its flags, are +modified by writing to the appropriate file in that cgroups +directory, as listed above. + +The named hierarchical structure of nested cgroups allows partitioning +a large system into nested, dynamically changeable, "soft-partitions". + +The attachment of each task, automatically inherited at fork by any +children of that task, to a cgroup allows organizing the work load +on a system into related sets of tasks. A task may be re-attached to +any other cgroup, if allowed by the permissions on the necessary +cgroup file system directories. + +When a task is moved from one cgroup to another, it gets a new +css_set pointer - if there's an already existing css_set with the +desired collection of cgroups then that group is reused, else a new +css_set is allocated. Note that the current implementation uses a +linear search to locate an appropriate existing css_set, so isn't +very efficient. A future version will use a hash table for better +performance. + +To allow access from a cgroup to the css_sets (and hence tasks) +that comprise it, a set of cg_cgroup_link objects form a lattice; +each cg_cgroup_link is linked into a list of cg_cgroup_links for +a single cgroup on its cgrp_link_list field, and a list of +cg_cgroup_links for a single css_set on its cg_link_list. + +Thus the set of tasks in a cgroup can be listed by iterating over +each css_set that references the cgroup, and sub-iterating over +each css_set's task set. + +The use of a Linux virtual file system (vfs) to represent the +cgroup hierarchy provides for a familiar permission and name space +for cgroups, with a minimum of additional kernel code. + +1.4 What does notify_on_release do ? +------------------------------------ + +If the notify_on_release flag is enabled (1) in a cgroup, then +whenever the last task in the cgroup leaves (exits or attaches to +some other cgroup) and the last child cgroup of that cgroup +is removed, then the kernel runs the command specified by the contents +of the "release_agent" file in that hierarchy's root directory, +supplying the pathname (relative to the mount point of the cgroup +file system) of the abandoned cgroup. This enables automatic +removal of abandoned cgroups. The default value of +notify_on_release in the root cgroup at system boot is disabled +(0). The default value of other cgroups at creation is the current +value of their parents notify_on_release setting. The default value of +a cgroup hierarchy's release_agent path is empty. + +1.5 How do I use cgroups ? +-------------------------- + +To start a new job that is to be contained within a cgroup, using +the "cpuset" cgroup subsystem, the steps are something like: + + 1) mkdir /dev/cgroup + 2) mount -t cgroup -ocpuset cpuset /dev/cgroup + 3) Create the new cgroup by doing mkdir's and write's (or echo's) in + the /dev/cgroup virtual file system. + 4) Start a task that will be the "founding father" of the new job. + 5) Attach that task to the new cgroup by writing its pid to the + /dev/cgroup tasks file for that cgroup. + 6) fork, exec or clone the job tasks from this founding father task. + +For example, the following sequence of commands will setup a cgroup +named "Charlie", containing just CPUs 2 and 3, and Memory Node 1, +and then start a subshell 'sh' in that cgroup: + + mount -t cgroup cpuset -ocpuset /dev/cgroup + cd /dev/cgroup + mkdir Charlie + cd Charlie + /bin/echo 2-3 > cpuset.cpus + /bin/echo 1 > cpuset.mems + /bin/echo $$ > tasks + sh + # The subshell 'sh' is now running in cgroup Charlie + # The next line should display '/Charlie' + cat /proc/self/cgroup + +2. Usage Examples and Syntax +============================ + +2.1 Basic Usage +--------------- + +Creating, modifying, using the cgroups can be done through the cgroup +virtual filesystem. + +To mount a cgroup hierarchy will all available subsystems, type: +# mount -t cgroup xxx /dev/cgroup + +The "xxx" is not interpreted by the cgroup code, but will appear in +/proc/mounts so may be any useful identifying string that you like. + +To mount a cgroup hierarchy with just the cpuset and numtasks +subsystems, type: +# mount -t cgroup -o cpuset,numtasks hier1 /dev/cgroup + +To change the set of subsystems bound to a mounted hierarchy, just +remount with different options: + +# mount -o remount,cpuset,ns /dev/cgroup + +Note that changing the set of subsystems is currently only supported +when the hierarchy consists of a single (root) cgroup. Supporting +the ability to arbitrarily bind/unbind subsystems from an existing +cgroup hierarchy is intended to be implemented in the future. + +Then under /dev/cgroup you can find a tree that corresponds to the +tree of the cgroups in the system. For instance, /dev/cgroup +is the cgroup that holds the whole system. + +If you want to create a new cgroup under /dev/cgroup: +# cd /dev/cgroup +# mkdir my_cgroup + +Now you want to do something with this cgroup. +# cd my_cgroup + +In this directory you can find several files: +# ls +notify_on_release releasable tasks +(plus whatever files added by the attached subsystems) + +Now attach your shell to this cgroup: +# /bin/echo $$ > tasks + +You can also create cgroups inside your cgroup by using mkdir in this +directory. +# mkdir my_sub_cs + +To remove a cgroup, just use rmdir: +# rmdir my_sub_cs + +This will fail if the cgroup is in use (has cgroups inside, or +has processes attached, or is held alive by other subsystem-specific +reference). + +2.2 Attaching processes +----------------------- + +# /bin/echo PID > tasks + +Note that it is PID, not PIDs. You can only attach ONE task at a time. +If you have several tasks to attach, you have to do it one after another: + +# /bin/echo PID1 > tasks +# /bin/echo PID2 > tasks + ... +# /bin/echo PIDn > tasks + +You can attach the current shell task by echoing 0: + +# echo 0 > tasks + +3. Kernel API +============= + +3.1 Overview +------------ + +Each kernel subsystem that wants to hook into the generic cgroup +system needs to create a cgroup_subsys object. This contains +various methods, which are callbacks from the cgroup system, along +with a subsystem id which will be assigned by the cgroup system. + +Other fields in the cgroup_subsys object include: + +- subsys_id: a unique array index for the subsystem, indicating which + entry in cgroup->subsys[] this subsystem should be managing. + +- name: should be initialized to a unique subsystem name. Should be + no longer than MAX_CGROUP_TYPE_NAMELEN. + +- early_init: indicate if the subsystem needs early initialization + at system boot. + +Each cgroup object created by the system has an array of pointers, +indexed by subsystem id; this pointer is entirely managed by the +subsystem; the generic cgroup code will never touch this pointer. + +3.2 Synchronization +------------------- + +There is a global mutex, cgroup_mutex, used by the cgroup +system. This should be taken by anything that wants to modify a +cgroup. It may also be taken to prevent cgroups from being +modified, but more specific locks may be more appropriate in that +situation. + +See kernel/cgroup.c for more details. + +Subsystems can take/release the cgroup_mutex via the functions +cgroup_lock()/cgroup_unlock(). + +Accessing a task's cgroup pointer may be done in the following ways: +- while holding cgroup_mutex +- while holding the task's alloc_lock (via task_lock()) +- inside an rcu_read_lock() section via rcu_dereference() + +3.3 Subsystem API +----------------- + +Each subsystem should: + +- add an entry in linux/cgroup_subsys.h +- define a cgroup_subsys object called <name>_subsys + +Each subsystem may export the following methods. The only mandatory +methods are create/destroy. Any others that are null are presumed to +be successful no-ops. + +struct cgroup_subsys_state *create(struct cgroup_subsys *ss, + struct cgroup *cgrp) +(cgroup_mutex held by caller) + +Called to create a subsystem state object for a cgroup. The +subsystem should allocate its subsystem state object for the passed +cgroup, returning a pointer to the new object on success or a +negative error code. On success, the subsystem pointer should point to +a structure of type cgroup_subsys_state (typically embedded in a +larger subsystem-specific object), which will be initialized by the +cgroup system. Note that this will be called at initialization to +create the root subsystem state for this subsystem; this case can be +identified by the passed cgroup object having a NULL parent (since +it's the root of the hierarchy) and may be an appropriate place for +initialization code. + +void destroy(struct cgroup_subsys *ss, struct cgroup *cgrp) +(cgroup_mutex held by caller) + +The cgroup system is about to destroy the passed cgroup; the subsystem +should do any necessary cleanup and free its subsystem state +object. By the time this method is called, the cgroup has already been +unlinked from the file system and from the child list of its parent; +cgroup->parent is still valid. (Note - can also be called for a +newly-created cgroup if an error occurs after this subsystem's +create() method has been called for the new cgroup). + +void pre_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp); +(cgroup_mutex held by caller) + +Called before checking the reference count on each subsystem. This may +be useful for subsystems which have some extra references even if +there are not tasks in the cgroup. + +int can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp, + struct task_struct *task) +(cgroup_mutex held by caller) + +Called prior to moving a task into a cgroup; if the subsystem +returns an error, this will abort the attach operation. If a NULL +task is passed, then a successful result indicates that *any* +unspecified task can be moved into the cgroup. Note that this isn't +called on a fork. If this method returns 0 (success) then this should +remain valid while the caller holds cgroup_mutex. + +void attach(struct cgroup_subsys *ss, struct cgroup *cgrp, + struct cgroup *old_cgrp, struct task_struct *task) + +Called after the task has been attached to the cgroup, to allow any +post-attachment activity that requires memory allocations or blocking. + +void fork(struct cgroup_subsy *ss, struct task_struct *task) + +Called when a task is forked into a cgroup. + +void exit(struct cgroup_subsys *ss, struct task_struct *task) + +Called during task exit. + +int populate(struct cgroup_subsys *ss, struct cgroup *cgrp) + +Called after creation of a cgroup to allow a subsystem to populate +the cgroup directory with file entries. The subsystem should make +calls to cgroup_add_file() with objects of type cftype (see +include/linux/cgroup.h for details). Note that although this +method can return an error code, the error code is currently not +always handled well. + +void post_clone(struct cgroup_subsys *ss, struct cgroup *cgrp) + +Called at the end of cgroup_clone() to do any paramater +initialization which might be required before a task could attach. For +example in cpusets, no task may attach before 'cpus' and 'mems' are set +up. + +void bind(struct cgroup_subsys *ss, struct cgroup *root) +(cgroup_mutex held by caller) + +Called when a cgroup subsystem is rebound to a different hierarchy +and root cgroup. Currently this will only involve movement between +the default hierarchy (which never has sub-cgroups) and a hierarchy +that is being created/destroyed (and hence has no sub-cgroups). + +4. Questions +============ + +Q: what's up with this '/bin/echo' ? +A: bash's builtin 'echo' command does not check calls to write() against + errors. If you use it in the cgroup file system, you won't be + able to tell whether a command succeeded or failed. + +Q: When I attach processes, only the first of the line gets really attached ! +A: We can only return one error code per call to write(). So you should also + put only ONE pid. + Index: linux-2.6.27-rc1-mm1/Documentation/cgroups/freezer-subsystem.txt =================================================================== --- /dev/null +++ linux-2.6.27-rc1-mm1/Documentation/cgroups/freezer-subsystem.txt @@ -0,0 +1,99 @@ + The cgroup freezer is useful to batch job management system which start +and stop sets of tasks in order to schedule the resources of a machine +according to the desires of a system administrator. This sort of program +is often used on HPC clusters to schedule access to the cluster as a +whole. The cgroup freezer uses cgroups to describe the set of tasks to +be started/stopped by the batch job management system. It also provides +a means to start and stop the tasks composing the job. + + The cgroup freezer will also be useful for checkpointing running groups +of tasks. The freezer allows the checkpoint code to obtain a consistent +image of the tasks by attempting to force the tasks in a cgroup into a +quiescent state. Once the tasks are quiescent another task can +walk /proc or invoke a kernel interface to gather information about the +quiesced tasks. Checkpointed tasks can be restarted later should a +recoverable error occur. This also allows the checkpointed tasks to be +migrated between nodes in a cluster by copying the gathered information +to another node and restarting the tasks there. + + Sequences of SIGSTOP and SIGCONT are not always sufficient for stopping +and resuming tasks in userspace. Both of these signals are observable +from within the tasks we wish to freeze. While SIGSTOP cannot be caught, +blocked, or ignored it can be seen by waiting or ptracing parent tasks. +SIGCONT is especially unsuitable since it can be caught by the task. Any +programs designed to watch for SIGSTOP and SIGCONT could be broken by +attempting to use SIGSTOP and SIGCONT to stop and resume tasks. We can +demonstrate this problem using nested bash shells: + + $ echo $$ + 16644 + $ bash + $ echo $$ + 16690 + + From a second, unrelated bash shell: + $ kill -SIGSTOP 16690 + $ kill -SIGCONT 16990 + + <at this point 16990 exits and causes 16644 to exit too> + + This happens because bash can observe both signals and choose how it +responds to them. + + Another example of a program which catches and responds to these +signals is gdb. In fact any program designed to use ptrace is likely to +have a problem with this method of stopping and resuming tasks. + + In contrast, the cgroup freezer uses the kernel freezer code to +prevent the freeze/unfreeze cycle from becoming visible to the tasks +being frozen. This allows the bash example above and gdb to run as +expected. + + The freezer subsystem in the container filesystem defines a file named +freezer.state. Writing "FROZEN" to the state file will freeze all tasks in the +cgroup. Subsequently writing "THAWED" will unfreeze the tasks in the cgroup. +Reading will return the current state. + +* Examples of usage : + + # mkdir /containers/freezer + # mount -t cgroup -ofreezer freezer /containers + # mkdir /containers/0 + # echo $some_pid > /containers/0/tasks + +to get status of the freezer subsystem : + + # cat /containers/0/freezer.state + THAWED + +to freeze all tasks in the container : + + # echo FROZEN > /containers/0/freezer.state + # cat /containers/0/freezer.state + FREEZING + # cat /containers/0/freezer.state + FROZEN + +to unfreeze all tasks in the container : + + # echo THAWED > /containers/0/freezer.state + # cat /containers/0/freezer.state + THAWED + +This is the basic mechanism which should do the right thing for user space task +in a simple scenario. + +It's important to note that freezing can be incomplete. In that case we return +EBUSY. This means that some tasks in the cgroup are busy doing something that +prevents us from completely freezing the cgroup at this time. After EBUSY, +the cgroup will remain partially frozen -- reflected by freezer.state reporting +"FREEZING" when read. The state will remain "FREEZING" until one of these +things happens: + + 1) Userspace cancels the freezing operation by writing "THAWED" to + the freezer.state file + 2) Userspace retries the freezing operation by writing "FROZEN" to + the freezer.state file (writing "FREEZING" is not legal + and returns EIO) + 3) The tasks that blocked the cgroup from entering the "FROZEN" + state disappear from the cgroup's set of tasks. Index: linux-2.6.27-rc1-mm1/Documentation/cpusets.txt =================================================================== --- linux-2.6.27-rc1-mm1.orig/Documentation/cpusets.txt +++ linux-2.6.27-rc1-mm1/Documentation/cpusets.txt @@ -48,7 +48,7 @@ hooks, beyond what is already present, r job placement on large systems. Cpusets use the generic cgroup subsystem described in -Documentation/cgroup.txt. +Documentation/cgroups/cgroups.txt. Requests by a task, using the sched_setaffinity(2) system call to include CPUs in its CPU affinity mask, and using the mbind(2) and -- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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