Documentation / scheduler / sched-rt-group.rst


Based on kernel version 6.9. Page generated on 2024-05-14 10:02 EST.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187
==========================
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
---------------

Real-time 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 real-time tasks, each group must be assigned a fixed portion
of the CPU time available.  Without a minimum guarantee a real-time 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 real-time group which
the other real-time groups will not be permitted to use.

Any time not allocated to a real-time 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 real-time 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
real-time 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 real-time scheduling may use. This is always
  less or equal to the period_us, as it denotes the time allocated from the
  period_us for the real-time tasks. Even without CONFIG_RT_GROUP_SCHED enabled,
  this will limit time reserved to real-time processes. With
  CONFIG_RT_GROUP_SCHED=y it signifies the total bandwidth available to all
  real-time 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 sched_rt_period_us.
  * 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
real-time 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.

Real-time group scheduling means you have to assign a portion of total CPU
bandwidth to the group before it will accept real-time tasks. Therefore you will
not be able to run real-time tasks as any user other than root until you have
done that, even if the user has the rights to run processes with real-time
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/admin-guide/cgroup-v1/cgroups.rst 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=50000us

	- this runs for 0.05s once every 0.1s

* group B: period= 50000us, runtime=25000us

	- this runs for 0.025s 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.