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cpusets.rst (38195B)


      1.. _cpusets:
      2
      3=======
      4CPUSETS
      5=======
      6
      7Copyright (C) 2004 BULL SA.
      8
      9Written by Simon.Derr@bull.net
     10
     11- Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
     12- Modified by Paul Jackson <pj@sgi.com>
     13- Modified by Christoph Lameter <cl@linux.com>
     14- Modified by Paul Menage <menage@google.com>
     15- Modified by Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
     16
     17.. CONTENTS:
     18
     19   1. Cpusets
     20     1.1 What are cpusets ?
     21     1.2 Why are cpusets needed ?
     22     1.3 How are cpusets implemented ?
     23     1.4 What are exclusive cpusets ?
     24     1.5 What is memory_pressure ?
     25     1.6 What is memory spread ?
     26     1.7 What is sched_load_balance ?
     27     1.8 What is sched_relax_domain_level ?
     28     1.9 How do I use cpusets ?
     29   2. Usage Examples and Syntax
     30     2.1 Basic Usage
     31     2.2 Adding/removing cpus
     32     2.3 Setting flags
     33     2.4 Attaching processes
     34   3. Questions
     35   4. Contact
     36
     371. Cpusets
     38==========
     39
     401.1 What are cpusets ?
     41----------------------
     42
     43Cpusets provide a mechanism for assigning a set of CPUs and Memory
     44Nodes to a set of tasks.   In this document "Memory Node" refers to
     45an on-line node that contains memory.
     46
     47Cpusets constrain the CPU and Memory placement of tasks to only
     48the resources within a task's current cpuset.  They form a nested
     49hierarchy visible in a virtual file system.  These are the essential
     50hooks, beyond what is already present, required to manage dynamic
     51job placement on large systems.
     52
     53Cpusets use the generic cgroup subsystem described in
     54Documentation/admin-guide/cgroup-v1/cgroups.rst.
     55
     56Requests by a task, using the sched_setaffinity(2) system call to
     57include CPUs in its CPU affinity mask, and using the mbind(2) and
     58set_mempolicy(2) system calls to include Memory Nodes in its memory
     59policy, are both filtered through that task's cpuset, filtering out any
     60CPUs or Memory Nodes not in that cpuset.  The scheduler will not
     61schedule a task on a CPU that is not allowed in its cpus_allowed
     62vector, and the kernel page allocator will not allocate a page on a
     63node that is not allowed in the requesting task's mems_allowed vector.
     64
     65User level code may create and destroy cpusets by name in the cgroup
     66virtual file system, manage the attributes and permissions of these
     67cpusets and which CPUs and Memory Nodes are assigned to each cpuset,
     68specify and query to which cpuset a task is assigned, and list the
     69task pids assigned to a cpuset.
     70
     71
     721.2 Why are cpusets needed ?
     73----------------------------
     74
     75The management of large computer systems, with many processors (CPUs),
     76complex memory cache hierarchies and multiple Memory Nodes having
     77non-uniform access times (NUMA) presents additional challenges for
     78the efficient scheduling and memory placement of processes.
     79
     80Frequently more modest sized systems can be operated with adequate
     81efficiency just by letting the operating system automatically share
     82the available CPU and Memory resources amongst the requesting tasks.
     83
     84But larger systems, which benefit more from careful processor and
     85memory placement to reduce memory access times and contention,
     86and which typically represent a larger investment for the customer,
     87can benefit from explicitly placing jobs on properly sized subsets of
     88the system.
     89
     90This can be especially valuable on:
     91
     92    * Web Servers running multiple instances of the same web application,
     93    * Servers running different applications (for instance, a web server
     94      and a database), or
     95    * NUMA systems running large HPC applications with demanding
     96      performance characteristics.
     97
     98These subsets, or "soft partitions" must be able to be dynamically
     99adjusted, as the job mix changes, without impacting other concurrently
    100executing jobs. The location of the running jobs pages may also be moved
    101when the memory locations are changed.
    102
    103The kernel cpuset patch provides the minimum essential kernel
    104mechanisms required to efficiently implement such subsets.  It
    105leverages existing CPU and Memory Placement facilities in the Linux
    106kernel to avoid any additional impact on the critical scheduler or
    107memory allocator code.
    108
    109
    1101.3 How are cpusets implemented ?
    111---------------------------------
    112
    113Cpusets provide a Linux kernel mechanism to constrain which CPUs and
    114Memory Nodes are used by a process or set of processes.
    115
    116The Linux kernel already has a pair of mechanisms to specify on which
    117CPUs a task may be scheduled (sched_setaffinity) and on which Memory
    118Nodes it may obtain memory (mbind, set_mempolicy).
    119
    120Cpusets extends these two mechanisms as follows:
    121
    122 - Cpusets are sets of allowed CPUs and Memory Nodes, known to the
    123   kernel.
    124 - Each task in the system is attached to a cpuset, via a pointer
    125   in the task structure to a reference counted cgroup structure.
    126 - Calls to sched_setaffinity are filtered to just those CPUs
    127   allowed in that task's cpuset.
    128 - Calls to mbind and set_mempolicy are filtered to just
    129   those Memory Nodes allowed in that task's cpuset.
    130 - The root cpuset contains all the systems CPUs and Memory
    131   Nodes.
    132 - For any cpuset, one can define child cpusets containing a subset
    133   of the parents CPU and Memory Node resources.
    134 - The hierarchy of cpusets can be mounted at /dev/cpuset, for
    135   browsing and manipulation from user space.
    136 - A cpuset may be marked exclusive, which ensures that no other
    137   cpuset (except direct ancestors and descendants) may contain
    138   any overlapping CPUs or Memory Nodes.
    139 - You can list all the tasks (by pid) attached to any cpuset.
    140
    141The implementation of cpusets requires a few, simple hooks
    142into the rest of the kernel, none in performance critical paths:
    143
    144 - in init/main.c, to initialize the root cpuset at system boot.
    145 - in fork and exit, to attach and detach a task from its cpuset.
    146 - in sched_setaffinity, to mask the requested CPUs by what's
    147   allowed in that task's cpuset.
    148 - in sched.c migrate_live_tasks(), to keep migrating tasks within
    149   the CPUs allowed by their cpuset, if possible.
    150 - in the mbind and set_mempolicy system calls, to mask the requested
    151   Memory Nodes by what's allowed in that task's cpuset.
    152 - in page_alloc.c, to restrict memory to allowed nodes.
    153 - in vmscan.c, to restrict page recovery to the current cpuset.
    154
    155You should mount the "cgroup" filesystem type in order to enable
    156browsing and modifying the cpusets presently known to the kernel.  No
    157new system calls are added for cpusets - all support for querying and
    158modifying cpusets is via this cpuset file system.
    159
    160The /proc/<pid>/status file for each task has four added lines,
    161displaying the task's cpus_allowed (on which CPUs it may be scheduled)
    162and mems_allowed (on which Memory Nodes it may obtain memory),
    163in the two formats seen in the following example::
    164
    165  Cpus_allowed:   ffffffff,ffffffff,ffffffff,ffffffff
    166  Cpus_allowed_list:      0-127
    167  Mems_allowed:   ffffffff,ffffffff
    168  Mems_allowed_list:      0-63
    169
    170Each cpuset is represented by a directory in the cgroup file system
    171containing (on top of the standard cgroup files) the following
    172files describing that cpuset:
    173
    174 - cpuset.cpus: list of CPUs in that cpuset
    175 - cpuset.mems: list of Memory Nodes in that cpuset
    176 - cpuset.memory_migrate flag: if set, move pages to cpusets nodes
    177 - cpuset.cpu_exclusive flag: is cpu placement exclusive?
    178 - cpuset.mem_exclusive flag: is memory placement exclusive?
    179 - cpuset.mem_hardwall flag:  is memory allocation hardwalled
    180 - cpuset.memory_pressure: measure of how much paging pressure in cpuset
    181 - cpuset.memory_spread_page flag: if set, spread page cache evenly on allowed nodes
    182 - cpuset.memory_spread_slab flag: if set, spread slab cache evenly on allowed nodes
    183 - cpuset.sched_load_balance flag: if set, load balance within CPUs on that cpuset
    184 - cpuset.sched_relax_domain_level: the searching range when migrating tasks
    185
    186In addition, only the root cpuset has the following file:
    187
    188 - cpuset.memory_pressure_enabled flag: compute memory_pressure?
    189
    190New cpusets are created using the mkdir system call or shell
    191command.  The properties of a cpuset, such as its flags, allowed
    192CPUs and Memory Nodes, and attached tasks, are modified by writing
    193to the appropriate file in that cpusets directory, as listed above.
    194
    195The named hierarchical structure of nested cpusets allows partitioning
    196a large system into nested, dynamically changeable, "soft-partitions".
    197
    198The attachment of each task, automatically inherited at fork by any
    199children of that task, to a cpuset allows organizing the work load
    200on a system into related sets of tasks such that each set is constrained
    201to using the CPUs and Memory Nodes of a particular cpuset.  A task
    202may be re-attached to any other cpuset, if allowed by the permissions
    203on the necessary cpuset file system directories.
    204
    205Such management of a system "in the large" integrates smoothly with
    206the detailed placement done on individual tasks and memory regions
    207using the sched_setaffinity, mbind and set_mempolicy system calls.
    208
    209The following rules apply to each cpuset:
    210
    211 - Its CPUs and Memory Nodes must be a subset of its parents.
    212 - It can't be marked exclusive unless its parent is.
    213 - If its cpu or memory is exclusive, they may not overlap any sibling.
    214
    215These rules, and the natural hierarchy of cpusets, enable efficient
    216enforcement of the exclusive guarantee, without having to scan all
    217cpusets every time any of them change to ensure nothing overlaps a
    218exclusive cpuset.  Also, the use of a Linux virtual file system (vfs)
    219to represent the cpuset hierarchy provides for a familiar permission
    220and name space for cpusets, with a minimum of additional kernel code.
    221
    222The cpus and mems files in the root (top_cpuset) cpuset are
    223read-only.  The cpus file automatically tracks the value of
    224cpu_online_mask using a CPU hotplug notifier, and the mems file
    225automatically tracks the value of node_states[N_MEMORY]--i.e.,
    226nodes with memory--using the cpuset_track_online_nodes() hook.
    227
    228The cpuset.effective_cpus and cpuset.effective_mems files are
    229normally read-only copies of cpuset.cpus and cpuset.mems files
    230respectively.  If the cpuset cgroup filesystem is mounted with the
    231special "cpuset_v2_mode" option, the behavior of these files will become
    232similar to the corresponding files in cpuset v2.  In other words, hotplug
    233events will not change cpuset.cpus and cpuset.mems.  Those events will
    234only affect cpuset.effective_cpus and cpuset.effective_mems which show
    235the actual cpus and memory nodes that are currently used by this cpuset.
    236See Documentation/admin-guide/cgroup-v2.rst for more information about
    237cpuset v2 behavior.
    238
    239
    2401.4 What are exclusive cpusets ?
    241--------------------------------
    242
    243If a cpuset is cpu or mem exclusive, no other cpuset, other than
    244a direct ancestor or descendant, may share any of the same CPUs or
    245Memory Nodes.
    246
    247A cpuset that is cpuset.mem_exclusive *or* cpuset.mem_hardwall is "hardwalled",
    248i.e. it restricts kernel allocations for page, buffer and other data
    249commonly shared by the kernel across multiple users.  All cpusets,
    250whether hardwalled or not, restrict allocations of memory for user
    251space.  This enables configuring a system so that several independent
    252jobs can share common kernel data, such as file system pages, while
    253isolating each job's user allocation in its own cpuset.  To do this,
    254construct a large mem_exclusive cpuset to hold all the jobs, and
    255construct child, non-mem_exclusive cpusets for each individual job.
    256Only a small amount of typical kernel memory, such as requests from
    257interrupt handlers, is allowed to be taken outside even a
    258mem_exclusive cpuset.
    259
    260
    2611.5 What is memory_pressure ?
    262-----------------------------
    263The memory_pressure of a cpuset provides a simple per-cpuset metric
    264of the rate that the tasks in a cpuset are attempting to free up in
    265use memory on the nodes of the cpuset to satisfy additional memory
    266requests.
    267
    268This enables batch managers monitoring jobs running in dedicated
    269cpusets to efficiently detect what level of memory pressure that job
    270is causing.
    271
    272This is useful both on tightly managed systems running a wide mix of
    273submitted jobs, which may choose to terminate or re-prioritize jobs that
    274are trying to use more memory than allowed on the nodes assigned to them,
    275and with tightly coupled, long running, massively parallel scientific
    276computing jobs that will dramatically fail to meet required performance
    277goals if they start to use more memory than allowed to them.
    278
    279This mechanism provides a very economical way for the batch manager
    280to monitor a cpuset for signs of memory pressure.  It's up to the
    281batch manager or other user code to decide what to do about it and
    282take action.
    283
    284==>
    285    Unless this feature is enabled by writing "1" to the special file
    286    /dev/cpuset/memory_pressure_enabled, the hook in the rebalance
    287    code of __alloc_pages() for this metric reduces to simply noticing
    288    that the cpuset_memory_pressure_enabled flag is zero.  So only
    289    systems that enable this feature will compute the metric.
    290
    291Why a per-cpuset, running average:
    292
    293    Because this meter is per-cpuset, rather than per-task or mm,
    294    the system load imposed by a batch scheduler monitoring this
    295    metric is sharply reduced on large systems, because a scan of
    296    the tasklist can be avoided on each set of queries.
    297
    298    Because this meter is a running average, instead of an accumulating
    299    counter, a batch scheduler can detect memory pressure with a
    300    single read, instead of having to read and accumulate results
    301    for a period of time.
    302
    303    Because this meter is per-cpuset rather than per-task or mm,
    304    the batch scheduler can obtain the key information, memory
    305    pressure in a cpuset, with a single read, rather than having to
    306    query and accumulate results over all the (dynamically changing)
    307    set of tasks in the cpuset.
    308
    309A per-cpuset simple digital filter (requires a spinlock and 3 words
    310of data per-cpuset) is kept, and updated by any task attached to that
    311cpuset, if it enters the synchronous (direct) page reclaim code.
    312
    313A per-cpuset file provides an integer number representing the recent
    314(half-life of 10 seconds) rate of direct page reclaims caused by
    315the tasks in the cpuset, in units of reclaims attempted per second,
    316times 1000.
    317
    318
    3191.6 What is memory spread ?
    320---------------------------
    321There are two boolean flag files per cpuset that control where the
    322kernel allocates pages for the file system buffers and related in
    323kernel data structures.  They are called 'cpuset.memory_spread_page' and
    324'cpuset.memory_spread_slab'.
    325
    326If the per-cpuset boolean flag file 'cpuset.memory_spread_page' is set, then
    327the kernel will spread the file system buffers (page cache) evenly
    328over all the nodes that the faulting task is allowed to use, instead
    329of preferring to put those pages on the node where the task is running.
    330
    331If the per-cpuset boolean flag file 'cpuset.memory_spread_slab' is set,
    332then the kernel will spread some file system related slab caches,
    333such as for inodes and dentries evenly over all the nodes that the
    334faulting task is allowed to use, instead of preferring to put those
    335pages on the node where the task is running.
    336
    337The setting of these flags does not affect anonymous data segment or
    338stack segment pages of a task.
    339
    340By default, both kinds of memory spreading are off, and memory
    341pages are allocated on the node local to where the task is running,
    342except perhaps as modified by the task's NUMA mempolicy or cpuset
    343configuration, so long as sufficient free memory pages are available.
    344
    345When new cpusets are created, they inherit the memory spread settings
    346of their parent.
    347
    348Setting memory spreading causes allocations for the affected page
    349or slab caches to ignore the task's NUMA mempolicy and be spread
    350instead.    Tasks using mbind() or set_mempolicy() calls to set NUMA
    351mempolicies will not notice any change in these calls as a result of
    352their containing task's memory spread settings.  If memory spreading
    353is turned off, then the currently specified NUMA mempolicy once again
    354applies to memory page allocations.
    355
    356Both 'cpuset.memory_spread_page' and 'cpuset.memory_spread_slab' are boolean flag
    357files.  By default they contain "0", meaning that the feature is off
    358for that cpuset.  If a "1" is written to that file, then that turns
    359the named feature on.
    360
    361The implementation is simple.
    362
    363Setting the flag 'cpuset.memory_spread_page' turns on a per-process flag
    364PFA_SPREAD_PAGE for each task that is in that cpuset or subsequently
    365joins that cpuset.  The page allocation calls for the page cache
    366is modified to perform an inline check for this PFA_SPREAD_PAGE task
    367flag, and if set, a call to a new routine cpuset_mem_spread_node()
    368returns the node to prefer for the allocation.
    369
    370Similarly, setting 'cpuset.memory_spread_slab' turns on the flag
    371PFA_SPREAD_SLAB, and appropriately marked slab caches will allocate
    372pages from the node returned by cpuset_mem_spread_node().
    373
    374The cpuset_mem_spread_node() routine is also simple.  It uses the
    375value of a per-task rotor cpuset_mem_spread_rotor to select the next
    376node in the current task's mems_allowed to prefer for the allocation.
    377
    378This memory placement policy is also known (in other contexts) as
    379round-robin or interleave.
    380
    381This policy can provide substantial improvements for jobs that need
    382to place thread local data on the corresponding node, but that need
    383to access large file system data sets that need to be spread across
    384the several nodes in the jobs cpuset in order to fit.  Without this
    385policy, especially for jobs that might have one thread reading in the
    386data set, the memory allocation across the nodes in the jobs cpuset
    387can become very uneven.
    388
    3891.7 What is sched_load_balance ?
    390--------------------------------
    391
    392The kernel scheduler (kernel/sched/core.c) automatically load balances
    393tasks.  If one CPU is underutilized, kernel code running on that
    394CPU will look for tasks on other more overloaded CPUs and move those
    395tasks to itself, within the constraints of such placement mechanisms
    396as cpusets and sched_setaffinity.
    397
    398The algorithmic cost of load balancing and its impact on key shared
    399kernel data structures such as the task list increases more than
    400linearly with the number of CPUs being balanced.  So the scheduler
    401has support to partition the systems CPUs into a number of sched
    402domains such that it only load balances within each sched domain.
    403Each sched domain covers some subset of the CPUs in the system;
    404no two sched domains overlap; some CPUs might not be in any sched
    405domain and hence won't be load balanced.
    406
    407Put simply, it costs less to balance between two smaller sched domains
    408than one big one, but doing so means that overloads in one of the
    409two domains won't be load balanced to the other one.
    410
    411By default, there is one sched domain covering all CPUs, including those
    412marked isolated using the kernel boot time "isolcpus=" argument. However,
    413the isolated CPUs will not participate in load balancing, and will not
    414have tasks running on them unless explicitly assigned.
    415
    416This default load balancing across all CPUs is not well suited for
    417the following two situations:
    418
    419 1) On large systems, load balancing across many CPUs is expensive.
    420    If the system is managed using cpusets to place independent jobs
    421    on separate sets of CPUs, full load balancing is unnecessary.
    422 2) Systems supporting realtime on some CPUs need to minimize
    423    system overhead on those CPUs, including avoiding task load
    424    balancing if that is not needed.
    425
    426When the per-cpuset flag "cpuset.sched_load_balance" is enabled (the default
    427setting), it requests that all the CPUs in that cpusets allowed 'cpuset.cpus'
    428be contained in a single sched domain, ensuring that load balancing
    429can move a task (not otherwised pinned, as by sched_setaffinity)
    430from any CPU in that cpuset to any other.
    431
    432When the per-cpuset flag "cpuset.sched_load_balance" is disabled, then the
    433scheduler will avoid load balancing across the CPUs in that cpuset,
    434--except-- in so far as is necessary because some overlapping cpuset
    435has "sched_load_balance" enabled.
    436
    437So, for example, if the top cpuset has the flag "cpuset.sched_load_balance"
    438enabled, then the scheduler will have one sched domain covering all
    439CPUs, and the setting of the "cpuset.sched_load_balance" flag in any other
    440cpusets won't matter, as we're already fully load balancing.
    441
    442Therefore in the above two situations, the top cpuset flag
    443"cpuset.sched_load_balance" should be disabled, and only some of the smaller,
    444child cpusets have this flag enabled.
    445
    446When doing this, you don't usually want to leave any unpinned tasks in
    447the top cpuset that might use non-trivial amounts of CPU, as such tasks
    448may be artificially constrained to some subset of CPUs, depending on
    449the particulars of this flag setting in descendant cpusets.  Even if
    450such a task could use spare CPU cycles in some other CPUs, the kernel
    451scheduler might not consider the possibility of load balancing that
    452task to that underused CPU.
    453
    454Of course, tasks pinned to a particular CPU can be left in a cpuset
    455that disables "cpuset.sched_load_balance" as those tasks aren't going anywhere
    456else anyway.
    457
    458There is an impedance mismatch here, between cpusets and sched domains.
    459Cpusets are hierarchical and nest.  Sched domains are flat; they don't
    460overlap and each CPU is in at most one sched domain.
    461
    462It is necessary for sched domains to be flat because load balancing
    463across partially overlapping sets of CPUs would risk unstable dynamics
    464that would be beyond our understanding.  So if each of two partially
    465overlapping cpusets enables the flag 'cpuset.sched_load_balance', then we
    466form a single sched domain that is a superset of both.  We won't move
    467a task to a CPU outside its cpuset, but the scheduler load balancing
    468code might waste some compute cycles considering that possibility.
    469
    470This mismatch is why there is not a simple one-to-one relation
    471between which cpusets have the flag "cpuset.sched_load_balance" enabled,
    472and the sched domain configuration.  If a cpuset enables the flag, it
    473will get balancing across all its CPUs, but if it disables the flag,
    474it will only be assured of no load balancing if no other overlapping
    475cpuset enables the flag.
    476
    477If two cpusets have partially overlapping 'cpuset.cpus' allowed, and only
    478one of them has this flag enabled, then the other may find its
    479tasks only partially load balanced, just on the overlapping CPUs.
    480This is just the general case of the top_cpuset example given a few
    481paragraphs above.  In the general case, as in the top cpuset case,
    482don't leave tasks that might use non-trivial amounts of CPU in
    483such partially load balanced cpusets, as they may be artificially
    484constrained to some subset of the CPUs allowed to them, for lack of
    485load balancing to the other CPUs.
    486
    487CPUs in "cpuset.isolcpus" were excluded from load balancing by the
    488isolcpus= kernel boot option, and will never be load balanced regardless
    489of the value of "cpuset.sched_load_balance" in any cpuset.
    490
    4911.7.1 sched_load_balance implementation details.
    492------------------------------------------------
    493
    494The per-cpuset flag 'cpuset.sched_load_balance' defaults to enabled (contrary
    495to most cpuset flags.)  When enabled for a cpuset, the kernel will
    496ensure that it can load balance across all the CPUs in that cpuset
    497(makes sure that all the CPUs in the cpus_allowed of that cpuset are
    498in the same sched domain.)
    499
    500If two overlapping cpusets both have 'cpuset.sched_load_balance' enabled,
    501then they will be (must be) both in the same sched domain.
    502
    503If, as is the default, the top cpuset has 'cpuset.sched_load_balance' enabled,
    504then by the above that means there is a single sched domain covering
    505the whole system, regardless of any other cpuset settings.
    506
    507The kernel commits to user space that it will avoid load balancing
    508where it can.  It will pick as fine a granularity partition of sched
    509domains as it can while still providing load balancing for any set
    510of CPUs allowed to a cpuset having 'cpuset.sched_load_balance' enabled.
    511
    512The internal kernel cpuset to scheduler interface passes from the
    513cpuset code to the scheduler code a partition of the load balanced
    514CPUs in the system. This partition is a set of subsets (represented
    515as an array of struct cpumask) of CPUs, pairwise disjoint, that cover
    516all the CPUs that must be load balanced.
    517
    518The cpuset code builds a new such partition and passes it to the
    519scheduler sched domain setup code, to have the sched domains rebuilt
    520as necessary, whenever:
    521
    522 - the 'cpuset.sched_load_balance' flag of a cpuset with non-empty CPUs changes,
    523 - or CPUs come or go from a cpuset with this flag enabled,
    524 - or 'cpuset.sched_relax_domain_level' value of a cpuset with non-empty CPUs
    525   and with this flag enabled changes,
    526 - or a cpuset with non-empty CPUs and with this flag enabled is removed,
    527 - or a cpu is offlined/onlined.
    528
    529This partition exactly defines what sched domains the scheduler should
    530setup - one sched domain for each element (struct cpumask) in the
    531partition.
    532
    533The scheduler remembers the currently active sched domain partitions.
    534When the scheduler routine partition_sched_domains() is invoked from
    535the cpuset code to update these sched domains, it compares the new
    536partition requested with the current, and updates its sched domains,
    537removing the old and adding the new, for each change.
    538
    539
    5401.8 What is sched_relax_domain_level ?
    541--------------------------------------
    542
    543In sched domain, the scheduler migrates tasks in 2 ways; periodic load
    544balance on tick, and at time of some schedule events.
    545
    546When a task is woken up, scheduler try to move the task on idle CPU.
    547For example, if a task A running on CPU X activates another task B
    548on the same CPU X, and if CPU Y is X's sibling and performing idle,
    549then scheduler migrate task B to CPU Y so that task B can start on
    550CPU Y without waiting task A on CPU X.
    551
    552And if a CPU run out of tasks in its runqueue, the CPU try to pull
    553extra tasks from other busy CPUs to help them before it is going to
    554be idle.
    555
    556Of course it takes some searching cost to find movable tasks and/or
    557idle CPUs, the scheduler might not search all CPUs in the domain
    558every time.  In fact, in some architectures, the searching ranges on
    559events are limited in the same socket or node where the CPU locates,
    560while the load balance on tick searches all.
    561
    562For example, assume CPU Z is relatively far from CPU X.  Even if CPU Z
    563is idle while CPU X and the siblings are busy, scheduler can't migrate
    564woken task B from X to Z since it is out of its searching range.
    565As the result, task B on CPU X need to wait task A or wait load balance
    566on the next tick.  For some applications in special situation, waiting
    5671 tick may be too long.
    568
    569The 'cpuset.sched_relax_domain_level' file allows you to request changing
    570this searching range as you like.  This file takes int value which
    571indicates size of searching range in levels ideally as follows,
    572otherwise initial value -1 that indicates the cpuset has no request.
    573
    574====== ===========================================================
    575  -1   no request. use system default or follow request of others.
    576   0   no search.
    577   1   search siblings (hyperthreads in a core).
    578   2   search cores in a package.
    579   3   search cpus in a node [= system wide on non-NUMA system]
    580   4   search nodes in a chunk of node [on NUMA system]
    581   5   search system wide [on NUMA system]
    582====== ===========================================================
    583
    584The system default is architecture dependent.  The system default
    585can be changed using the relax_domain_level= boot parameter.
    586
    587This file is per-cpuset and affect the sched domain where the cpuset
    588belongs to.  Therefore if the flag 'cpuset.sched_load_balance' of a cpuset
    589is disabled, then 'cpuset.sched_relax_domain_level' have no effect since
    590there is no sched domain belonging the cpuset.
    591
    592If multiple cpusets are overlapping and hence they form a single sched
    593domain, the largest value among those is used.  Be careful, if one
    594requests 0 and others are -1 then 0 is used.
    595
    596Note that modifying this file will have both good and bad effects,
    597and whether it is acceptable or not depends on your situation.
    598Don't modify this file if you are not sure.
    599
    600If your situation is:
    601
    602 - The migration costs between each cpu can be assumed considerably
    603   small(for you) due to your special application's behavior or
    604   special hardware support for CPU cache etc.
    605 - The searching cost doesn't have impact(for you) or you can make
    606   the searching cost enough small by managing cpuset to compact etc.
    607 - The latency is required even it sacrifices cache hit rate etc.
    608   then increasing 'sched_relax_domain_level' would benefit you.
    609
    610
    6111.9 How do I use cpusets ?
    612--------------------------
    613
    614In order to minimize the impact of cpusets on critical kernel
    615code, such as the scheduler, and due to the fact that the kernel
    616does not support one task updating the memory placement of another
    617task directly, the impact on a task of changing its cpuset CPU
    618or Memory Node placement, or of changing to which cpuset a task
    619is attached, is subtle.
    620
    621If a cpuset has its Memory Nodes modified, then for each task attached
    622to that cpuset, the next time that the kernel attempts to allocate
    623a page of memory for that task, the kernel will notice the change
    624in the task's cpuset, and update its per-task memory placement to
    625remain within the new cpusets memory placement.  If the task was using
    626mempolicy MPOL_BIND, and the nodes to which it was bound overlap with
    627its new cpuset, then the task will continue to use whatever subset
    628of MPOL_BIND nodes are still allowed in the new cpuset.  If the task
    629was using MPOL_BIND and now none of its MPOL_BIND nodes are allowed
    630in the new cpuset, then the task will be essentially treated as if it
    631was MPOL_BIND bound to the new cpuset (even though its NUMA placement,
    632as queried by get_mempolicy(), doesn't change).  If a task is moved
    633from one cpuset to another, then the kernel will adjust the task's
    634memory placement, as above, the next time that the kernel attempts
    635to allocate a page of memory for that task.
    636
    637If a cpuset has its 'cpuset.cpus' modified, then each task in that cpuset
    638will have its allowed CPU placement changed immediately.  Similarly,
    639if a task's pid is written to another cpuset's 'tasks' file, then its
    640allowed CPU placement is changed immediately.  If such a task had been
    641bound to some subset of its cpuset using the sched_setaffinity() call,
    642the task will be allowed to run on any CPU allowed in its new cpuset,
    643negating the effect of the prior sched_setaffinity() call.
    644
    645In summary, the memory placement of a task whose cpuset is changed is
    646updated by the kernel, on the next allocation of a page for that task,
    647and the processor placement is updated immediately.
    648
    649Normally, once a page is allocated (given a physical page
    650of main memory) then that page stays on whatever node it
    651was allocated, so long as it remains allocated, even if the
    652cpusets memory placement policy 'cpuset.mems' subsequently changes.
    653If the cpuset flag file 'cpuset.memory_migrate' is set true, then when
    654tasks are attached to that cpuset, any pages that task had
    655allocated to it on nodes in its previous cpuset are migrated
    656to the task's new cpuset. The relative placement of the page within
    657the cpuset is preserved during these migration operations if possible.
    658For example if the page was on the second valid node of the prior cpuset
    659then the page will be placed on the second valid node of the new cpuset.
    660
    661Also if 'cpuset.memory_migrate' is set true, then if that cpuset's
    662'cpuset.mems' file is modified, pages allocated to tasks in that
    663cpuset, that were on nodes in the previous setting of 'cpuset.mems',
    664will be moved to nodes in the new setting of 'mems.'
    665Pages that were not in the task's prior cpuset, or in the cpuset's
    666prior 'cpuset.mems' setting, will not be moved.
    667
    668There is an exception to the above.  If hotplug functionality is used
    669to remove all the CPUs that are currently assigned to a cpuset,
    670then all the tasks in that cpuset will be moved to the nearest ancestor
    671with non-empty cpus.  But the moving of some (or all) tasks might fail if
    672cpuset is bound with another cgroup subsystem which has some restrictions
    673on task attaching.  In this failing case, those tasks will stay
    674in the original cpuset, and the kernel will automatically update
    675their cpus_allowed to allow all online CPUs.  When memory hotplug
    676functionality for removing Memory Nodes is available, a similar exception
    677is expected to apply there as well.  In general, the kernel prefers to
    678violate cpuset placement, over starving a task that has had all
    679its allowed CPUs or Memory Nodes taken offline.
    680
    681There is a second exception to the above.  GFP_ATOMIC requests are
    682kernel internal allocations that must be satisfied, immediately.
    683The kernel may drop some request, in rare cases even panic, if a
    684GFP_ATOMIC alloc fails.  If the request cannot be satisfied within
    685the current task's cpuset, then we relax the cpuset, and look for
    686memory anywhere we can find it.  It's better to violate the cpuset
    687than stress the kernel.
    688
    689To start a new job that is to be contained within a cpuset, the steps are:
    690
    691 1) mkdir /sys/fs/cgroup/cpuset
    692 2) mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpuset
    693 3) Create the new cpuset by doing mkdir's and write's (or echo's) in
    694    the /sys/fs/cgroup/cpuset virtual file system.
    695 4) Start a task that will be the "founding father" of the new job.
    696 5) Attach that task to the new cpuset by writing its pid to the
    697    /sys/fs/cgroup/cpuset tasks file for that cpuset.
    698 6) fork, exec or clone the job tasks from this founding father task.
    699
    700For example, the following sequence of commands will setup a cpuset
    701named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
    702and then start a subshell 'sh' in that cpuset::
    703
    704  mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpuset
    705  cd /sys/fs/cgroup/cpuset
    706  mkdir Charlie
    707  cd Charlie
    708  /bin/echo 2-3 > cpuset.cpus
    709  /bin/echo 1 > cpuset.mems
    710  /bin/echo $$ > tasks
    711  sh
    712  # The subshell 'sh' is now running in cpuset Charlie
    713  # The next line should display '/Charlie'
    714  cat /proc/self/cpuset
    715
    716There are ways to query or modify cpusets:
    717
    718 - via the cpuset file system directly, using the various cd, mkdir, echo,
    719   cat, rmdir commands from the shell, or their equivalent from C.
    720 - via the C library libcpuset.
    721 - via the C library libcgroup.
    722   (http://sourceforge.net/projects/libcg/)
    723 - via the python application cset.
    724   (http://code.google.com/p/cpuset/)
    725
    726The sched_setaffinity calls can also be done at the shell prompt using
    727SGI's runon or Robert Love's taskset.  The mbind and set_mempolicy
    728calls can be done at the shell prompt using the numactl command
    729(part of Andi Kleen's numa package).
    730
    7312. Usage Examples and Syntax
    732============================
    733
    7342.1 Basic Usage
    735---------------
    736
    737Creating, modifying, using the cpusets can be done through the cpuset
    738virtual filesystem.
    739
    740To mount it, type:
    741# mount -t cgroup -o cpuset cpuset /sys/fs/cgroup/cpuset
    742
    743Then under /sys/fs/cgroup/cpuset you can find a tree that corresponds to the
    744tree of the cpusets in the system. For instance, /sys/fs/cgroup/cpuset
    745is the cpuset that holds the whole system.
    746
    747If you want to create a new cpuset under /sys/fs/cgroup/cpuset::
    748
    749  # cd /sys/fs/cgroup/cpuset
    750  # mkdir my_cpuset
    751
    752Now you want to do something with this cpuset::
    753
    754  # cd my_cpuset
    755
    756In this directory you can find several files::
    757
    758  # ls
    759  cgroup.clone_children  cpuset.memory_pressure
    760  cgroup.event_control   cpuset.memory_spread_page
    761  cgroup.procs           cpuset.memory_spread_slab
    762  cpuset.cpu_exclusive   cpuset.mems
    763  cpuset.cpus            cpuset.sched_load_balance
    764  cpuset.mem_exclusive   cpuset.sched_relax_domain_level
    765  cpuset.mem_hardwall    notify_on_release
    766  cpuset.memory_migrate  tasks
    767
    768Reading them will give you information about the state of this cpuset:
    769the CPUs and Memory Nodes it can use, the processes that are using
    770it, its properties.  By writing to these files you can manipulate
    771the cpuset.
    772
    773Set some flags::
    774
    775  # /bin/echo 1 > cpuset.cpu_exclusive
    776
    777Add some cpus::
    778
    779  # /bin/echo 0-7 > cpuset.cpus
    780
    781Add some mems::
    782
    783  # /bin/echo 0-7 > cpuset.mems
    784
    785Now attach your shell to this cpuset::
    786
    787  # /bin/echo $$ > tasks
    788
    789You can also create cpusets inside your cpuset by using mkdir in this
    790directory::
    791
    792  # mkdir my_sub_cs
    793
    794To remove a cpuset, just use rmdir::
    795
    796  # rmdir my_sub_cs
    797
    798This will fail if the cpuset is in use (has cpusets inside, or has
    799processes attached).
    800
    801Note that for legacy reasons, the "cpuset" filesystem exists as a
    802wrapper around the cgroup filesystem.
    803
    804The command::
    805
    806  mount -t cpuset X /sys/fs/cgroup/cpuset
    807
    808is equivalent to::
    809
    810  mount -t cgroup -ocpuset,noprefix X /sys/fs/cgroup/cpuset
    811  echo "/sbin/cpuset_release_agent" > /sys/fs/cgroup/cpuset/release_agent
    812
    8132.2 Adding/removing cpus
    814------------------------
    815
    816This is the syntax to use when writing in the cpus or mems files
    817in cpuset directories::
    818
    819  # /bin/echo 1-4 > cpuset.cpus		-> set cpus list to cpus 1,2,3,4
    820  # /bin/echo 1,2,3,4 > cpuset.cpus	-> set cpus list to cpus 1,2,3,4
    821
    822To add a CPU to a cpuset, write the new list of CPUs including the
    823CPU to be added. To add 6 to the above cpuset::
    824
    825  # /bin/echo 1-4,6 > cpuset.cpus	-> set cpus list to cpus 1,2,3,4,6
    826
    827Similarly to remove a CPU from a cpuset, write the new list of CPUs
    828without the CPU to be removed.
    829
    830To remove all the CPUs::
    831
    832  # /bin/echo "" > cpuset.cpus		-> clear cpus list
    833
    8342.3 Setting flags
    835-----------------
    836
    837The syntax is very simple::
    838
    839  # /bin/echo 1 > cpuset.cpu_exclusive 	-> set flag 'cpuset.cpu_exclusive'
    840  # /bin/echo 0 > cpuset.cpu_exclusive 	-> unset flag 'cpuset.cpu_exclusive'
    841
    8422.4 Attaching processes
    843-----------------------
    844
    845::
    846
    847  # /bin/echo PID > tasks
    848
    849Note that it is PID, not PIDs. You can only attach ONE task at a time.
    850If you have several tasks to attach, you have to do it one after another::
    851
    852  # /bin/echo PID1 > tasks
    853  # /bin/echo PID2 > tasks
    854	...
    855  # /bin/echo PIDn > tasks
    856
    857
    8583. Questions
    859============
    860
    861Q:
    862   what's up with this '/bin/echo' ?
    863
    864A:
    865   bash's builtin 'echo' command does not check calls to write() against
    866   errors. If you use it in the cpuset file system, you won't be
    867   able to tell whether a command succeeded or failed.
    868
    869Q:
    870   When I attach processes, only the first of the line gets really attached !
    871
    872A:
    873   We can only return one error code per call to write(). So you should also
    874   put only ONE pid.
    875
    8764. Contact
    877==========
    878
    879Web: http://www.bullopensource.org/cpuset