linux/Documentation/scheduler/sched-rt-group.txt
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   1                                Real-Time group scheduling
   2                                --------------------------
   3
   4CONTENTS
   5========
   6
   70. WARNING
   81. Overview
   9  1.1 The problem
  10  1.2 The solution
  112. The interface
  12  2.1 System-wide settings
  13  2.2 Default behaviour
  14  2.3 Basis for grouping tasks
  153. Future plans
  16
  17
  180. WARNING
  19==========
  20
  21 Fiddling with these settings can result in an unstable system, the knobs are
  22 root only and assumes root knows what he is doing.
  23
  24Most notable:
  25
  26 * very small values in sched_rt_period_us can result in an unstable
  27   system when the period is smaller than either the available hrtimer
  28   resolution, or the time it takes to handle the budget refresh itself.
  29
  30 * very small values in sched_rt_runtime_us can result in an unstable
  31   system when the runtime is so small the system has difficulty making
  32   forward progress (NOTE: the migration thread and kstopmachine both
  33   are real-time processes).
  34
  351. Overview
  36===========
  37
  38
  391.1 The problem
  40---------------
  41
  42Realtime scheduling is all about determinism, a group has to be able to rely on
  43the amount of bandwidth (eg. CPU time) being constant. In order to schedule
  44multiple groups of realtime tasks, each group must be assigned a fixed portion
  45of the CPU time available.  Without a minimum guarantee a realtime group can
  46obviously fall short. A fuzzy upper limit is of no use since it cannot be
  47relied upon. Which leaves us with just the single fixed portion.
  48
  491.2 The solution
  50----------------
  51
  52CPU time is divided by means of specifying how much time can be spent running
  53in a given period. We allocate this "run time" for each realtime group which
  54the other realtime groups will not be permitted to use.
  55
  56Any time not allocated to a realtime group will be used to run normal priority
  57tasks (SCHED_OTHER). Any allocated run time not used will also be picked up by
  58SCHED_OTHER.
  59
  60Let's consider an example: a frame fixed realtime renderer must deliver 25
  61frames a second, which yields a period of 0.04s per frame. Now say it will also
  62have to play some music and respond to input, leaving it with around 80% CPU
  63time dedicated for the graphics. We can then give this group a run time of 0.8
  64* 0.04s = 0.032s.
  65
  66This way the graphics group will have a 0.04s period with a 0.032s run time
  67limit. Now if the audio thread needs to refill the DMA buffer every 0.005s, but
  68needs only about 3% CPU time to do so, it can do with a 0.03 * 0.005s =
  690.00015s. So this group can be scheduled with a period of 0.005s and a run time
  70of 0.00015s.
  71
  72The remaining CPU time will be used for user input and other tasks. Because
  73realtime tasks have explicitly allocated the CPU time they need to perform
  74their tasks, buffer underruns in the graphics or audio can be eliminated.
  75
  76NOTE: the above example is not fully implemented yet. We still
  77lack an EDF scheduler to make non-uniform periods usable.
  78
  79
  802. The Interface
  81================
  82
  83
  842.1 System wide settings
  85------------------------
  86
  87The system wide settings are configured under the /proc virtual file system:
  88
  89/proc/sys/kernel/sched_rt_period_us:
  90  The scheduling period that is equivalent to 100% CPU bandwidth
  91
  92/proc/sys/kernel/sched_rt_runtime_us:
  93  A global limit on how much time realtime scheduling may use.  Even without
  94  CONFIG_RT_GROUP_SCHED enabled, this will limit time reserved to realtime
  95  processes. With CONFIG_RT_GROUP_SCHED it signifies the total bandwidth
  96  available to all realtime groups.
  97
  98  * Time is specified in us because the interface is s32. This gives an
  99    operating range from 1us to about 35 minutes.
 100  * sched_rt_period_us takes values from 1 to INT_MAX.
 101  * sched_rt_runtime_us takes values from -1 to (INT_MAX - 1).
 102  * A run time of -1 specifies runtime == period, ie. no limit.
 103
 104
 1052.2 Default behaviour
 106---------------------
 107
 108The default values for sched_rt_period_us (1000000 or 1s) and
 109sched_rt_runtime_us (950000 or 0.95s).  This gives 0.05s to be used by
 110SCHED_OTHER (non-RT tasks). These defaults were chosen so that a run-away
 111realtime tasks will not lock up the machine but leave a little time to recover
 112it.  By setting runtime to -1 you'd get the old behaviour back.
 113
 114By default all bandwidth is assigned to the root group and new groups get the
 115period from /proc/sys/kernel/sched_rt_period_us and a run time of 0. If you
 116want to assign bandwidth to another group, reduce the root group's bandwidth
 117and assign some or all of the difference to another group.
 118
 119Realtime group scheduling means you have to assign a portion of total CPU
 120bandwidth to the group before it will accept realtime tasks. Therefore you will
 121not be able to run realtime tasks as any user other than root until you have
 122done that, even if the user has the rights to run processes with realtime
 123priority!
 124
 125
 1262.3 Basis for grouping tasks
 127----------------------------
 128
 129Enabling CONFIG_RT_GROUP_SCHED lets you explicitly allocate real
 130CPU bandwidth to task groups.
 131
 132This uses the cgroup virtual file system and "<cgroup>/cpu.rt_runtime_us"
 133to control the CPU time reserved for each control group.
 134
 135For more information on working with control groups, you should read
 136Documentation/cgroups/cgroups.txt as well.
 137
 138Group settings are checked against the following limits in order to keep the
 139configuration schedulable:
 140
 141   \Sum_{i} runtime_{i} / global_period <= global_runtime / global_period
 142
 143For now, this can be simplified to just the following (but see Future plans):
 144
 145   \Sum_{i} runtime_{i} <= global_runtime
 146
 147
 1483. Future plans
 149===============
 150
 151There is work in progress to make the scheduling period for each group
 152("<cgroup>/cpu.rt_period_us") configurable as well.
 153
 154The constraint on the period is that a subgroup must have a smaller or
 155equal period to its parent. But realistically its not very useful _yet_
 156as its prone to starvation without deadline scheduling.
 157
 158Consider two sibling groups A and B; both have 50% bandwidth, but A's
 159period is twice the length of B's.
 160
 161* group A: period=100000us, runtime=10000us
 162        - this runs for 0.01s once every 0.1s
 163
 164* group B: period= 50000us, runtime=10000us
 165        - this runs for 0.01s twice every 0.1s (or once every 0.05 sec).
 166
 167This means that currently a while (1) loop in A will run for the full period of
 168B and can starve B's tasks (assuming they are of lower priority) for a whole
 169period.
 170
 171The next project will be SCHED_EDF (Earliest Deadline First scheduling) to bring
 172full deadline scheduling to the linux kernel. Deadline scheduling the above
 173groups and treating end of the period as a deadline will ensure that they both
 174get their allocated time.
 175
 176Implementing SCHED_EDF might take a while to complete. Priority Inheritance is
 177the biggest challenge as the current linux PI infrastructure is geared towards
 178the limited static priority levels 0-99. With deadline scheduling you need to
 179do deadline inheritance (since priority is inversely proportional to the
 180deadline delta (deadline - now)).
 181
 182This means the whole PI machinery will have to be reworked - and that is one of
 183the most complex pieces of code we have.
 184
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