Documentation/cpusets.txt - kernel/msm-5.4 - Gitiles

 				CPUSETS
 				-------

 Copyright (C) 2004 BULL SA.
 Written by Simon.Derr@bull.net

 Portions Copyright (c) 2004 Silicon Graphics, Inc.
 Modified by Paul Jackson <pj@sgi.com>

 CONTENTS:
 =========

 1. Cpusets
   1.1 What are cpusets ?
   1.2 Why are cpusets needed ?
   1.3 How are cpusets implemented ?
   1.4 How do I use cpusets ?
 2. Usage Examples and Syntax
   2.1 Basic Usage
   2.2 Adding/removing cpus
   2.3 Setting flags
   2.4 Attaching processes
 3. Questions
 4. Contact

 1. Cpusets
 ==========

 1.1 What are cpusets ?
 ----------------------

 Cpusets provide a mechanism for assigning a set of CPUs and Memory
 Nodes to a set of tasks.

 Cpusets constrain the CPU and Memory placement of tasks to only
 the resources within a tasks current cpuset.  They form a nested
 hierarchy visible in a virtual file system.  These are the essential
 hooks, beyond what is already present, required to manage dynamic
 job placement on large systems.

 Each task has a pointer to a cpuset.  Multiple tasks may reference
 the same cpuset.  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 set_mempolicy(2) system calls to include Memory Nodes
 in its memory policy, are both filtered through that tasks cpuset,
 filtering out any CPUs or Memory Nodes not in that cpuset.  The
 scheduler will not schedule a task on a CPU that is not allowed in
 its cpus_allowed vector, and the kernel page allocator will not
 allocate a page on a node that is not allowed in the requesting tasks
 mems_allowed vector.

 If a cpuset is cpu or mem exclusive, no other cpuset, other than a direct
 ancestor or descendent, may share any of the same CPUs or Memory Nodes.

 User level code may create and destroy cpusets by name in the cpuset
 virtual file system, manage the attributes and permissions of these
 cpusets and which CPUs and Memory Nodes are assigned to each cpuset,
 specify and query to which cpuset a task is assigned, and list the
 task pids assigned to a cpuset.


 1.2 Why are cpusets needed ?
 ----------------------------

 The management of large computer systems, with many processors (CPUs),
 complex memory cache hierarchies and multiple Memory Nodes having
 non-uniform access times (NUMA) presents additional challenges for
 the efficient scheduling and memory placement of processes.

 Frequently more modest sized systems can be operated with adequate
 efficiency just by letting the operating system automatically share
 the available CPU and Memory resources amongst the requesting tasks.

 But larger systems, which benefit more from careful processor and
 memory placement to reduce memory access times and contention,
 and which typically represent a larger investment for the customer,
 can benefit from explictly placing jobs on properly sized subsets of
 the system.

 This can be especially valuable on:

     * Web Servers running multiple instances of the same web application,
     * Servers running different applications (for instance, a web server
       and a database), or
     * NUMA systems running large HPC applications with demanding
       performance characteristics.

 These subsets, or "soft partitions" must be able to be dynamically
 adjusted, as the job mix changes, without impacting other concurrently
 executing jobs.

 The kernel cpuset patch provides the minimum essential kernel
 mechanisms required to efficiently implement such subsets.  It
 leverages existing CPU and Memory Placement facilities in the Linux
 kernel to avoid any additional impact on the critical scheduler or
 memory allocator code.


 1.3 How are cpusets implemented ?
 ---------------------------------

 Cpusets provide a Linux kernel (2.6.7 and above) mechanism to constrain
 which CPUs and Memory Nodes are used by a process or set of processes.

 The Linux kernel already has a pair of mechanisms to specify on which
 CPUs a task may be scheduled (sched_setaffinity) and on which Memory
 Nodes it may obtain memory (mbind, set_mempolicy).

 Cpusets extends these two mechanisms as follows:

  - Cpusets are sets of allowed CPUs and Memory Nodes, known to the
    kernel.
  - Each task in the system is attached to a cpuset, via a pointer
    in the task structure to a reference counted cpuset structure.
  - Calls to sched_setaffinity are filtered to just those CPUs
    allowed in that tasks cpuset.
  - Calls to mbind and set_mempolicy are filtered to just
    those Memory Nodes allowed in that tasks cpuset.
  - The root cpuset contains all the systems CPUs and Memory
    Nodes.
  - For any cpuset, one can define child cpusets containing a subset
    of the parents CPU and Memory Node resources.
  - The hierarchy of cpusets can be mounted at /dev/cpuset, for
    browsing and manipulation from user space.
  - A cpuset may be marked exclusive, which ensures that no other
    cpuset (except direct ancestors and descendents) may contain
    any overlapping CPUs or Memory Nodes.
  - You can list all the tasks (by pid) attached to any cpuset.

 The implementation of cpusets requires a few, simple hooks
 into the rest of the kernel, none in performance critical paths:

  - in main/init.c, to initialize the root cpuset at system boot.
  - in fork and exit, to attach and detach a task from its cpuset.
  - in sched_setaffinity, to mask the requested CPUs by what's
    allowed in that tasks cpuset.
  - in sched.c migrate_all_tasks(), to keep migrating tasks within
    the CPUs allowed by their cpuset, if possible.
  - in the mbind and set_mempolicy system calls, to mask the requested
    Memory Nodes by what's allowed in that tasks cpuset.
  - in page_alloc, to restrict memory to allowed nodes.
  - in vmscan.c, to restrict page recovery to the current cpuset.

 In addition a new file system, of type "cpuset" may be mounted,
 typically at /dev/cpuset, to enable browsing and modifying the cpusets
 presently known to the kernel.  No new system calls are added for
 cpusets - all support for querying and modifying cpusets is via
 this cpuset file system.

 Each task under /proc has an added file named 'cpuset', displaying
 the cpuset name, as the path relative to the root of the cpuset file
 system.

 The /proc/<pid>/status file for each task has two added lines,
 displaying the tasks cpus_allowed (on which CPUs it may be scheduled)
 and mems_allowed (on which Memory Nodes it may obtain memory),
 in the format seen in the following example:

   Cpus_allowed:   ffffffff,ffffffff,ffffffff,ffffffff
   Mems_allowed:   ffffffff,ffffffff

 Each cpuset is represented by a directory in the cpuset file system
 containing the following files describing that cpuset:

  - cpus: list of CPUs in that cpuset
  - mems: list of Memory Nodes in that cpuset
  - cpu_exclusive flag: is cpu placement exclusive?
  - mem_exclusive flag: is memory placement exclusive?
  - tasks: list of tasks (by pid) attached to that cpuset

 New cpusets are created using the mkdir system call or shell
 command.  The properties of a cpuset, such as its flags, allowed
 CPUs and Memory Nodes, and attached tasks, are modified by writing
 to the appropriate file in that cpusets directory, as listed above.

 The named hierarchical structure of nested cpusets 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 cpuset allows organizing the work load
 on a system into related sets of tasks such that each set is constrained
 to using the CPUs and Memory Nodes of a particular cpuset.  A task
 may be re-attached to any other cpuset, if allowed by the permissions
 on the necessary cpuset file system directories.

 Such management of a system "in the large" integrates smoothly with
 the detailed placement done on individual tasks and memory regions
 using the sched_setaffinity, mbind and set_mempolicy system calls.

 The following rules apply to each cpuset:

  - Its CPUs and Memory Nodes must be a subset of its parents.
  - It can only be marked exclusive if its parent is.
  - If its cpu or memory is exclusive, they may not overlap any sibling.

 These rules, and the natural hierarchy of cpusets, enable efficient
 enforcement of the exclusive guarantee, without having to scan all
 cpusets every time any of them change to ensure nothing overlaps a
 exclusive cpuset.  Also, the use of a Linux virtual file system (vfs)
 to represent the cpuset hierarchy provides for a familiar permission
 and name space for cpusets, with a minimum of additional kernel code.

 1.4 How do I use cpusets ?
 --------------------------

 In order to minimize the impact of cpusets on critical kernel
 code, such as the scheduler, and due to the fact that the kernel
 does not support one task updating the memory placement of another
 task directly, the impact on a task of changing its cpuset CPU
 or Memory Node placement, or of changing to which cpuset a task
 is attached, is subtle.

 If a cpuset has its Memory Nodes modified, then for each task attached
 to that cpuset, the next time that the kernel attempts to allocate
 a page of memory for that task, the kernel will notice the change
 in the tasks cpuset, and update its per-task memory placement to
 remain within the new cpusets memory placement.  If the task was using
 mempolicy MPOL_BIND, and the nodes to which it was bound overlap with
 its new cpuset, then the task will continue to use whatever subset
 of MPOL_BIND nodes are still allowed in the new cpuset.  If the task
 was using MPOL_BIND and now none of its MPOL_BIND nodes are allowed
 in the new cpuset, then the task will be essentially treated as if it
 was MPOL_BIND bound to the new cpuset (even though its numa placement,
 as queried by get_mempolicy(), doesn't change).  If a task is moved
 from one cpuset to another, then the kernel will adjust the tasks
 memory placement, as above, the next time that the kernel attempts
 to allocate a page of memory for that task.

 If a cpuset has its CPUs modified, then each task using that
 cpuset does _not_ change its behavior automatically.  In order to
 minimize the impact on the critical scheduling code in the kernel,
 tasks will continue to use their prior CPU placement until they
 are rebound to their cpuset, by rewriting their pid to the 'tasks'
 file of their cpuset.  If a task had been bound to some subset of its
 cpuset using the sched_setaffinity() call, and if any of that subset
 is still allowed in its new cpuset settings, then the task will be
 restricted to the intersection of the CPUs it was allowed on before,
 and its new cpuset CPU placement.  If, on the other hand, there is
 no overlap between a tasks prior placement and its new cpuset CPU
 placement, then the task will be allowed to run on any CPU allowed
 in its new cpuset.  If a task is moved from one cpuset to another,
 its CPU placement is updated in the same way as if the tasks pid is
 rewritten to the 'tasks' file of its current cpuset.

 In summary, the memory placement of a task whose cpuset is changed is
 updated by the kernel, on the next allocation of a page for that task,
 but the processor placement is not updated, until that tasks pid is
 rewritten to the 'tasks' file of its cpuset.  This is done to avoid
 impacting the scheduler code in the kernel with a check for changes
 in a tasks processor placement.

 There is an exception to the above.  If hotplug funtionality is used
 to remove all the CPUs that are currently assigned to a cpuset,
 then the kernel will automatically update the cpus_allowed of all
 tasks attached to CPUs in that cpuset to allow all CPUs.  When memory
 hotplug functionality for removing Memory Nodes is available, a
 similar exception is expected to apply there as well.  In general,
 the kernel prefers to violate cpuset placement, over starving a task
 that has had all its allowed CPUs or Memory Nodes taken offline.  User
 code should reconfigure cpusets to only refer to online CPUs and Memory
 Nodes when using hotplug to add or remove such resources.

 There is a second exception to the above.  GFP_ATOMIC requests are
 kernel internal allocations that must be satisfied, immediately.
 The kernel may drop some request, in rare cases even panic, if a
 GFP_ATOMIC alloc fails.  If the request cannot be satisfied within
 the current tasks cpuset, then we relax the cpuset, and look for
 memory anywhere we can find it.  It's better to violate the cpuset
 than stress the kernel.

 To start a new job that is to be contained within a cpuset, the steps are:

  1) mkdir /dev/cpuset
  2) mount -t cpuset none /dev/cpuset
  3) Create the new cpuset by doing mkdir's and write's (or echo's) in
     the /dev/cpuset virtual file system.
  4) Start a task that will be the "founding father" of the new job.
  5) Attach that task to the new cpuset by writing its pid to the
     /dev/cpuset tasks file for that cpuset.
  6) fork, exec or clone the job tasks from this founding father task.

 For example, the following sequence of commands will setup a cpuset
 named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
 and then start a subshell 'sh' in that cpuset:

   mount -t cpuset none /dev/cpuset
   cd /dev/cpuset
   mkdir Charlie
   cd Charlie
   /bin/echo 2-3 > cpus
   /bin/echo 1 > mems
   /bin/echo $$ > tasks
   sh
   # The subshell 'sh' is now running in cpuset Charlie
   # The next line should display '/Charlie'
   cat /proc/self/cpuset

 In the case that a change of cpuset includes wanting to move already
 allocated memory pages, consider further the work of IWAMOTO
 Toshihiro <iwamoto@valinux.co.jp> for page remapping and memory
 hotremoval, which can be found at:

   http://people.valinux.co.jp/~iwamoto/mh.html

 The integration of cpusets with such memory migration is not yet
 available.

 In the future, a C library interface to cpusets will likely be
 available.  For now, the only way to query or modify cpusets is
 via the cpuset file system, using the various cd, mkdir, echo, cat,
 rmdir commands from the shell, or their equivalent from C.

 The sched_setaffinity calls can also be done at the shell prompt using
 SGI's runon or Robert Love's taskset.  The mbind and set_mempolicy
 calls can be done at the shell prompt using the numactl command
 (part of Andi Kleen's numa package).

 2. Usage Examples and Syntax
 ============================

 2.1 Basic Usage
 ---------------

 Creating, modifying, using the cpusets can be done through the cpuset
 virtual filesystem.

 To mount it, type:
 # mount -t cpuset none /dev/cpuset

 Then under /dev/cpuset you can find a tree that corresponds to the
 tree of the cpusets in the system. For instance, /dev/cpuset
 is the cpuset that holds the whole system.

 If you want to create a new cpuset under /dev/cpuset:
 # cd /dev/cpuset
 # mkdir my_cpuset

 Now you want to do something with this cpuset.
 # cd my_cpuset

 In this directory you can find several files:
 # ls
 cpus  cpu_exclusive  mems  mem_exclusive  tasks

 Reading them will give you information about the state of this cpuset:
 the CPUs and Memory Nodes it can use, the processes that are using
 it, its properties.  By writing to these files you can manipulate
 the cpuset.

 Set some flags:
 # /bin/echo 1 > cpu_exclusive

 Add some cpus:
 # /bin/echo 0-7 > cpus

 Now attach your shell to this cpuset:
 # /bin/echo $$ > tasks

 You can also create cpusets inside your cpuset by using mkdir in this
 directory.
 # mkdir my_sub_cs

 To remove a cpuset, just use rmdir:
 # rmdir my_sub_cs
 This will fail if the cpuset is in use (has cpusets inside, or has
 processes attached).

 2.2 Adding/removing cpus
 ------------------------

 This is the syntax to use when writing in the cpus or mems files
 in cpuset directories:

 # /bin/echo 1-4 > cpus		-> set cpus list to cpus 1,2,3,4
 # /bin/echo 1,2,3,4 > cpus	-> set cpus list to cpus 1,2,3,4

 2.3 Setting flags
 -----------------

 The syntax is very simple:

 # /bin/echo 1 > cpu_exclusive 	-> set flag 'cpu_exclusive'
 # /bin/echo 0 > cpu_exclusive 	-> unset flag 'cpu_exclusive'

 2.4 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


 3. 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 cpuset 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.

 4. Contact
 ==========

 Web: http://www.bullopensource.org/cpuset
	CPUSETS
	-------

	Copyright (C) 2004 BULL SA.
	Written by Simon.Derr@bull.net

	Portions Copyright (c) 2004 Silicon Graphics, Inc.
	Modified by Paul Jackson <pj@sgi.com>

	CONTENTS:
	=========

	1. Cpusets
	1.1 What are cpusets ?
	1.2 Why are cpusets needed ?
	1.3 How are cpusets implemented ?
	1.4 How do I use cpusets ?
	2. Usage Examples and Syntax
	2.1 Basic Usage
	2.2 Adding/removing cpus
	2.3 Setting flags
	2.4 Attaching processes
	3. Questions
	4. Contact

	1. Cpusets
	==========

	1.1 What are cpusets ?
	----------------------

	Cpusets provide a mechanism for assigning a set of CPUs and Memory
	Nodes to a set of tasks.

	Cpusets constrain the CPU and Memory placement of tasks to only
	the resources within a tasks current cpuset. They form a nested
	hierarchy visible in a virtual file system. These are the essential
	hooks, beyond what is already present, required to manage dynamic
	job placement on large systems.

	Each task has a pointer to a cpuset. Multiple tasks may reference
	the same cpuset. 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 set_mempolicy(2) system calls to include Memory Nodes
	in its memory policy, are both filtered through that tasks cpuset,
	filtering out any CPUs or Memory Nodes not in that cpuset. The
	scheduler will not schedule a task on a CPU that is not allowed in
	its cpus_allowed vector, and the kernel page allocator will not
	allocate a page on a node that is not allowed in the requesting tasks
	mems_allowed vector.

	If a cpuset is cpu or mem exclusive, no other cpuset, other than a direct
	ancestor or descendent, may share any of the same CPUs or Memory Nodes.

	User level code may create and destroy cpusets by name in the cpuset
	virtual file system, manage the attributes and permissions of these
	cpusets and which CPUs and Memory Nodes are assigned to each cpuset,
	specify and query to which cpuset a task is assigned, and list the
	task pids assigned to a cpuset.


	1.2 Why are cpusets needed ?
	----------------------------

	The management of large computer systems, with many processors (CPUs),
	complex memory cache hierarchies and multiple Memory Nodes having
	non-uniform access times (NUMA) presents additional challenges for
	the efficient scheduling and memory placement of processes.

	Frequently more modest sized systems can be operated with adequate
	efficiency just by letting the operating system automatically share
	the available CPU and Memory resources amongst the requesting tasks.

	But larger systems, which benefit more from careful processor and
	memory placement to reduce memory access times and contention,
	and which typically represent a larger investment for the customer,
	can benefit from explictly placing jobs on properly sized subsets of
	the system.

	This can be especially valuable on:

	* Web Servers running multiple instances of the same web application,
	* Servers running different applications (for instance, a web server
	and a database), or
	* NUMA systems running large HPC applications with demanding
	performance characteristics.

	These subsets, or "soft partitions" must be able to be dynamically
	adjusted, as the job mix changes, without impacting other concurrently
	executing jobs.

	The kernel cpuset patch provides the minimum essential kernel
	mechanisms required to efficiently implement such subsets. It
	leverages existing CPU and Memory Placement facilities in the Linux
	kernel to avoid any additional impact on the critical scheduler or
	memory allocator code.


	1.3 How are cpusets implemented ?
	---------------------------------

	Cpusets provide a Linux kernel (2.6.7 and above) mechanism to constrain
	which CPUs and Memory Nodes are used by a process or set of processes.

	The Linux kernel already has a pair of mechanisms to specify on which
	CPUs a task may be scheduled (sched_setaffinity) and on which Memory
	Nodes it may obtain memory (mbind, set_mempolicy).

	Cpusets extends these two mechanisms as follows:

	- Cpusets are sets of allowed CPUs and Memory Nodes, known to the
	kernel.
	- Each task in the system is attached to a cpuset, via a pointer
	in the task structure to a reference counted cpuset structure.
	- Calls to sched_setaffinity are filtered to just those CPUs
	allowed in that tasks cpuset.
	- Calls to mbind and set_mempolicy are filtered to just
	those Memory Nodes allowed in that tasks cpuset.
	- The root cpuset contains all the systems CPUs and Memory
	Nodes.
	- For any cpuset, one can define child cpusets containing a subset
	of the parents CPU and Memory Node resources.
	- The hierarchy of cpusets can be mounted at /dev/cpuset, for
	browsing and manipulation from user space.
	- A cpuset may be marked exclusive, which ensures that no other
	cpuset (except direct ancestors and descendents) may contain
	any overlapping CPUs or Memory Nodes.
	- You can list all the tasks (by pid) attached to any cpuset.

	The implementation of cpusets requires a few, simple hooks
	into the rest of the kernel, none in performance critical paths:

	- in main/init.c, to initialize the root cpuset at system boot.
	- in fork and exit, to attach and detach a task from its cpuset.
	- in sched_setaffinity, to mask the requested CPUs by what's
	allowed in that tasks cpuset.
	- in sched.c migrate_all_tasks(), to keep migrating tasks within
	the CPUs allowed by their cpuset, if possible.
	- in the mbind and set_mempolicy system calls, to mask the requested
	Memory Nodes by what's allowed in that tasks cpuset.
	- in page_alloc, to restrict memory to allowed nodes.
	- in vmscan.c, to restrict page recovery to the current cpuset.

	In addition a new file system, of type "cpuset" may be mounted,
	typically at /dev/cpuset, to enable browsing and modifying the cpusets
	presently known to the kernel. No new system calls are added for
	cpusets - all support for querying and modifying cpusets is via
	this cpuset file system.

	Each task under /proc has an added file named 'cpuset', displaying
	the cpuset name, as the path relative to the root of the cpuset file
	system.

	The /proc/<pid>/status file for each task has two added lines,
	displaying the tasks cpus_allowed (on which CPUs it may be scheduled)
	and mems_allowed (on which Memory Nodes it may obtain memory),
	in the format seen in the following example:

	Cpus_allowed: ffffffff,ffffffff,ffffffff,ffffffff
	Mems_allowed: ffffffff,ffffffff

	Each cpuset is represented by a directory in the cpuset file system
	containing the following files describing that cpuset:

	- cpus: list of CPUs in that cpuset
	- mems: list of Memory Nodes in that cpuset
	- cpu_exclusive flag: is cpu placement exclusive?
	- mem_exclusive flag: is memory placement exclusive?
	- tasks: list of tasks (by pid) attached to that cpuset

	New cpusets are created using the mkdir system call or shell
	command. The properties of a cpuset, such as its flags, allowed
	CPUs and Memory Nodes, and attached tasks, are modified by writing
	to the appropriate file in that cpusets directory, as listed above.

	The named hierarchical structure of nested cpusets 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 cpuset allows organizing the work load
	on a system into related sets of tasks such that each set is constrained
	to using the CPUs and Memory Nodes of a particular cpuset. A task
	may be re-attached to any other cpuset, if allowed by the permissions
	on the necessary cpuset file system directories.

	Such management of a system "in the large" integrates smoothly with
	the detailed placement done on individual tasks and memory regions
	using the sched_setaffinity, mbind and set_mempolicy system calls.

	The following rules apply to each cpuset:

	- Its CPUs and Memory Nodes must be a subset of its parents.
	- It can only be marked exclusive if its parent is.
	- If its cpu or memory is exclusive, they may not overlap any sibling.

	These rules, and the natural hierarchy of cpusets, enable efficient
	enforcement of the exclusive guarantee, without having to scan all
	cpusets every time any of them change to ensure nothing overlaps a
	exclusive cpuset. Also, the use of a Linux virtual file system (vfs)
	to represent the cpuset hierarchy provides for a familiar permission
	and name space for cpusets, with a minimum of additional kernel code.

	1.4 How do I use cpusets ?
	--------------------------

	In order to minimize the impact of cpusets on critical kernel
	code, such as the scheduler, and due to the fact that the kernel
	does not support one task updating the memory placement of another
	task directly, the impact on a task of changing its cpuset CPU
	or Memory Node placement, or of changing to which cpuset a task
	is attached, is subtle.

	If a cpuset has its Memory Nodes modified, then for each task attached
	to that cpuset, the next time that the kernel attempts to allocate
	a page of memory for that task, the kernel will notice the change
	in the tasks cpuset, and update its per-task memory placement to
	remain within the new cpusets memory placement. If the task was using
	mempolicy MPOL_BIND, and the nodes to which it was bound overlap with
	its new cpuset, then the task will continue to use whatever subset
	of MPOL_BIND nodes are still allowed in the new cpuset. If the task
	was using MPOL_BIND and now none of its MPOL_BIND nodes are allowed
	in the new cpuset, then the task will be essentially treated as if it
	was MPOL_BIND bound to the new cpuset (even though its numa placement,
	as queried by get_mempolicy(), doesn't change). If a task is moved
	from one cpuset to another, then the kernel will adjust the tasks
	memory placement, as above, the next time that the kernel attempts
	to allocate a page of memory for that task.

	If a cpuset has its CPUs modified, then each task using that
	cpuset does _not_ change its behavior automatically. In order to
	minimize the impact on the critical scheduling code in the kernel,
	tasks will continue to use their prior CPU placement until they
	are rebound to their cpuset, by rewriting their pid to the 'tasks'
	file of their cpuset. If a task had been bound to some subset of its
	cpuset using the sched_setaffinity() call, and if any of that subset
	is still allowed in its new cpuset settings, then the task will be
	restricted to the intersection of the CPUs it was allowed on before,
	and its new cpuset CPU placement. If, on the other hand, there is
	no overlap between a tasks prior placement and its new cpuset CPU
	placement, then the task will be allowed to run on any CPU allowed
	in its new cpuset. If a task is moved from one cpuset to another,
	its CPU placement is updated in the same way as if the tasks pid is
	rewritten to the 'tasks' file of its current cpuset.

	In summary, the memory placement of a task whose cpuset is changed is
	updated by the kernel, on the next allocation of a page for that task,
	but the processor placement is not updated, until that tasks pid is
	rewritten to the 'tasks' file of its cpuset. This is done to avoid
	impacting the scheduler code in the kernel with a check for changes
	in a tasks processor placement.

	There is an exception to the above. If hotplug funtionality is used
	to remove all the CPUs that are currently assigned to a cpuset,
	then the kernel will automatically update the cpus_allowed of all
	tasks attached to CPUs in that cpuset to allow all CPUs. When memory
	hotplug functionality for removing Memory Nodes is available, a
	similar exception is expected to apply there as well. In general,
	the kernel prefers to violate cpuset placement, over starving a task
	that has had all its allowed CPUs or Memory Nodes taken offline. User
	code should reconfigure cpusets to only refer to online CPUs and Memory
	Nodes when using hotplug to add or remove such resources.

	There is a second exception to the above. GFP_ATOMIC requests are
	kernel internal allocations that must be satisfied, immediately.
	The kernel may drop some request, in rare cases even panic, if a
	GFP_ATOMIC alloc fails. If the request cannot be satisfied within
	the current tasks cpuset, then we relax the cpuset, and look for
	memory anywhere we can find it. It's better to violate the cpuset
	than stress the kernel.

	To start a new job that is to be contained within a cpuset, the steps are:

	1) mkdir /dev/cpuset
	2) mount -t cpuset none /dev/cpuset
	3) Create the new cpuset by doing mkdir's and write's (or echo's) in
	the /dev/cpuset virtual file system.
	4) Start a task that will be the "founding father" of the new job.
	5) Attach that task to the new cpuset by writing its pid to the
	/dev/cpuset tasks file for that cpuset.
	6) fork, exec or clone the job tasks from this founding father task.

	For example, the following sequence of commands will setup a cpuset
	named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
	and then start a subshell 'sh' in that cpuset:

	mount -t cpuset none /dev/cpuset
	cd /dev/cpuset
	mkdir Charlie
	cd Charlie
	/bin/echo 2-3 > cpus
	/bin/echo 1 > mems
	/bin/echo $$ > tasks
	sh
	# The subshell 'sh' is now running in cpuset Charlie
	# The next line should display '/Charlie'
	cat /proc/self/cpuset

	In the case that a change of cpuset includes wanting to move already
	allocated memory pages, consider further the work of IWAMOTO
	Toshihiro <iwamoto@valinux.co.jp> for page remapping and memory
	hotremoval, which can be found at:

	http://people.valinux.co.jp/~iwamoto/mh.html

	The integration of cpusets with such memory migration is not yet
	available.

	In the future, a C library interface to cpusets will likely be
	available. For now, the only way to query or modify cpusets is
	via the cpuset file system, using the various cd, mkdir, echo, cat,
	rmdir commands from the shell, or their equivalent from C.

	The sched_setaffinity calls can also be done at the shell prompt using
	SGI's runon or Robert Love's taskset. The mbind and set_mempolicy
	calls can be done at the shell prompt using the numactl command
	(part of Andi Kleen's numa package).

	2. Usage Examples and Syntax
	============================

	2.1 Basic Usage
	---------------

	Creating, modifying, using the cpusets can be done through the cpuset
	virtual filesystem.

	To mount it, type:
	# mount -t cpuset none /dev/cpuset

	Then under /dev/cpuset you can find a tree that corresponds to the
	tree of the cpusets in the system. For instance, /dev/cpuset
	is the cpuset that holds the whole system.

	If you want to create a new cpuset under /dev/cpuset:
	# cd /dev/cpuset
	# mkdir my_cpuset

	Now you want to do something with this cpuset.
	# cd my_cpuset

	In this directory you can find several files:
	# ls
	cpus cpu_exclusive mems mem_exclusive tasks

	Reading them will give you information about the state of this cpuset:
	the CPUs and Memory Nodes it can use, the processes that are using
	it, its properties. By writing to these files you can manipulate
	the cpuset.

	Set some flags:
	# /bin/echo 1 > cpu_exclusive

	Add some cpus:
	# /bin/echo 0-7 > cpus

	Now attach your shell to this cpuset:
	# /bin/echo $$ > tasks

	You can also create cpusets inside your cpuset by using mkdir in this
	directory.
	# mkdir my_sub_cs

	To remove a cpuset, just use rmdir:
	# rmdir my_sub_cs
	This will fail if the cpuset is in use (has cpusets inside, or has
	processes attached).

	2.2 Adding/removing cpus
	------------------------

	This is the syntax to use when writing in the cpus or mems files
	in cpuset directories:

	# /bin/echo 1-4 > cpus -> set cpus list to cpus 1,2,3,4
	# /bin/echo 1,2,3,4 > cpus -> set cpus list to cpus 1,2,3,4

	2.3 Setting flags
	-----------------

	The syntax is very simple:

	# /bin/echo 1 > cpu_exclusive -> set flag 'cpu_exclusive'
	# /bin/echo 0 > cpu_exclusive -> unset flag 'cpu_exclusive'

	2.4 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


	3. 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 cpuset 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.

	4. Contact
	==========

	Web: http://www.bullopensource.org/cpuset