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Documentation / scheduler / sched-deadline.txt

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

1				  Deadline Task Scheduling
2				  ------------------------
5	========
8	 1. Overview
9	 2. Scheduling algorithm
10	 3. Scheduling Real-Time Tasks
11	 4. Bandwidth management
12	   4.1 System-wide settings
13	   4.2 Task interface
14	   4.3 Default behavior
15	 5. Tasks CPU affinity
16	   5.1 SCHED_DEADLINE and cpusets HOWTO
17	 6. Future plans
21	==========
23	 Fiddling with these settings can result in an unpredictable or even unstable
24	 system behavior. As for -rt (group) scheduling, it is assumed that root users
25	 know what they're doing.
28	1. Overview
29	===========
31	 The SCHED_DEADLINE policy contained inside the sched_dl scheduling class is
32	 basically an implementation of the Earliest Deadline First (EDF) scheduling
33	 algorithm, augmented with a mechanism (called Constant Bandwidth Server, CBS)
34	 that makes it possible to isolate the behavior of tasks between each other.
37	2. Scheduling algorithm
38	==================
40	 SCHED_DEADLINE uses three parameters, named "runtime", "period", and
41	 "deadline" to schedule tasks. A SCHED_DEADLINE task is guaranteed to receive
42	 "runtime" microseconds of execution time every "period" microseconds, and
43	 these "runtime" microseconds are available within "deadline" microseconds
44	 from the beginning of the period.  In order to implement this behaviour,
45	 every time the task wakes up, the scheduler computes a "scheduling deadline"
46	 consistent with the guarantee (using the CBS[2,3] algorithm). Tasks are then
47	 scheduled using EDF[1] on these scheduling deadlines (the task with the
48	 smallest scheduling deadline is selected for execution). Notice that this
49	 guaranteed is respected if a proper "admission control" strategy (see Section
50	 "4. Bandwidth management") is used.
52	 Summing up, the CBS[2,3] algorithms assigns scheduling deadlines to tasks so
53	 that each task runs for at most its runtime every period, avoiding any
54	 interference between different tasks (bandwidth isolation), while the EDF[1]
55	 algorithm selects the task with the smallest scheduling deadline as the one
56	 to be executed first.  Thanks to this feature, also tasks that do not
57	 strictly comply with the "traditional" real-time task model (see Section 3)
58	 can effectively use the new policy.
60	 In more details, the CBS algorithm assigns scheduling deadlines to
61	 tasks in the following way:
63	  - Each SCHED_DEADLINE task is characterised by the "runtime",
64	    "deadline", and "period" parameters;
66	  - The state of the task is described by a "scheduling deadline", and
67	    a "current runtime". These two parameters are initially set to 0;
69	  - When a SCHED_DEADLINE task wakes up (becomes ready for execution),
70	    the scheduler checks if
72	                    current runtime                runtime
73	         ---------------------------------- > ----------------
74	         scheduling deadline - current time         period
76	    then, if the scheduling deadline is smaller than the current time, or
77	    this condition is verified, the scheduling deadline and the
78	    current budget are re-initialised as
80	         scheduling deadline = current time + deadline
81	         current runtime = runtime
83	    otherwise, the scheduling deadline and the current runtime are
84	    left unchanged;
86	  - When a SCHED_DEADLINE task executes for an amount of time t, its
87	    current runtime is decreased as
89	         current runtime = current runtime - t
91	    (technically, the runtime is decreased at every tick, or when the
92	    task is descheduled / preempted);
94	  - When the current runtime becomes less or equal than 0, the task is
95	    said to be "throttled" (also known as "depleted" in real-time literature)
96	    and cannot be scheduled until its scheduling deadline. The "replenishment
97	    time" for this task (see next item) is set to be equal to the current
98	    value of the scheduling deadline;
100	  - When the current time is equal to the replenishment time of a
101	    throttled task, the scheduling deadline and the current runtime are
102	    updated as
104	         scheduling deadline = scheduling deadline + period
105	         current runtime = current runtime + runtime
108	3. Scheduling Real-Time Tasks
109	=============================
111	 * BIG FAT WARNING ******************************************************
112	 *
113	 * This section contains a (not-thorough) summary on classical deadline
114	 * scheduling theory, and how it applies to SCHED_DEADLINE.
115	 * The reader can "safely" skip to Section 4 if only interested in seeing
116	 * how the scheduling policy can be used. Anyway, we strongly recommend
117	 * to come back here and continue reading (once the urge for testing is
118	 * satisfied :P) to be sure of fully understanding all technical details.
119	 ************************************************************************
121	 There are no limitations on what kind of task can exploit this new
122	 scheduling discipline, even if it must be said that it is particularly
123	 suited for periodic or sporadic real-time tasks that need guarantees on their
124	 timing behavior, e.g., multimedia, streaming, control applications, etc.
126	 A typical real-time task is composed of a repetition of computation phases
127	 (task instances, or jobs) which are activated on a periodic or sporadic
128	 fashion.
129	 Each job J_j (where J_j is the j^th job of the task) is characterised by an
130	 arrival time r_j (the time when the job starts), an amount of computation
131	 time c_j needed to finish the job, and a job absolute deadline d_j, which
132	 is the time within which the job should be finished. The maximum execution
133	 time max_j{c_j} is called "Worst Case Execution Time" (WCET) for the task.
134	 A real-time task can be periodic with period P if r_{j+1} = r_j + P, or
135	 sporadic with minimum inter-arrival time P is r_{j+1} >= r_j + P. Finally,
136	 d_j = r_j + D, where D is the task's relative deadline.
138	 SCHED_DEADLINE can be used to schedule real-time tasks guaranteeing that
139	 the jobs' deadlines of a task are respected. In order to do this, a task
140	 must be scheduled by setting:
142	  - runtime >= WCET
143	  - deadline = D
144	  - period <= P
146	 IOW, if runtime >= WCET and if period is >= P, then the scheduling deadlines
147	 and the absolute deadlines (d_j) coincide, so a proper admission control
148	 allows to respect the jobs' absolute deadlines for this task (this is what is
149	 called "hard schedulability property" and is an extension of Lemma 1 of [2]).
151	 References:
152	  1 - C. L. Liu and J. W. Layland. Scheduling algorithms for multiprogram-
153	      ming in a hard-real-time environment. Journal of the Association for
154	      Computing Machinery, 20(1), 1973.
155	  2 - L. Abeni , G. Buttazzo. Integrating Multimedia Applications in Hard
156	      Real-Time Systems. Proceedings of the 19th IEEE Real-time Systems
157	      Symposium, 1998. http://retis.sssup.it/~giorgio/paps/1998/rtss98-cbs.pdf
158	  3 - L. Abeni. Server Mechanisms for Multimedia Applications. ReTiS Lab
159	      Technical Report. http://xoomer.virgilio.it/lucabe72/pubs/tr-98-01.ps
161	4. Bandwidth management
162	=======================
164	 In order for the -deadline scheduling to be effective and useful, it is
165	 important to have some method to keep the allocation of the available CPU
166	 bandwidth to the tasks under control.
167	 This is usually called "admission control" and if it is not performed at all,
168	 no guarantee can be given on the actual scheduling of the -deadline tasks.
170	 Since when RT-throttling has been introduced each task group has a bandwidth
171	 associated, calculated as a certain amount of runtime over a period.
172	 Moreover, to make it possible to manipulate such bandwidth, readable/writable
173	 controls have been added to both procfs (for system wide settings) and cgroupfs
174	 (for per-group settings).
175	 Therefore, the same interface is being used for controlling the bandwidth
176	 distrubution to -deadline tasks.
178	 However, more discussion is needed in order to figure out how we want to manage
179	 SCHED_DEADLINE bandwidth at the task group level. Therefore, SCHED_DEADLINE
180	 uses (for now) a less sophisticated, but actually very sensible, mechanism to
181	 ensure that a certain utilization cap is not overcome per each root_domain.
183	 Another main difference between deadline bandwidth management and RT-throttling
184	 is that -deadline tasks have bandwidth on their own (while -rt ones don't!),
185	 and thus we don't need an higher level throttling mechanism to enforce the
186	 desired bandwidth.
188	4.1 System wide settings
189	------------------------
191	 The system wide settings are configured under the /proc virtual file system.
193	 For now the -rt knobs are used for dl admission control and the -deadline
194	 runtime is accounted against the -rt runtime. We realise that this isn't
195	 entirely desirable; however, it is better to have a small interface for now,
196	 and be able to change it easily later. The ideal situation (see 5.) is to run
197	 -rt tasks from a -deadline server; in which case the -rt bandwidth is a direct
198	 subset of dl_bw.
200	 This means that, for a root_domain comprising M CPUs, -deadline tasks
201	 can be created while the sum of their bandwidths stays below:
203	   M * (sched_rt_runtime_us / sched_rt_period_us)
205	 It is also possible to disable this bandwidth management logic, and
206	 be thus free of oversubscribing the system up to any arbitrary level.
207	 This is done by writing -1 in /proc/sys/kernel/sched_rt_runtime_us.
210	4.2 Task interface
211	------------------
213	 Specifying a periodic/sporadic task that executes for a given amount of
214	 runtime at each instance, and that is scheduled according to the urgency of
215	 its own timing constraints needs, in general, a way of declaring:
216	  - a (maximum/typical) instance execution time,
217	  - a minimum interval between consecutive instances,
218	  - a time constraint by which each instance must be completed.
220	 Therefore:
221	  * a new struct sched_attr, containing all the necessary fields is
222	    provided;
223	  * the new scheduling related syscalls that manipulate it, i.e.,
224	    sched_setattr() and sched_getattr() are implemented.
227	4.3 Default behavior
228	---------------------
230	 The default value for SCHED_DEADLINE bandwidth is to have rt_runtime equal to
231	 950000. With rt_period equal to 1000000, by default, it means that -deadline
232	 tasks can use at most 95%, multiplied by the number of CPUs that compose the
233	 root_domain, for each root_domain.
235	 A -deadline task cannot fork.
237	5. Tasks CPU affinity
238	=====================
240	 -deadline tasks cannot have an affinity mask smaller that the entire
241	 root_domain they are created on. However, affinities can be specified
242	 through the cpuset facility (Documentation/cgroups/cpusets.txt).
244	5.1 SCHED_DEADLINE and cpusets HOWTO
245	------------------------------------
247	 An example of a simple configuration (pin a -deadline task to CPU0)
248	 follows (rt-app is used to create a -deadline task).
250	 mkdir /dev/cpuset
251	 mount -t cgroup -o cpuset cpuset /dev/cpuset
252	 cd /dev/cpuset
253	 mkdir cpu0
254	 echo 0 > cpu0/cpuset.cpus
255	 echo 0 > cpu0/cpuset.mems
256	 echo 1 > cpuset.cpu_exclusive
257	 echo 0 > cpuset.sched_load_balance
258	 echo 1 > cpu0/cpuset.cpu_exclusive
259	 echo 1 > cpu0/cpuset.mem_exclusive
260	 echo $$ > cpu0/tasks
261	 rt-app -t 100000:10000:d:0 -D5 (it is now actually superfluous to specify
262	 task affinity)
264	6. Future plans
265	===============
267	 Still missing:
269	  - refinements to deadline inheritance, especially regarding the possibility
270	    of retaining bandwidth isolation among non-interacting tasks. This is
271	    being studied from both theoretical and practical points of view, and
272	    hopefully we should be able to produce some demonstrative code soon;
273	  - (c)group based bandwidth management, and maybe scheduling;
274	  - access control for non-root users (and related security concerns to
275	    address), which is the best way to allow unprivileged use of the mechanisms
276	    and how to prevent non-root users "cheat" the system?
278	 As already discussed, we are planning also to merge this work with the EDF
279	 throttling patches [https://lkml.org/lkml/2010/2/23/239] but we still are in
280	 the preliminary phases of the merge and we really seek feedback that would
281	 help us decide on the direction it should take.
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