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Diffstat (limited to 'Documentation/scheduler/sched-deadline.txt')
| -rw-r--r-- | Documentation/scheduler/sched-deadline.txt | 184 |
1 files changed, 154 insertions, 30 deletions
diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt index 21461a0441c1..e114513a2731 100644 --- a/Documentation/scheduler/sched-deadline.txt +++ b/Documentation/scheduler/sched-deadline.txt @@ -8,6 +8,10 @@ CONTENTS 1. Overview 2. Scheduling algorithm 3. Scheduling Real-Time Tasks + 3.1 Definitions + 3.2 Schedulability Analysis for Uniprocessor Systems + 3.3 Schedulability Analysis for Multiprocessor Systems + 3.4 Relationship with SCHED_DEADLINE Parameters 4. Bandwidth management 4.1 System-wide settings 4.2 Task interface @@ -43,7 +47,7 @@ CONTENTS "deadline", to schedule tasks. A SCHED_DEADLINE task should receive "runtime" microseconds of execution time every "period" microseconds, and these "runtime" microseconds are available within "deadline" microseconds - from the beginning of the period. In order to implement this behaviour, + from the beginning of the period. In order to implement this behavior, every time the task wakes up, the scheduler computes a "scheduling deadline" consistent with the guarantee (using the CBS[2,3] algorithm). Tasks are then scheduled using EDF[1] on these scheduling deadlines (the task with the @@ -52,7 +56,7 @@ CONTENTS "admission control" strategy (see Section "4. Bandwidth management") is used (clearly, if the system is overloaded this guarantee cannot be respected). - Summing up, the CBS[2,3] algorithms assigns scheduling deadlines to tasks so + Summing up, the CBS[2,3] algorithm assigns scheduling deadlines to tasks so that each task runs for at most its runtime every period, avoiding any interference between different tasks (bandwidth isolation), while the EDF[1] algorithm selects the task with the earliest scheduling deadline as the one @@ -63,7 +67,7 @@ CONTENTS In more details, the CBS algorithm assigns scheduling deadlines to tasks in the following way: - - Each SCHED_DEADLINE task is characterised by the "runtime", + - Each SCHED_DEADLINE task is characterized by the "runtime", "deadline", and "period" parameters; - The state of the task is described by a "scheduling deadline", and @@ -78,7 +82,7 @@ CONTENTS then, if the scheduling deadline is smaller than the current time, or this condition is verified, the scheduling deadline and the - remaining runtime are re-initialised as + remaining runtime are re-initialized as scheduling deadline = current time + deadline remaining runtime = runtime @@ -126,31 +130,37 @@ CONTENTS suited for periodic or sporadic real-time tasks that need guarantees on their timing behavior, e.g., multimedia, streaming, control applications, etc. +3.1 Definitions +------------------------ + A typical real-time task is composed of a repetition of computation phases (task instances, or jobs) which are activated on a periodic or sporadic fashion. - Each job J_j (where J_j is the j^th job of the task) is characterised by an + Each job J_j (where J_j is the j^th job of the task) is characterized by an arrival time r_j (the time when the job starts), an amount of computation time c_j needed to finish the job, and a job absolute deadline d_j, which is the time within which the job should be finished. The maximum execution - time max_j{c_j} is called "Worst Case Execution Time" (WCET) for the task. + time max{c_j} is called "Worst Case Execution Time" (WCET) for the task. A real-time task can be periodic with period P if r_{j+1} = r_j + P, or sporadic with minimum inter-arrival time P is r_{j+1} >= r_j + P. Finally, d_j = r_j + D, where D is the task's relative deadline. - The utilisation of a real-time task is defined as the ratio between its + Summing up, a real-time task can be described as + Task = (WCET, D, P) + + The utilization of a real-time task is defined as the ratio between its WCET and its period (or minimum inter-arrival time), and represents the fraction of CPU time needed to execute the task. - If the total utilisation sum_i(WCET_i/P_i) is larger than M (with M equal + If the total utilization U=sum(WCET_i/P_i) is larger than M (with M equal to the number of CPUs), then the scheduler is unable to respect all the deadlines. - Note that total utilisation is defined as the sum of the utilisations + Note that total utilization is defined as the sum of the utilizations WCET_i/P_i over all the real-time tasks in the system. When considering multiple real-time tasks, the parameters of the i-th task are indicated with the "_i" suffix. - Moreover, if the total utilisation is larger than M, then we risk starving + Moreover, if the total utilization is larger than M, then we risk starving non- real-time tasks by real-time tasks. - If, instead, the total utilisation is smaller than M, then non real-time + If, instead, the total utilization is smaller than M, then non real-time tasks will not be starved and the system might be able to respect all the deadlines. As a matter of fact, in this case it is possible to provide an upper bound @@ -159,38 +169,119 @@ CONTENTS More precisely, it can be proven that using a global EDF scheduler the maximum tardiness of each task is smaller or equal than ((M − 1) · WCET_max − WCET_min)/(M − (M − 2) · U_max) + WCET_max - where WCET_max = max_i{WCET_i} is the maximum WCET, WCET_min=min_i{WCET_i} - is the minimum WCET, and U_max = max_i{WCET_i/P_i} is the maximum utilisation. + where WCET_max = max{WCET_i} is the maximum WCET, WCET_min=min{WCET_i} + is the minimum WCET, and U_max = max{WCET_i/P_i} is the maximum + utilization[12]. + +3.2 Schedulability Analysis for Uniprocessor Systems +------------------------ If M=1 (uniprocessor system), or in case of partitioned scheduling (each real-time task is statically assigned to one and only one CPU), it is possible to formally check if all the deadlines are respected. If D_i = P_i for all tasks, then EDF is able to respect all the deadlines - of all the tasks executing on a CPU if and only if the total utilisation + of all the tasks executing on a CPU if and only if the total utilization of the tasks running on such a CPU is smaller or equal than 1. If D_i != P_i for some task, then it is possible to define the density of - a task as C_i/min{D_i,T_i}, and EDF is able to respect all the deadlines - of all the tasks running on a CPU if the sum sum_i C_i/min{D_i,T_i} of the - densities of the tasks running on such a CPU is smaller or equal than 1 - (notice that this condition is only sufficient, and not necessary). + a task as WCET_i/min{D_i,P_i}, and EDF is able to respect all the deadlines + of all the tasks running on a CPU if the sum of the densities of the tasks + running on such a CPU is smaller or equal than 1: + sum(WCET_i / min{D_i, P_i}) <= 1 + It is important to notice that this condition is only sufficient, and not + necessary: there are task sets that are schedulable, but do not respect the + condition. For example, consider the task set {Task_1,Task_2} composed by + Task_1=(50ms,50ms,100ms) and Task_2=(10ms,100ms,100ms). + EDF is clearly able to schedule the two tasks without missing any deadline + (Task_1 is scheduled as soon as it is released, and finishes just in time + to respect its deadline; Task_2 is scheduled immediately after Task_1, hence + its response time cannot be larger than 50ms + 10ms = 60ms) even if + 50 / min{50,100} + 10 / min{100, 100} = 50 / 50 + 10 / 100 = 1.1 + Of course it is possible to test the exact schedulability of tasks with + D_i != P_i (checking a condition that is both sufficient and necessary), + but this cannot be done by comparing the total utilization or density with + a constant. Instead, the so called "processor demand" approach can be used, + computing the total amount of CPU time h(t) needed by all the tasks to + respect all of their deadlines in a time interval of size t, and comparing + such a time with the interval size t. If h(t) is smaller than t (that is, + the amount of time needed by the tasks in a time interval of size t is + smaller than the size of the interval) for all the possible values of t, then + EDF is able to schedule the tasks respecting all of their deadlines. Since + performing this check for all possible values of t is impossible, it has been + proven[4,5,6] that it is sufficient to perform the test for values of t + between 0 and a maximum value L. The cited papers contain all of the + mathematical details and explain how to compute h(t) and L. + In any case, this kind of analysis is too complex as well as too + time-consuming to be performed on-line. Hence, as explained in Section + 4 Linux uses an admission test based on the tasks' utilizations. + +3.3 Schedulability Analysis for Multiprocessor Systems +------------------------ On multiprocessor systems with global EDF scheduling (non partitioned systems), a sufficient test for schedulability can not be based on the - utilisations (it can be shown that task sets with utilisations slightly - larger than 1 can miss deadlines regardless of the number of CPUs M). - However, as previously stated, enforcing that the total utilisation is smaller - than M is enough to guarantee that non real-time tasks are not starved and - that the tardiness of real-time tasks has an upper bound. + utilizations or densities: it can be shown that even if D_i = P_i task + sets with utilizations slightly larger than 1 can miss deadlines regardless + of the number of CPUs. + + Consider a set {Task_1,...Task_{M+1}} of M+1 tasks on a system with M + CPUs, with the first task Task_1=(P,P,P) having period, relative deadline + and WCET equal to P. The remaining M tasks Task_i=(e,P-1,P-1) have an + arbitrarily small worst case execution time (indicated as "e" here) and a + period smaller than the one of the first task. Hence, if all the tasks + activate at the same time t, global EDF schedules these M tasks first + (because their absolute deadlines are equal to t + P - 1, hence they are + smaller than the absolute deadline of Task_1, which is t + P). As a + result, Task_1 can be scheduled only at time t + e, and will finish at + time t + e + P, after its absolute deadline. The total utilization of the + task set is U = M · e / (P - 1) + P / P = M · e / (P - 1) + 1, and for small + values of e this can become very close to 1. This is known as "Dhall's + effect"[7]. Note: the example in the original paper by Dhall has been + slightly simplified here (for example, Dhall more correctly computed + lim_{e->0}U). + + More complex schedulability tests for global EDF have been developed in + real-time literature[8,9], but they are not based on a simple comparison + between total utilization (or density) and a fixed constant. If all tasks + have D_i = P_i, a sufficient schedulability condition can be expressed in + a simple way: + sum(WCET_i / P_i) <= M - (M - 1) · U_max + where U_max = max{WCET_i / P_i}[10]. Notice that for U_max = 1, + M - (M - 1) · U_max becomes M - M + 1 = 1 and this schedulability condition + just confirms the Dhall's effect. A more complete survey of the literature + about schedulability tests for multi-processor real-time scheduling can be + found in [11]. + + As seen, enforcing that the total utilization is smaller than M does not + guarantee that global EDF schedules the tasks without missing any deadline + (in other words, global EDF is not an optimal scheduling algorithm). However, + a total utilization smaller than M is enough to guarantee that non real-time + tasks are not starved and that the tardiness of real-time tasks has an upper + bound[12] (as previously noted). Different bounds on the maximum tardiness + experienced by real-time tasks have been developed in various papers[13,14], + but the theoretical result that is important for SCHED_DEADLINE is that if + the total utilization is smaller or equal than M then the response times of + the tasks are limited. + +3.4 Relationship with SCHED_DEADLINE Parameters +------------------------ - SCHED_DEADLINE can be used to schedule real-time tasks guaranteeing that - the jobs' deadlines of a task are respected. In order to do this, a task - must be scheduled by setting: + Finally, it is important to understand the relationship between the + SCHED_DEADLINE scheduling parameters described in Section 2 (runtime, + deadline and period) and the real-time task parameters (WCET, D, P) + described in this section. Note that the tasks' temporal constraints are + represented by its absolute deadlines d_j = r_j + D described above, while + SCHED_DEADLINE schedules the tasks according to scheduling deadlines (see + Section 2). + If an admission test is used to guarantee that the scheduling deadlines + are respected, then SCHED_DEADLINE can be used to schedule real-time tasks + guaranteeing that all the jobs' deadlines of a task are respected. + In order to do this, a task must be scheduled by setting: - runtime >= WCET - deadline = D - period <= P - IOW, if runtime >= WCET and if period is >= P, then the scheduling deadlines + IOW, if runtime >= WCET and if period is <= P, then the scheduling deadlines and the absolute deadlines (d_j) coincide, so a proper admission control allows to respect the jobs' absolute deadlines for this task (this is what is called "hard schedulability property" and is an extension of Lemma 1 of [2]). @@ -206,6 +297,39 @@ CONTENTS Symposium, 1998. http://retis.sssup.it/~giorgio/paps/1998/rtss98-cbs.pdf 3 - L. Abeni. Server Mechanisms for Multimedia Applications. ReTiS Lab Technical Report. http://disi.unitn.it/~abeni/tr-98-01.pdf + 4 - J. Y. Leung and M.L. Merril. A Note on Preemptive Scheduling of + Periodic, Real-Time Tasks. Information Processing Letters, vol. 11, + no. 3, pp. 115-118, 1980. + 5 - S. K. Baruah, A. K. Mok and L. E. Rosier. Preemptively Scheduling + Hard-Real-Time Sporadic Tasks on One Processor. Proceedings of the + 11th IEEE Real-time Systems Symposium, 1990. + 6 - S. K. Baruah, L. E. Rosier and R. R. Howell. Algorithms and Complexity + Concerning the Preemptive Scheduling of Periodic Real-Time tasks on + One Processor. Real-Time Systems Journal, vol. 4, no. 2, pp 301-324, + 1990. + 7 - S. J. Dhall and C. L. Liu. On a real-time scheduling problem. Operations + research, vol. 26, no. 1, pp 127-140, 1978. + 8 - T. Baker. Multiprocessor EDF and Deadline Monotonic Schedulability + Analysis. Proceedings of the 24th IEEE Real-Time Systems Symposium, 2003. + 9 - T. Baker. An Analysis of EDF Schedulability on a Multiprocessor. + IEEE Transactions on Parallel and Distributed Systems, vol. 16, no. 8, + pp 760-768, 2005. + 10 - J. Goossens, S. Funk and S. Baruah, Priority-Driven Scheduling of + Periodic Task Systems on Multiprocessors. Real-Time Systems Journal, + vol. 25, no. 2–3, pp. 187–205, 2003. + 11 - R. Davis and A. Burns. A Survey of Hard Real-Time Scheduling for + Multiprocessor Systems. ACM Computing Surveys, vol. 43, no. 4, 2011. + http://www-users.cs.york.ac.uk/~robdavis/papers/MPSurveyv5.0.pdf + 12 - U. C. Devi and J. H. Anderson. Tardiness Bounds under Global EDF + Scheduling on a Multiprocessor. Real-Time Systems Journal, vol. 32, + no. 2, pp 133-189, 2008. + 13 - P. Valente and G. Lipari. An Upper Bound to the Lateness of Soft + Real-Time Tasks Scheduled by EDF on Multiprocessors. Proceedings of + the 26th IEEE Real-Time Systems Symposium, 2005. + 14 - J. Erickson, U. Devi and S. Baruah. Improved tardiness bounds for + Global EDF. Proceedings of the 22nd Euromicro Conference on + Real-Time Systems, 2010. + 4. Bandwidth management ======================= @@ -218,10 +342,10 @@ CONTENTS no guarantee can be given on the actual scheduling of the -deadline tasks. As already stated in Section 3, a necessary condition to be respected to - correctly schedule a set of real-time tasks is that the total utilisation + correctly schedule a set of real-time tasks is that the total utilization is smaller than M. When talking about -deadline tasks, this requires that the sum of the ratio between runtime and period for all tasks is smaller - than M. Notice that the ratio runtime/period is equivalent to the utilisation + than M. Notice that the ratio runtime/period is equivalent to the utilization of a "traditional" real-time task, and is also often referred to as "bandwidth". The interface used to control the CPU bandwidth that can be allocated @@ -251,7 +375,7 @@ CONTENTS The system wide settings are configured under the /proc virtual file system. For now the -rt knobs are used for -deadline admission control and the - -deadline runtime is accounted against the -rt runtime. We realise that this + -deadline runtime is accounted against the -rt runtime. We realize that this isn't entirely desirable; however, it is better to have a small interface for now, and be able to change it easily later. The ideal situation (see 5.) is to run -rt tasks from a -deadline server; in which case the -rt bandwidth is a |