1==========================
2Real-Time group scheduling
3==========================
4
5.. CONTENTS
6
7   0. WARNING
8   1. Overview
9     1.1 The problem
10     1.2 The solution
11   2. The interface
12     2.1 System-wide settings
13     2.2 Default behaviour
14     2.3 Basis for grouping tasks
15   3. 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/admin-guide/cgroup-v1/cgroups.rst 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=50000us
162
163	- this runs for 0.05s once every 0.1s
164
165* group B: period= 50000us, runtime=25000us
166
167	- this runs for 0.025s twice every 0.1s (or once every 0.05 sec).
168
169This means that currently a while (1) loop in A will run for the full period of
170B and can starve B's tasks (assuming they are of lower priority) for a whole
171period.
172
173The next project will be SCHED_EDF (Earliest Deadline First scheduling) to bring
174full deadline scheduling to the linux kernel. Deadline scheduling the above
175groups and treating end of the period as a deadline will ensure that they both
176get their allocated time.
177
178Implementing SCHED_EDF might take a while to complete. Priority Inheritance is
179the biggest challenge as the current linux PI infrastructure is geared towards
180the limited static priority levels 0-99. With deadline scheduling you need to
181do deadline inheritance (since priority is inversely proportional to the
182deadline delta (deadline - now)).
183
184This means the whole PI machinery will have to be reworked - and that is one of
185the most complex pieces of code we have.
186