linux/Documentation/scheduler/sched-design-CFS.txt
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   1                      =============
   2                      CFS Scheduler
   3                      =============
   4
   5
   61.  OVERVIEW
   7
   8CFS stands for "Completely Fair Scheduler," and is the new "desktop" process
   9scheduler implemented by Ingo Molnar and merged in Linux 2.6.23.  It is the
  10replacement for the previous vanilla scheduler's SCHED_OTHER interactivity
  11code.
  12
  1380% of CFS's design can be summed up in a single sentence: CFS basically models
  14an "ideal, precise multi-tasking CPU" on real hardware.
  15
  16"Ideal multi-tasking CPU" is a (non-existent  :-)) CPU that has 100% physical
  17power and which can run each task at precise equal speed, in parallel, each at
  181/nr_running speed.  For example: if there are 2 tasks running, then it runs
  19each at 50% physical power --- i.e., actually in parallel.
  20
  21On real hardware, we can run only a single task at once, so we have to
  22introduce the concept of "virtual runtime."  The virtual runtime of a task
  23specifies when its next timeslice would start execution on the ideal
  24multi-tasking CPU described above.  In practice, the virtual runtime of a task
  25is its actual runtime normalized to the total number of running tasks.
  26
  27
  28
  292.  FEW IMPLEMENTATION DETAILS
  30
  31In CFS the virtual runtime is expressed and tracked via the per-task
  32p->se.vruntime (nanosec-unit) value.  This way, it's possible to accurately
  33timestamp and measure the "expected CPU time" a task should have gotten.
  34
  35[ small detail: on "ideal" hardware, at any time all tasks would have the same
  36  p->se.vruntime value --- i.e., tasks would execute simultaneously and no task
  37  would ever get "out of balance" from the "ideal" share of CPU time.  ]
  38
  39CFS's task picking logic is based on this p->se.vruntime value and it is thus
  40very simple: it always tries to run the task with the smallest p->se.vruntime
  41value (i.e., the task which executed least so far).  CFS always tries to split
  42up CPU time between runnable tasks as close to "ideal multitasking hardware" as
  43possible.
  44
  45Most of the rest of CFS's design just falls out of this really simple concept,
  46with a few add-on embellishments like nice levels, multiprocessing and various
  47algorithm variants to recognize sleepers.
  48
  49
  50
  513.  THE RBTREE
  52
  53CFS's design is quite radical: it does not use the old data structures for the
  54runqueues, but it uses a time-ordered rbtree to build a "timeline" of future
  55task execution, and thus has no "array switch" artifacts (by which both the
  56previous vanilla scheduler and RSDL/SD are affected).
  57
  58CFS also maintains the rq->cfs.min_vruntime value, which is a monotonic
  59increasing value tracking the smallest vruntime among all tasks in the
  60runqueue.  The total amount of work done by the system is tracked using
  61min_vruntime; that value is used to place newly activated entities on the left
  62side of the tree as much as possible.
  63
  64The total number of running tasks in the runqueue is accounted through the
  65rq->cfs.load value, which is the sum of the weights of the tasks queued on the
  66runqueue.
  67
  68CFS maintains a time-ordered rbtree, where all runnable tasks are sorted by the
  69p->se.vruntime key (there is a subtraction using rq->cfs.min_vruntime to
  70account for possible wraparounds).  CFS picks the "leftmost" task from this
  71tree and sticks to it.
  72As the system progresses forwards, the executed tasks are put into the tree
  73more and more to the right --- slowly but surely giving a chance for every task
  74to become the "leftmost task" and thus get on the CPU within a deterministic
  75amount of time.
  76
  77Summing up, CFS works like this: it runs a task a bit, and when the task
  78schedules (or a scheduler tick happens) the task's CPU usage is "accounted
  79for": the (small) time it just spent using the physical CPU is added to
  80p->se.vruntime.  Once p->se.vruntime gets high enough so that another task
  81becomes the "leftmost task" of the time-ordered rbtree it maintains (plus a
  82small amount of "granularity" distance relative to the leftmost task so that we
  83do not over-schedule tasks and trash the cache), then the new leftmost task is
  84picked and the current task is preempted.
  85
  86
  87
  884.  SOME FEATURES OF CFS
  89
  90CFS uses nanosecond granularity accounting and does not rely on any jiffies or
  91other HZ detail.  Thus the CFS scheduler has no notion of "timeslices" in the
  92way the previous scheduler had, and has no heuristics whatsoever.  There is
  93only one central tunable (you have to switch on CONFIG_SCHED_DEBUG):
  94
  95   /proc/sys/kernel/sched_min_granularity_ns
  96
  97which can be used to tune the scheduler from "desktop" (i.e., low latencies) to
  98"server" (i.e., good batching) workloads.  It defaults to a setting suitable
  99for desktop workloads.  SCHED_BATCH is handled by the CFS scheduler module too.
 100
 101Due to its design, the CFS scheduler is not prone to any of the "attacks" that
 102exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c,
 103chew.c, ring-test.c, massive_intr.c all work fine and do not impact
 104interactivity and produce the expected behavior.
 105
 106The CFS scheduler has a much stronger handling of nice levels and SCHED_BATCH
 107than the previous vanilla scheduler: both types of workloads are isolated much
 108more aggressively.
 109
 110SMP load-balancing has been reworked/sanitized: the runqueue-walking
 111assumptions are gone from the load-balancing code now, and iterators of the
 112scheduling modules are used.  The balancing code got quite a bit simpler as a
 113result.
 114
 115
 116
 1175. Scheduling policies
 118
 119CFS implements three scheduling policies:
 120
 121  - SCHED_NORMAL (traditionally called SCHED_OTHER): The scheduling
 122    policy that is used for regular tasks.
 123
 124  - SCHED_BATCH: Does not preempt nearly as often as regular tasks
 125    would, thereby allowing tasks to run longer and make better use of
 126    caches but at the cost of interactivity. This is well suited for
 127    batch jobs.
 128
 129  - SCHED_IDLE: This is even weaker than nice 19, but its not a true
 130    idle timer scheduler in order to avoid to get into priority
 131    inversion problems which would deadlock the machine.
 132
 133SCHED_FIFO/_RR are implemented in sched_rt.c and are as specified by
 134POSIX.
 135
 136The command chrt from util-linux-ng 2.13.1.1 can set all of these except
 137SCHED_IDLE.
 138
 139
 140
 1416.  SCHEDULING CLASSES
 142
 143The new CFS scheduler has been designed in such a way to introduce "Scheduling
 144Classes," an extensible hierarchy of scheduler modules.  These modules
 145encapsulate scheduling policy details and are handled by the scheduler core
 146without the core code assuming too much about them.
 147
 148sched_fair.c implements the CFS scheduler described above.
 149
 150sched_rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler way than
 151the previous vanilla scheduler did.  It uses 100 runqueues (for all 100 RT
 152priority levels, instead of 140 in the previous scheduler) and it needs no
 153expired array.
 154
 155Scheduling classes are implemented through the sched_class structure, which
 156contains hooks to functions that must be called whenever an interesting event
 157occurs.
 158
 159This is the (partial) list of the hooks:
 160
 161 - enqueue_task(...)
 162
 163   Called when a task enters a runnable state.
 164   It puts the scheduling entity (task) into the red-black tree and
 165   increments the nr_running variable.
 166
 167 - dequeue_tree(...)
 168
 169   When a task is no longer runnable, this function is called to keep the
 170   corresponding scheduling entity out of the red-black tree.  It decrements
 171   the nr_running variable.
 172
 173 - yield_task(...)
 174
 175   This function is basically just a dequeue followed by an enqueue, unless the
 176   compat_yield sysctl is turned on; in that case, it places the scheduling
 177   entity at the right-most end of the red-black tree.
 178
 179 - check_preempt_curr(...)
 180
 181   This function checks if a task that entered the runnable state should
 182   preempt the currently running task.
 183
 184 - pick_next_task(...)
 185
 186   This function chooses the most appropriate task eligible to run next.
 187
 188 - set_curr_task(...)
 189
 190   This function is called when a task changes its scheduling class or changes
 191   its task group.
 192
 193 - task_tick(...)
 194
 195   This function is mostly called from time tick functions; it might lead to
 196   process switch.  This drives the running preemption.
 197
 198 - task_new(...)
 199
 200   The core scheduler gives the scheduling module an opportunity to manage new
 201   task startup.  The CFS scheduling module uses it for group scheduling, while
 202   the scheduling module for a real-time task does not use it.
 203
 204
 205
 2067.  GROUP SCHEDULER EXTENSIONS TO CFS
 207
 208Normally, the scheduler operates on individual tasks and strives to provide
 209fair CPU time to each task.  Sometimes, it may be desirable to group tasks and
 210provide fair CPU time to each such task group.  For example, it may be
 211desirable to first provide fair CPU time to each user on the system and then to
 212each task belonging to a user.
 213
 214CONFIG_GROUP_SCHED strives to achieve exactly that.  It lets tasks to be
 215grouped and divides CPU time fairly among such groups.
 216
 217CONFIG_RT_GROUP_SCHED permits to group real-time (i.e., SCHED_FIFO and
 218SCHED_RR) tasks.
 219
 220CONFIG_FAIR_GROUP_SCHED permits to group CFS (i.e., SCHED_NORMAL and
 221SCHED_BATCH) tasks.
 222
 223At present, there are two (mutually exclusive) mechanisms to group tasks for
 224CPU bandwidth control purposes:
 225
 226 - Based on user id (CONFIG_USER_SCHED)
 227
 228   With this option, tasks are grouped according to their user id.
 229
 230 - Based on "cgroup" pseudo filesystem (CONFIG_CGROUP_SCHED)
 231
 232   This options needs CONFIG_CGROUPS to be defined, and lets the administrator
 233   create arbitrary groups of tasks, using the "cgroup" pseudo filesystem.  See
 234   Documentation/cgroups/cgroups.txt for more information about this filesystem.
 235
 236Only one of these options to group tasks can be chosen and not both.
 237
 238When CONFIG_USER_SCHED is defined, a directory is created in sysfs for each new
 239user and a "cpu_share" file is added in that directory.
 240
 241        # cd /sys/kernel/uids
 242        # cat 512/cpu_share             # Display user 512's CPU share
 243        1024
 244        # echo 2048 > 512/cpu_share     # Modify user 512's CPU share
 245        # cat 512/cpu_share             # Display user 512's CPU share
 246        2048
 247        #
 248
 249CPU bandwidth between two users is divided in the ratio of their CPU shares.
 250For example: if you would like user "root" to get twice the bandwidth of user
 251"guest," then set the cpu_share for both the users such that "root"'s cpu_share
 252is twice "guest"'s cpu_share.
 253
 254When CONFIG_CGROUP_SCHED is defined, a "cpu.shares" file is created for each
 255group created using the pseudo filesystem.  See example steps below to create
 256task groups and modify their CPU share using the "cgroups" pseudo filesystem.
 257
 258        # mkdir /dev/cpuctl
 259        # mount -t cgroup -ocpu none /dev/cpuctl
 260        # cd /dev/cpuctl
 261
 262        # mkdir multimedia      # create "multimedia" group of tasks
 263        # mkdir browser         # create "browser" group of tasks
 264
 265        # #Configure the multimedia group to receive twice the CPU bandwidth
 266        # #that of browser group
 267
 268        # echo 2048 > multimedia/cpu.shares
 269        # echo 1024 > browser/cpu.shares
 270
 271        # firefox &     # Launch firefox and move it to "browser" group
 272        # echo <firefox_pid> > browser/tasks
 273
 274        # #Launch gmplayer (or your favourite movie player)
 275        # echo <movie_player_pid> > multimedia/tasks
 276
 2778. Implementation note: user namespaces
 278
 279User namespaces are intended to be hierarchical.  But they are currently
 280only partially implemented.  Each of those has ramifications for CFS.
 281
 282First, since user namespaces are hierarchical, the /sys/kernel/uids
 283presentation is inadequate.  Eventually we will likely want to use sysfs
 284tagging to provide private views of /sys/kernel/uids within each user
 285namespace.
 286
 287Second, the hierarchical nature is intended to support completely
 288unprivileged use of user namespaces.  So if using user groups, then
 289we want the users in a user namespace to be children of the user
 290who created it.
 291
 292That is currently unimplemented.  So instead, every user in a new
 293user namespace will receive 1024 shares just like any user in the
 294initial user namespace.  Note that at the moment creation of a new
 295user namespace requires each of CAP_SYS_ADMIN, CAP_SETUID, and
 296CAP_SETGID.
 297
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