FreeBSD/Linux Kernel Cross Reference
sys/Documentation/cpusets.txt

                                     CPUSETS
                                     -------
     
     Copyright (C) 2004 BULL SA.
     Written by Simon.Derr@bull.net
     
     Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
     Modified by Paul Jackson <pj@sgi.com>
     Modified by Christoph Lameter <clameter@sgi.com>
    Modified by Paul Menage <menage@google.com>
    Modified by Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
    
    CONTENTS:
    =========
    
    1. Cpusets
      1.1 What are cpusets ?
      1.2 Why are cpusets needed ?
      1.3 How are cpusets implemented ?
      1.4 What are exclusive cpusets ?
      1.5 What is memory_pressure ?
      1.6 What is memory spread ?
      1.7 What is sched_load_balance ?
      1.8 What is sched_relax_domain_level ?
      1.9 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.   In this document "Memory Node" refers to
    an on-line node that contains memory.
    
    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.
    
    Cpusets use the generic cgroup subsystem described in
    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
    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.
    
    User level code may create and destroy cpusets by name in the cgroup
    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 explicitly 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 location of the running jobs pages may also be moved
    when the memory locations are changed.
    
   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 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 cgroup 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 init/main.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.c, to restrict memory to allowed nodes.
    - in vmscan.c, to restrict page recovery to the current cpuset.
   
   You should mount the "cgroup" filesystem type in order 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.
   
   The /proc/<pid>/status file for each task has four 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 two formats seen in the following example:
   
     Cpus_allowed:   ffffffff,ffffffff,ffffffff,ffffffff
     Cpus_allowed_list:      0-127
     Mems_allowed:   ffffffff,ffffffff
     Mems_allowed_list:      0-63
   
   Each cpuset is represented by a directory in the cgroup file system
   containing (on top of the standard cgroup files) the following
   files describing that cpuset:
   
    - cpus: list of CPUs in that cpuset
    - mems: list of Memory Nodes in that cpuset
    - memory_migrate flag: if set, move pages to cpusets nodes
    - cpu_exclusive flag: is cpu placement exclusive?
    - mem_exclusive flag: is memory placement exclusive?
    - mem_hardwall flag:  is memory allocation hardwalled
    - memory_pressure: measure of how much paging pressure in cpuset
   
   In addition, the root cpuset only has the following file:
    - memory_pressure_enabled flag: compute memory_pressure?
   
   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't be marked exclusive unless 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.
   
   The cpus and mems files in the root (top_cpuset) cpuset are
   read-only.  The cpus file automatically tracks the value of
   cpu_online_map using a CPU hotplug notifier, and the mems file
   automatically tracks the value of node_states[N_HIGH_MEMORY]--i.e.,
   nodes with memory--using the cpuset_track_online_nodes() hook.
   
   
   1.4 What are exclusive cpusets ?
   --------------------------------
   
   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.
   
   A cpuset that is mem_exclusive *or* mem_hardwall is "hardwalled",
   i.e. it restricts kernel allocations for page, buffer and other data
   commonly shared by the kernel across multiple users.  All cpusets,
   whether hardwalled or not, restrict allocations of memory for user
   space.  This enables configuring a system so that several independent
   jobs can share common kernel data, such as file system pages, while
   isolating each job's user allocation in its own cpuset.  To do this,
   construct a large mem_exclusive cpuset to hold all the jobs, and
   construct child, non-mem_exclusive cpusets for each individual job.
   Only a small amount of typical kernel memory, such as requests from
   interrupt handlers, is allowed to be taken outside even a
   mem_exclusive cpuset.
   
   
   1.5 What is memory_pressure ?
   -----------------------------
   The memory_pressure of a cpuset provides a simple per-cpuset metric
   of the rate that the tasks in a cpuset are attempting to free up in
   use memory on the nodes of the cpuset to satisfy additional memory
   requests.
   
   This enables batch managers monitoring jobs running in dedicated
   cpusets to efficiently detect what level of memory pressure that job
   is causing.
   
   This is useful both on tightly managed systems running a wide mix of
   submitted jobs, which may choose to terminate or re-prioritize jobs that
   are trying to use more memory than allowed on the nodes assigned them,
   and with tightly coupled, long running, massively parallel scientific
   computing jobs that will dramatically fail to meet required performance
   goals if they start to use more memory than allowed to them.
   
   This mechanism provides a very economical way for the batch manager
   to monitor a cpuset for signs of memory pressure.  It's up to the
   batch manager or other user code to decide what to do about it and
   take action.
   
   ==> Unless this feature is enabled by writing "1" to the special file
       /dev/cpuset/memory_pressure_enabled, the hook in the rebalance
       code of __alloc_pages() for this metric reduces to simply noticing
       that the cpuset_memory_pressure_enabled flag is zero.  So only
       systems that enable this feature will compute the metric.
   
   Why a per-cpuset, running average:
   
       Because this meter is per-cpuset, rather than per-task or mm,
       the system load imposed by a batch scheduler monitoring this
       metric is sharply reduced on large systems, because a scan of
       the tasklist can be avoided on each set of queries.
   
       Because this meter is a running average, instead of an accumulating
       counter, a batch scheduler can detect memory pressure with a
       single read, instead of having to read and accumulate results
       for a period of time.
   
       Because this meter is per-cpuset rather than per-task or mm,
       the batch scheduler can obtain the key information, memory
       pressure in a cpuset, with a single read, rather than having to
       query and accumulate results over all the (dynamically changing)
       set of tasks in the cpuset.
   
   A per-cpuset simple digital filter (requires a spinlock and 3 words
   of data per-cpuset) is kept, and updated by any task attached to that
   cpuset, if it enters the synchronous (direct) page reclaim code.
   
   A per-cpuset file provides an integer number representing the recent
   (half-life of 10 seconds) rate of direct page reclaims caused by
   the tasks in the cpuset, in units of reclaims attempted per second,
   times 1000.
   
   
   1.6 What is memory spread ?
   ---------------------------
   There are two boolean flag files per cpuset that control where the
   kernel allocates pages for the file system buffers and related in
   kernel data structures.  They are called 'memory_spread_page' and
   'memory_spread_slab'.
   
   If the per-cpuset boolean flag file 'memory_spread_page' is set, then
   the kernel will spread the file system buffers (page cache) evenly
   over all the nodes that the faulting task is allowed to use, instead
   of preferring to put those pages on the node where the task is running.
   
   If the per-cpuset boolean flag file 'memory_spread_slab' is set,
   then the kernel will spread some file system related slab caches,
   such as for inodes and dentries evenly over all the nodes that the
   faulting task is allowed to use, instead of preferring to put those
   pages on the node where the task is running.
   
   The setting of these flags does not affect anonymous data segment or
   stack segment pages of a task.
   
   By default, both kinds of memory spreading are off, and memory
   pages are allocated on the node local to where the task is running,
   except perhaps as modified by the tasks NUMA mempolicy or cpuset
   configuration, so long as sufficient free memory pages are available.
   
   When new cpusets are created, they inherit the memory spread settings
   of their parent.
   
   Setting memory spreading causes allocations for the affected page
   or slab caches to ignore the tasks NUMA mempolicy and be spread
   instead.    Tasks using mbind() or set_mempolicy() calls to set NUMA
   mempolicies will not notice any change in these calls as a result of
   their containing tasks memory spread settings.  If memory spreading
   is turned off, then the currently specified NUMA mempolicy once again
   applies to memory page allocations.
   
   Both 'memory_spread_page' and 'memory_spread_slab' are boolean flag
   files.  By default they contain "0", meaning that the feature is off
   for that cpuset.  If a "1" is written to that file, then that turns
   the named feature on.
   
   The implementation is simple.
   
   Setting the flag 'memory_spread_page' turns on a per-process flag
   PF_SPREAD_PAGE for each task that is in that cpuset or subsequently
   joins that cpuset.  The page allocation calls for the page cache
   is modified to perform an inline check for this PF_SPREAD_PAGE task
   flag, and if set, a call to a new routine cpuset_mem_spread_node()
   returns the node to prefer for the allocation.
   
   Similarly, setting 'memory_spread_slab' turns on the flag
   PF_SPREAD_SLAB, and appropriately marked slab caches will allocate
   pages from the node returned by cpuset_mem_spread_node().
   
   The cpuset_mem_spread_node() routine is also simple.  It uses the
   value of a per-task rotor cpuset_mem_spread_rotor to select the next
   node in the current tasks mems_allowed to prefer for the allocation.
   
   This memory placement policy is also known (in other contexts) as
   round-robin or interleave.
   
   This policy can provide substantial improvements for jobs that need
   to place thread local data on the corresponding node, but that need
   to access large file system data sets that need to be spread across
   the several nodes in the jobs cpuset in order to fit.  Without this
   policy, especially for jobs that might have one thread reading in the
   data set, the memory allocation across the nodes in the jobs cpuset
   can become very uneven.
   
   1.7 What is sched_load_balance ?
   --------------------------------
   
   The kernel scheduler (kernel/sched.c) automatically load balances
   tasks.  If one CPU is underutilized, kernel code running on that
   CPU will look for tasks on other more overloaded CPUs and move those
   tasks to itself, within the constraints of such placement mechanisms
   as cpusets and sched_setaffinity.
   
   The algorithmic cost of load balancing and its impact on key shared
   kernel data structures such as the task list increases more than
   linearly with the number of CPUs being balanced.  So the scheduler
   has support to  partition the systems CPUs into a number of sched
   domains such that it only load balances within each sched domain.
   Each sched domain covers some subset of the CPUs in the system;
   no two sched domains overlap; some CPUs might not be in any sched
   domain and hence won't be load balanced.
   
   Put simply, it costs less to balance between two smaller sched domains
   than one big one, but doing so means that overloads in one of the
   two domains won't be load balanced to the other one.
   
   By default, there is one sched domain covering all CPUs, except those
   marked isolated using the kernel boot time "isolcpus=" argument.
   
   This default load balancing across all CPUs is not well suited for
   the following two situations:
    1) On large systems, load balancing across many CPUs is expensive.
       If the system is managed using cpusets to place independent jobs
       on separate sets of CPUs, full load balancing is unnecessary.
    2) Systems supporting realtime on some CPUs need to minimize
       system overhead on those CPUs, including avoiding task load
       balancing if that is not needed.
   
   When the per-cpuset flag "sched_load_balance" is enabled (the default
   setting), it requests that all the CPUs in that cpusets allowed 'cpus'
   be contained in a single sched domain, ensuring that load balancing
   can move a task (not otherwised pinned, as by sched_setaffinity)
   from any CPU in that cpuset to any other.
   
   When the per-cpuset flag "sched_load_balance" is disabled, then the
   scheduler will avoid load balancing across the CPUs in that cpuset,
   --except-- in so far as is necessary because some overlapping cpuset
   has "sched_load_balance" enabled.
   
   So, for example, if the top cpuset has the flag "sched_load_balance"
   enabled, then the scheduler will have one sched domain covering all
   CPUs, and the setting of the "sched_load_balance" flag in any other
   cpusets won't matter, as we're already fully load balancing.
   
   Therefore in the above two situations, the top cpuset flag
   "sched_load_balance" should be disabled, and only some of the smaller,
   child cpusets have this flag enabled.
   
   When doing this, you don't usually want to leave any unpinned tasks in
   the top cpuset that might use non-trivial amounts of CPU, as such tasks
   may be artificially constrained to some subset of CPUs, depending on
   the particulars of this flag setting in descendent cpusets.  Even if
   such a task could use spare CPU cycles in some other CPUs, the kernel
   scheduler might not consider the possibility of load balancing that
   task to that underused CPU.
   
   Of course, tasks pinned to a particular CPU can be left in a cpuset
   that disables "sched_load_balance" as those tasks aren't going anywhere
   else anyway.
   
   There is an impedance mismatch here, between cpusets and sched domains.
   Cpusets are hierarchical and nest.  Sched domains are flat; they don't
   overlap and each CPU is in at most one sched domain.
   
   It is necessary for sched domains to be flat because load balancing
   across partially overlapping sets of CPUs would risk unstable dynamics
   that would be beyond our understanding.  So if each of two partially
   overlapping cpusets enables the flag 'sched_load_balance', then we
   form a single sched domain that is a superset of both.  We won't move
   a task to a CPU outside it cpuset, but the scheduler load balancing
   code might waste some compute cycles considering that possibility.
   
   This mismatch is why there is not a simple one-to-one relation
   between which cpusets have the flag "sched_load_balance" enabled,
   and the sched domain configuration.  If a cpuset enables the flag, it
   will get balancing across all its CPUs, but if it disables the flag,
   it will only be assured of no load balancing if no other overlapping
   cpuset enables the flag.
   
   If two cpusets have partially overlapping 'cpus' allowed, and only
   one of them has this flag enabled, then the other may find its
   tasks only partially load balanced, just on the overlapping CPUs.
   This is just the general case of the top_cpuset example given a few
   paragraphs above.  In the general case, as in the top cpuset case,
   don't leave tasks that might use non-trivial amounts of CPU in
   such partially load balanced cpusets, as they may be artificially
   constrained to some subset of the CPUs allowed to them, for lack of
   load balancing to the other CPUs.
   
   1.7.1 sched_load_balance implementation details.
   ------------------------------------------------
   
   The per-cpuset flag 'sched_load_balance' defaults to enabled (contrary
   to most cpuset flags.)  When enabled for a cpuset, the kernel will
   ensure that it can load balance across all the CPUs in that cpuset
   (makes sure that all the CPUs in the cpus_allowed of that cpuset are
   in the same sched domain.)
   
   If two overlapping cpusets both have 'sched_load_balance' enabled,
   then they will be (must be) both in the same sched domain.
   
   If, as is the default, the top cpuset has 'sched_load_balance' enabled,
   then by the above that means there is a single sched domain covering
   the whole system, regardless of any other cpuset settings.
   
   The kernel commits to user space that it will avoid load balancing
   where it can.  It will pick as fine a granularity partition of sched
   domains as it can while still providing load balancing for any set
   of CPUs allowed to a cpuset having 'sched_load_balance' enabled.
   
   The internal kernel cpuset to scheduler interface passes from the
   cpuset code to the scheduler code a partition of the load balanced
   CPUs in the system. This partition is a set of subsets (represented
   as an array of cpumask_t) of CPUs, pairwise disjoint, that cover all
   the CPUs that must be load balanced.
   
   Whenever the 'sched_load_balance' flag changes, or CPUs come or go
   from a cpuset with this flag enabled, or a cpuset with this flag
   enabled is removed, the cpuset code builds a new such partition and
   passes it to the scheduler sched domain setup code, to have the sched
   domains rebuilt as necessary.
   
   This partition exactly defines what sched domains the scheduler should
   setup - one sched domain for each element (cpumask_t) in the partition.
   
   The scheduler remembers the currently active sched domain partitions.
   When the scheduler routine partition_sched_domains() is invoked from
   the cpuset code to update these sched domains, it compares the new
   partition requested with the current, and updates its sched domains,
   removing the old and adding the new, for each change.
   
   
   1.8 What is sched_relax_domain_level ?
   --------------------------------------
   
   In sched domain, the scheduler migrates tasks in 2 ways; periodic load
   balance on tick, and at time of some schedule events.
   
   When a task is woken up, scheduler try to move the task on idle CPU.
   For example, if a task A running on CPU X activates another task B
   on the same CPU X, and if CPU Y is X's sibling and performing idle,
   then scheduler migrate task B to CPU Y so that task B can start on
   CPU Y without waiting task A on CPU X.
   
   And if a CPU run out of tasks in its runqueue, the CPU try to pull
   extra tasks from other busy CPUs to help them before it is going to
   be idle.
   
   Of course it takes some searching cost to find movable tasks and/or
   idle CPUs, the scheduler might not search all CPUs in the domain
   everytime.  In fact, in some architectures, the searching ranges on
   events are limited in the same socket or node where the CPU locates,
   while the load balance on tick searchs all.
   
   For example, assume CPU Z is relatively far from CPU X.  Even if CPU Z
   is idle while CPU X and the siblings are busy, scheduler can't migrate
   woken task B from X to Z since it is out of its searching range.
   As the result, task B on CPU X need to wait task A or wait load balance
   on the next tick.  For some applications in special situation, waiting
   1 tick may be too long.
   
   The 'sched_relax_domain_level' file allows you to request changing
   this searching range as you like.  This file takes int value which
   indicates size of searching range in levels ideally as follows,
   otherwise initial value -1 that indicates the cpuset has no request.
   
     -1  : no request. use system default or follow request of others.
      0  : no search.
      1  : search siblings (hyperthreads in a core).
      2  : search cores in a package.
      3  : search cpus in a node [= system wide on non-NUMA system]
    ( 4  : search nodes in a chunk of node [on NUMA system] )
    ( 5  : search system wide [on NUMA system] )
   
   The system default is architecture dependent.  The system default
   can be changed using the relax_domain_level= boot parameter.
   
   This file is per-cpuset and affect the sched domain where the cpuset
   belongs to.  Therefore if the flag 'sched_load_balance' of a cpuset
   is disabled, then 'sched_relax_domain_level' have no effect since
   there is no sched domain belonging the cpuset.
   
   If multiple cpusets are overlapping and hence they form a single sched
   domain, the largest value among those is used.  Be careful, if one
   requests 0 and others are -1 then 0 is used.
   
   Note that modifying this file will have both good and bad effects,
   and whether it is acceptable or not will be depend on your situation.
   Don't modify this file if you are not sure.
   
   If your situation is:
    - The migration costs between each cpu can be assumed considerably
      small(for you) due to your special application's behavior or
      special hardware support for CPU cache etc.
    - The searching cost doesn't have impact(for you) or you can make
      the searching cost enough small by managing cpuset to compact etc.
    - The latency is required even it sacrifices cache hit rate etc.
   then increasing 'sched_relax_domain_level' would benefit you.
   
   
   1.9 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 in that cpuset
   will have its allowed CPU placement changed immediately.  Similarly,
   if a tasks pid is written to a cpusets 'tasks' file, in either its
   current cpuset or another cpuset, then its allowed CPU placement is
   changed immediately.  If such a task had been bound to some subset
   of its cpuset using the sched_setaffinity() call, the task will be
   allowed to run on any CPU allowed in its new cpuset, negating the
   affect of the prior sched_setaffinity() call.
   
   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.
   
   Normally, once a page is allocated (given a physical page
   of main memory) then that page stays on whatever node it
   was allocated, so long as it remains allocated, even if the
   cpusets memory placement policy 'mems' subsequently changes.
   If the cpuset flag file 'memory_migrate' is set true, then when
   tasks are attached to that cpuset, any pages that task had
   allocated to it on nodes in its previous cpuset are migrated
   to the tasks new cpuset. The relative placement of the page within
   the cpuset is preserved during these migration operations if possible.
   For example if the page was on the second valid node of the prior cpuset
   then the page will be placed on the second valid node of the new cpuset.
   
   Also if 'memory_migrate' is set true, then if that cpusets
   'mems' file is modified, pages allocated to tasks in that
   cpuset, that were on nodes in the previous setting of 'mems',
   will be moved to nodes in the new setting of 'mems.'
   Pages that were not in the tasks prior cpuset, or in the cpusets
   prior 'mems' setting, will not be moved.
   
   There is an exception to the above.  If hotplug functionality is used
   to remove all the CPUs that are currently assigned to a cpuset,
   then all the tasks in that cpuset will be moved to the nearest ancestor
   with non-empty cpus.  But the moving of some (or all) tasks might fail if
   cpuset is bound with another cgroup subsystem which has some restrictions
   on task attaching.  In this failing case, those tasks will stay
   in the original cpuset, and the kernel will automatically update
   their cpus_allowed to allow all online 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.
   
   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 cgroup -ocpuset cpuset /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 cgroup -ocpuset cpuset /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 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 cgroup -o cpuset cpuset /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
   cpu_exclusive  memory_migrate      mems                      tasks
   cpus           memory_pressure     notify_on_release
   mem_exclusive  memory_spread_page  sched_load_balance
   mem_hardwall   memory_spread_slab  sched_relax_domain_level
   
   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
   
   Add some mems:
   # /bin/echo 0-7 > mems
   
   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).
   
   Note that for legacy reasons, the "cpuset" filesystem exists as a
   wrapper around the cgroup filesystem.
   
   The command
   
   mount -t cpuset X /dev/cpuset
   
   is equivalent to
   
   mount -t cgroup -ocpuset X /dev/cpuset
   echo "/sbin/cpuset_release_agent" > /dev/cpuset/release_agent
   
   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

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