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1 | Real-Time group scheduling |
2 | -------------------------- | |
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4 | CONTENTS |
5 | ======== | |
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7 | 1. Overview |
8 | 1.1 The problem | |
9 | 1.2 The solution | |
10 | 2. The interface | |
11 | 2.1 System-wide settings | |
12 | 2.2 Default behaviour | |
13 | 2.3 Basis for grouping tasks | |
14 | 3. Future plans | |
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17 | 1. Overview |
18 | =========== | |
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21 | 1.1 The problem |
22 | --------------- | |
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24 | Realtime scheduling is all about determinism, a group has to be able to rely on |
25 | the amount of bandwidth (eg. CPU time) being constant. In order to schedule | |
26 | multiple groups of realtime tasks, each group must be assigned a fixed portion | |
27 | of the CPU time available. Without a minimum guarantee a realtime group can | |
28 | obviously fall short. A fuzzy upper limit is of no use since it cannot be | |
29 | relied upon. Which leaves us with just the single fixed portion. | |
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31 | 1.2 The solution |
32 | ---------------- | |
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34 | CPU time is divided by means of specifying how much time can be spent running |
35 | in a given period. We allocate this "run time" for each realtime group which | |
36 | the other realtime groups will not be permitted to use. | |
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38 | Any time not allocated to a realtime group will be used to run normal priority |
39 | tasks (SCHED_OTHER). Any allocated run time not used will also be picked up by | |
40 | SCHED_OTHER. | |
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42 | Let's consider an example: a frame fixed realtime renderer must deliver 25 |
43 | frames a second, which yields a period of 0.04s per frame. Now say it will also | |
44 | have to play some music and respond to input, leaving it with around 80% CPU | |
45 | time dedicated for the graphics. We can then give this group a run time of 0.8 | |
46 | * 0.04s = 0.032s. | |
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48 | This way the graphics group will have a 0.04s period with a 0.032s run time |
49 | limit. Now if the audio thread needs to refill the DMA buffer every 0.005s, but | |
50 | needs only about 3% CPU time to do so, it can do with a 0.03 * 0.005s = | |
51 | 0.00015s. So this group can be scheduled with a period of 0.005s and a run time | |
52 | of 0.00015s. | |
9f0c1e56 | 53 | |
f7d62364 | 54 | The remaining CPU time will be used for user input and other tasks. Because |
b9b158fe | 55 | realtime tasks have explicitly allocated the CPU time they need to perform |
f7d62364 | 56 | their tasks, buffer underruns in the graphics or audio can be eliminated. |
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58 | NOTE: the above example is not fully implemented as of yet (2.6.25). We still |
59 | lack an EDF scheduler to make non-uniform periods usable. | |
9f0c1e56 | 60 | |
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62 | 2. The Interface |
63 | ================ | |
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66 | 2.1 System wide settings |
67 | ------------------------ | |
9f0c1e56 | 68 | |
b9b158fe | 69 | The system wide settings are configured under the /proc virtual file system: |
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71 | /proc/sys/kernel/sched_rt_period_us: |
72 | The scheduling period that is equivalent to 100% CPU bandwidth | |
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74 | /proc/sys/kernel/sched_rt_runtime_us: |
75 | A global limit on how much time realtime scheduling may use. Even without | |
76 | CONFIG_RT_GROUP_SCHED enabled, this will limit time reserved to realtime | |
77 | processes. With CONFIG_RT_GROUP_SCHED it signifies the total bandwidth | |
78 | available to all realtime groups. | |
79 | ||
80 | * Time is specified in us because the interface is s32. This gives an | |
81 | operating range from 1us to about 35 minutes. | |
82 | * sched_rt_period_us takes values from 1 to INT_MAX. | |
83 | * sched_rt_runtime_us takes values from -1 to (INT_MAX - 1). | |
84 | * A run time of -1 specifies runtime == period, ie. no limit. | |
85 | ||
86 | ||
87 | 2.2 Default behaviour | |
88 | --------------------- | |
89 | ||
90 | The default values for sched_rt_period_us (1000000 or 1s) and | |
91 | sched_rt_runtime_us (950000 or 0.95s). This gives 0.05s to be used by | |
92 | SCHED_OTHER (non-RT tasks). These defaults were chosen so that a run-away | |
93 | realtime tasks will not lock up the machine but leave a little time to recover | |
94 | it. By setting runtime to -1 you'd get the old behaviour back. | |
95 | ||
96 | By default all bandwidth is assigned to the root group and new groups get the | |
97 | period from /proc/sys/kernel/sched_rt_period_us and a run time of 0. If you | |
98 | want to assign bandwidth to another group, reduce the root group's bandwidth | |
99 | and assign some or all of the difference to another group. | |
100 | ||
101 | Realtime group scheduling means you have to assign a portion of total CPU | |
102 | bandwidth to the group before it will accept realtime tasks. Therefore you will | |
103 | not be able to run realtime tasks as any user other than root until you have | |
104 | done that, even if the user has the rights to run processes with realtime | |
105 | priority! | |
106 | ||
107 | ||
108 | 2.3 Basis for grouping tasks | |
109 | ---------------------------- | |
110 | ||
111 | There are two compile-time settings for allocating CPU bandwidth. These are | |
112 | configured using the "Basis for grouping tasks" multiple choice menu under | |
113 | General setup > Group CPU Scheduler: | |
114 | ||
115 | a. CONFIG_USER_SCHED (aka "Basis for grouping tasks" = "user id") | |
116 | ||
117 | This lets you use the virtual files under | |
118 | "/sys/kernel/uids/<uid>/cpu_rt_runtime_us" to control he CPU time reserved for | |
119 | each user . | |
120 | ||
121 | The other option is: | |
122 | ||
123 | .o CONFIG_CGROUP_SCHED (aka "Basis for grouping tasks" = "Control groups") | |
124 | ||
125 | This uses the /cgroup virtual file system and "/cgroup/<cgroup>/cpu.rt_runtime_us" | |
126 | to control the CPU time reserved for each control group instead. | |
127 | ||
128 | For more information on working with control groups, you should read | |
129 | Documentation/cgroups.txt as well. | |
130 | ||
131 | Group settings are checked against the following limits in order to keep the configuration | |
132 | schedulable: | |
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133 | |
134 | \Sum_{i} runtime_{i} / global_period <= global_runtime / global_period | |
135 | ||
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136 | For now, this can be simplified to just the following (but see Future plans): |
137 | ||
138 | \Sum_{i} runtime_{i} <= global_runtime | |
139 | ||
140 | ||
141 | 3. Future plans | |
142 | =============== | |
143 | ||
144 | There is work in progress to make the scheduling period for each group | |
145 | ("/sys/kernel/uids/<uid>/cpu_rt_period_us" or | |
146 | "/cgroup/<cgroup>/cpu.rt_period_us" respectively) configurable as well. | |
147 | ||
148 | The constraint on the period is that a subgroup must have a smaller or | |
149 | equal period to its parent. But realistically its not very useful _yet_ | |
150 | as its prone to starvation without deadline scheduling. | |
151 | ||
152 | Consider two sibling groups A and B; both have 50% bandwidth, but A's | |
153 | period is twice the length of B's. | |
154 | ||
155 | * group A: period=100000us, runtime=10000us | |
156 | - this runs for 0.01s once every 0.1s | |
157 | ||
158 | * group B: period= 50000us, runtime=10000us | |
159 | - this runs for 0.01s twice every 0.1s (or once every 0.05 sec). | |
160 | ||
161 | This means that currently a while (1) loop in A will run for the full period of | |
162 | B and can starve B's tasks (assuming they are of lower priority) for a whole | |
163 | period. | |
164 | ||
165 | The next project will be SCHED_EDF (Earliest Deadline First scheduling) to bring | |
166 | full deadline scheduling to the linux kernel. Deadline scheduling the above | |
167 | groups and treating end of the period as a deadline will ensure that they both | |
168 | get their allocated time. | |
169 | ||
170 | Implementing SCHED_EDF might take a while to complete. Priority Inheritance is | |
171 | the biggest challenge as the current linux PI infrastructure is geared towards | |
172 | the limited static priority levels 0-139. With deadline scheduling you need to | |
173 | do deadline inheritance (since priority is inversely proportional to the | |
174 | deadline delta (deadline - now). | |
175 | ||
176 | This means the whole PI machinery will have to be reworked - and that is one of | |
177 | the most complex pieces of code we have. |