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Documentation / scheduler / sched-rt-group.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1					Real-Time group scheduling
2					--------------------------
3	
4	CONTENTS
5	========
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	
18	0. 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	
24	Most 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	
35	1. Overview
36	===========
37	
38	
39	1.1 The problem
40	---------------
41	
42	Realtime scheduling is all about determinism, a group has to be able to rely on
43	the amount of bandwidth (eg. CPU time) being constant. In order to schedule
44	multiple groups of realtime tasks, each group must be assigned a fixed portion
45	of the CPU time available.  Without a minimum guarantee a realtime group can
46	obviously fall short. A fuzzy upper limit is of no use since it cannot be
47	relied upon. Which leaves us with just the single fixed portion.
48	
49	1.2 The solution
50	----------------
51	
52	CPU time is divided by means of specifying how much time can be spent running
53	in a given period. We allocate this "run time" for each realtime group which
54	the other realtime groups will not be permitted to use.
55	
56	Any time not allocated to a realtime group will be used to run normal priority
57	tasks (SCHED_OTHER). Any allocated run time not used will also be picked up by
58	SCHED_OTHER.
59	
60	Let's consider an example: a frame fixed realtime renderer must deliver 25
61	frames a second, which yields a period of 0.04s per frame. Now say it will also
62	have to play some music and respond to input, leaving it with around 80% CPU
63	time dedicated for the graphics. We can then give this group a run time of 0.8
64	* 0.04s = 0.032s.
65	
66	This way the graphics group will have a 0.04s period with a 0.032s run time
67	limit. Now if the audio thread needs to refill the DMA buffer every 0.005s, but
68	needs only about 3% CPU time to do so, it can do with a 0.03 * 0.005s =
69	0.00015s. So this group can be scheduled with a period of 0.005s and a run time
70	of 0.00015s.
71	
72	The remaining CPU time will be used for user input and other tasks. Because
73	realtime tasks have explicitly allocated the CPU time they need to perform
74	their tasks, buffer underruns in the graphics or audio can be eliminated.
75	
76	NOTE: the above example is not fully implemented yet. We still
77	lack an EDF scheduler to make non-uniform periods usable.
78	
79	
80	2. The Interface
81	================
82	
83	
84	2.1 System wide settings
85	------------------------
86	
87	The 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	
105	2.2 Default behaviour
106	---------------------
107	
108	The default values for sched_rt_period_us (1000000 or 1s) and
109	sched_rt_runtime_us (950000 or 0.95s).  This gives 0.05s to be used by
110	SCHED_OTHER (non-RT tasks). These defaults were chosen so that a run-away
111	realtime tasks will not lock up the machine but leave a little time to recover
112	it.  By setting runtime to -1 you'd get the old behaviour back.
113	
114	By default all bandwidth is assigned to the root group and new groups get the
115	period from /proc/sys/kernel/sched_rt_period_us and a run time of 0. If you
116	want to assign bandwidth to another group, reduce the root group's bandwidth
117	and assign some or all of the difference to another group.
118	
119	Realtime group scheduling means you have to assign a portion of total CPU
120	bandwidth to the group before it will accept realtime tasks. Therefore you will
121	not be able to run realtime tasks as any user other than root until you have
122	done that, even if the user has the rights to run processes with realtime
123	priority!
124	
125	
126	2.3 Basis for grouping tasks
127	----------------------------
128	
129	Enabling CONFIG_RT_GROUP_SCHED lets you explicitly allocate real
130	CPU bandwidth to task groups.
131	
132	This uses the cgroup virtual file system and "<cgroup>/cpu.rt_runtime_us"
133	to control the CPU time reserved for each control group.
134	
135	For more information on working with control groups, you should read
136	Documentation/cgroups/cgroups.txt as well.
137	
138	Group settings are checked against the following limits in order to keep the
139	configuration schedulable:
140	
141	   \Sum_{i} runtime_{i} / global_period <= global_runtime / global_period
142	
143	For now, this can be simplified to just the following (but see Future plans):
144	
145	   \Sum_{i} runtime_{i} <= global_runtime
146	
147	
148	3. Future plans
149	===============
150	
151	There is work in progress to make the scheduling period for each group
152	("<cgroup>/cpu.rt_period_us") configurable as well.
153	
154	The constraint on the period is that a subgroup must have a smaller or
155	equal period to its parent. But realistically its not very useful _yet_
156	as its prone to starvation without deadline scheduling.
157	
158	Consider two sibling groups A and B; both have 50% bandwidth, but A's
159	period 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	
167	This means that currently a while (1) loop in A will run for the full period of
168	B and can starve B's tasks (assuming they are of lower priority) for a whole
169	period.
170	
171	The next project will be SCHED_EDF (Earliest Deadline First scheduling) to bring
172	full deadline scheduling to the linux kernel. Deadline scheduling the above
173	groups and treating end of the period as a deadline will ensure that they both
174	get their allocated time.
175	
176	Implementing SCHED_EDF might take a while to complete. Priority Inheritance is
177	the biggest challenge as the current linux PI infrastructure is geared towards
178	the limited static priority levels 0-99. With deadline scheduling you need to
179	do deadline inheritance (since priority is inversely proportional to the
180	deadline delta (deadline - now)).
181	
182	This means the whole PI machinery will have to be reworked - and that is one of
183	the most complex pieces of code we have.
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