Real-Time group scheduling
  				--------------------------

  CONTENTS
  ========

  0. WARNING

  1. Overview
    1.1 The problem
    1.2 The solution
  2. The interface
    2.1 System-wide settings
    2.2 Default behaviour
    2.3 Basis for grouping tasks
  3. Future plans

  0. WARNING
  ==========
  
   Fiddling with these settings can result in an unstable system, the knobs are
   root only and assumes root knows what he is doing.
  
  Most notable:
  
   * very small values in sched_rt_period_us can result in an unstable
     system when the period is smaller than either the available hrtimer
     resolution, or the time it takes to handle the budget refresh itself.
  
   * very small values in sched_rt_runtime_us can result in an unstable
     system when the runtime is so small the system has difficulty making
     forward progress (NOTE: the migration thread and kstopmachine both
     are real-time processes).

  1. Overview
  ===========

  1.1 The problem
  ---------------

  Realtime scheduling is all about determinism, a group has to be able to rely on
  the amount of bandwidth (eg. CPU time) being constant. In order to schedule
  multiple groups of realtime tasks, each group must be assigned a fixed portion
  of the CPU time available.  Without a minimum guarantee a realtime group can
  obviously fall short. A fuzzy upper limit is of no use since it cannot be
  relied upon. Which leaves us with just the single fixed portion.

  1.2 The solution
  ----------------

  CPU time is divided by means of specifying how much time can be spent running
  in a given period. We allocate this "run time" for each realtime group which
  the other realtime groups will not be permitted to use.

  Any time not allocated to a realtime group will be used to run normal priority
  tasks (SCHED_OTHER). Any allocated run time not used will also be picked up by
  SCHED_OTHER.

  Let's consider an example: a frame fixed realtime renderer must deliver 25
  frames a second, which yields a period of 0.04s per frame. Now say it will also
  have to play some music and respond to input, leaving it with around 80% CPU
  time dedicated for the graphics. We can then give this group a run time of 0.8
  * 0.04s = 0.032s.

  This way the graphics group will have a 0.04s period with a 0.032s run time
  limit. Now if the audio thread needs to refill the DMA buffer every 0.005s, but
  needs only about 3% CPU time to do so, it can do with a 0.03 * 0.005s =
  0.00015s. So this group can be scheduled with a period of 0.005s and a run time
  of 0.00015s.

  The remaining CPU time will be used for user input and other tasks. Because

  realtime tasks have explicitly allocated the CPU time they need to perform

  their tasks, buffer underruns in the graphics or audio can be eliminated.

  NOTE: the above example is not fully implemented yet. We still

  lack an EDF scheduler to make non-uniform periods usable.

  2. The Interface
  ================

  2.1 System wide settings
  ------------------------

  The system wide settings are configured under the /proc virtual file system:

  /proc/sys/kernel/sched_rt_period_us:
    The scheduling period that is equivalent to 100% CPU bandwidth

  /proc/sys/kernel/sched_rt_runtime_us:
    A global limit on how much time realtime scheduling may use.  Even without
    CONFIG_RT_GROUP_SCHED enabled, this will limit time reserved to realtime
    processes. With CONFIG_RT_GROUP_SCHED it signifies the total bandwidth
    available to all realtime groups.
  
    * Time is specified in us because the interface is s32. This gives an
      operating range from 1us to about 35 minutes.
    * sched_rt_period_us takes values from 1 to INT_MAX.
    * sched_rt_runtime_us takes values from -1 to (INT_MAX - 1).
    * A run time of -1 specifies runtime == period, ie. no limit.
  
  
  2.2 Default behaviour
  ---------------------
  
  The default values for sched_rt_period_us (1000000 or 1s) and
  sched_rt_runtime_us (950000 or 0.95s).  This gives 0.05s to be used by
  SCHED_OTHER (non-RT tasks). These defaults were chosen so that a run-away
  realtime tasks will not lock up the machine but leave a little time to recover
  it.  By setting runtime to -1 you'd get the old behaviour back.
  
  By default all bandwidth is assigned to the root group and new groups get the
  period from /proc/sys/kernel/sched_rt_period_us and a run time of 0. If you
  want to assign bandwidth to another group, reduce the root group's bandwidth
  and assign some or all of the difference to another group.
  
  Realtime group scheduling means you have to assign a portion of total CPU
  bandwidth to the group before it will accept realtime tasks. Therefore you will
  not be able to run realtime tasks as any user other than root until you have
  done that, even if the user has the rights to run processes with realtime
  priority!
  
  
  2.3 Basis for grouping tasks
  ----------------------------

  Enabling CONFIG_RT_GROUP_SCHED lets you explicitly allocate real
  CPU bandwidth to task groups.

  This uses the cgroup virtual file system and "<cgroup>/cpu.rt_runtime_us"
  to control the CPU time reserved for each control group.

  
  For more information on working with control groups, you should read

  Documentation/cgroup-v1/cgroups.txt as well.

  Group settings are checked against the following limits in order to keep the
  configuration schedulable:

  
     \Sum_{i} runtime_{i} / global_period <= global_runtime / global_period

  For now, this can be simplified to just the following (but see Future plans):
  
     \Sum_{i} runtime_{i} <= global_runtime
  
  
  3. Future plans
  ===============
  
  There is work in progress to make the scheduling period for each group

  ("<cgroup>/cpu.rt_period_us") configurable as well.

  
  The constraint on the period is that a subgroup must have a smaller or
  equal period to its parent. But realistically its not very useful _yet_
  as its prone to starvation without deadline scheduling.
  
  Consider two sibling groups A and B; both have 50% bandwidth, but A's
  period is twice the length of B's.
  
  * group A: period=100000us, runtime=10000us
  	- this runs for 0.01s once every 0.1s
  
  * group B: period= 50000us, runtime=10000us
  	- this runs for 0.01s twice every 0.1s (or once every 0.05 sec).
  
  This means that currently a while (1) loop in A will run for the full period of
  B and can starve B's tasks (assuming they are of lower priority) for a whole
  period.
  
  The next project will be SCHED_EDF (Earliest Deadline First scheduling) to bring
  full deadline scheduling to the linux kernel. Deadline scheduling the above
  groups and treating end of the period as a deadline will ensure that they both
  get their allocated time.
  
  Implementing SCHED_EDF might take a while to complete. Priority Inheritance is
  the biggest challenge as the current linux PI infrastructure is geared towards

  the limited static priority levels 0-99. With deadline scheduling you need to

  do deadline inheritance (since priority is inversely proportional to the

  deadline delta (deadline - now)).

  
  This means the whole PI machinery will have to be reworked - and that is one of
  the most complex pieces of code we have.