Deadline Task Scheduling ------------------------ CONTENTS ======== 0. WARNING 1. Overview 2. Scheduling algorithm 3. Scheduling Real-Time Tasks 4. Bandwidth management 4.1 System-wide settings 4.2 Task interface 4.3 Default behavior 5. Tasks CPU affinity 5.1 SCHED_DEADLINE and cpusets HOWTO 6. Future plans 0. WARNING ========== Fiddling with these settings can result in an unpredictable or even unstable system behavior. As for -rt (group) scheduling, it is assumed that root users know what they're doing. 1. Overview =========== The SCHED_DEADLINE policy contained inside the sched_dl scheduling class is basically an implementation of the Earliest Deadline First (EDF) scheduling algorithm, augmented with a mechanism (called Constant Bandwidth Server, CBS) that makes it possible to isolate the behavior of tasks between each other. 2. Scheduling algorithm ================== SCHED_DEADLINE uses three parameters, named "runtime", "period", and "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, 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 earliest scheduling deadline is selected for execution). Notice that the task actually receives "runtime" time units within "deadline" if a proper "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 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 to be executed next. Thanks to this feature, tasks that do not strictly comply with the "traditional" real-time task model (see Section 3) can effectively use the new policy. In more details, the CBS algorithm assigns scheduling deadlines to tasks in the following way: - Each SCHED_DEADLINE task is characterised by the "runtime", "deadline", and "period" parameters; - The state of the task is described by a "scheduling deadline", and a "remaining runtime". These two parameters are initially set to 0; - When a SCHED_DEADLINE task wakes up (becomes ready for execution), the scheduler checks if remaining runtime runtime ---------------------------------- > --------- scheduling deadline - current time period 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 scheduling deadline = current time + deadline remaining runtime = runtime otherwise, the scheduling deadline and the remaining runtime are left unchanged; - When a SCHED_DEADLINE task executes for an amount of time t, its remaining runtime is decreased as remaining runtime = remaining runtime - t (technically, the runtime is decreased at every tick, or when the task is descheduled / preempted); - When the remaining runtime becomes less or equal than 0, the task is said to be "throttled" (also known as "depleted" in real-time literature) and cannot be scheduled until its scheduling deadline. The "replenishment time" for this task (see next item) is set to be equal to the current value of the scheduling deadline; - When the current time is equal to the replenishment time of a throttled task, the scheduling deadline and the remaining runtime are updated as scheduling deadline = scheduling deadline + period remaining runtime = remaining runtime + runtime 3. Scheduling Real-Time Tasks ============================= * BIG FAT WARNING ****************************************************** * * This section contains a (not-thorough) summary on classical deadline * scheduling theory, and how it applies to SCHED_DEADLINE. * The reader can "safely" skip to Section 4 if only interested in seeing * how the scheduling policy can be used. Anyway, we strongly recommend * to come back here and continue reading (once the urge for testing is * satisfied :P) to be sure of fully understanding all technical details. ************************************************************************ There are no limitations on what kind of task can exploit this new scheduling discipline, even if it must be said that it is particularly suited for periodic or sporadic real-time tasks that need guarantees on their timing behavior, e.g., multimedia, streaming, control applications, etc. 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 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. 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 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 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 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 non- real-time tasks by real-time tasks. If, instead, the total utilisation 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 for tardiness (defined as the maximum between 0 and the difference between the finishing time of a job and its absolute deadline). 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. 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 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). 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. 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: - runtime >= WCET - deadline = D - period <= P 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]). Notice that if runtime > deadline the admission control will surely reject this task, as it is not possible to respect its temporal constraints. References: 1 - C. L. Liu and J. W. Layland. Scheduling algorithms for multiprogram- ming in a hard-real-time environment. Journal of the Association for Computing Machinery, 20(1), 1973. 2 - L. Abeni , G. Buttazzo. Integrating Multimedia Applications in Hard Real-Time Systems. Proceedings of the 19th IEEE Real-time Systems 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. Bandwidth management ======================= As previously mentioned, in order for -deadline scheduling to be effective and useful (that is, to be able to provide "runtime" time units within "deadline"), it is important to have some method to keep the allocation of the available fractions of CPU time to the various tasks under control. This is usually called "admission control" and if it is not performed, then 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 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 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 to -deadline tasks is similar to the one already used for -rt tasks with real-time group scheduling (a.k.a. RT-throttling - see Documentation/scheduler/sched-rt-group.txt), and is based on readable/ writable control files located in procfs (for system wide settings). Notice that per-group settings (controlled through cgroupfs) are still not defined for -deadline tasks, because more discussion is needed in order to figure out how we want to manage SCHED_DEADLINE bandwidth at the task group level. A main difference between deadline bandwidth management and RT-throttling is that -deadline tasks have bandwidth on their own (while -rt ones don't!), and thus we don't need a higher level throttling mechanism to enforce the desired bandwidth. In other words, this means that interface parameters are only used at admission control time (i.e., when the user calls sched_setattr()). Scheduling is then performed considering actual tasks' parameters, so that CPU bandwidth is allocated to SCHED_DEADLINE tasks respecting their needs in terms of granularity. Therefore, using this simple interface we can put a cap on total utilization of -deadline tasks (i.e., \Sum (runtime_i / period_i) < global_dl_utilization_cap). 4.1 System wide settings ------------------------ 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 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 direct subset of dl_bw. This means that, for a root_domain comprising M CPUs, -deadline tasks can be created while the sum of their bandwidths stays below: M * (sched_rt_runtime_us / sched_rt_period_us) It is also possible to disable this bandwidth management logic, and be thus free of oversubscribing the system up to any arbitrary level. This is done by writing -1 in /proc/sys/kernel/sched_rt_runtime_us. 4.2 Task interface ------------------ Specifying a periodic/sporadic task that executes for a given amount of runtime at each instance, and that is scheduled according to the urgency of its own timing constraints needs, in general, a way of declaring: - a (maximum/typical) instance execution time, - a minimum interval between consecutive instances, - a time constraint by which each instance must be completed. Therefore: * a new struct sched_attr, containing all the necessary fields is provided; * the new scheduling related syscalls that manipulate it, i.e., sched_setattr() and sched_getattr() are implemented. 4.3 Default behavior --------------------- The default value for SCHED_DEADLINE bandwidth is to have rt_runtime equal to 950000. With rt_period equal to 1000000, by default, it means that -deadline tasks can use at most 95%, multiplied by the number of CPUs that compose the root_domain, for each root_domain. This means that non -deadline tasks will receive at least 5% of the CPU time, and that -deadline tasks will receive their runtime with a guaranteed worst-case delay respect to the "deadline" parameter. If "deadline" = "period" and the cpuset mechanism is used to implement partitioned scheduling (see Section 5), then this simple setting of the bandwidth management is able to deterministically guarantee that -deadline tasks will receive their runtime in a period. Finally, notice that in order not to jeopardize the admission control a -deadline task cannot fork. 5. Tasks CPU affinity ===================== -deadline tasks cannot have an affinity mask smaller that the entire root_domain they are created on. However, affinities can be specified through the cpuset facility (Documentation/cgroups/cpusets.txt). 5.1 SCHED_DEADLINE and cpusets HOWTO ------------------------------------ An example of a simple configuration (pin a -deadline task to CPU0) follows (rt-app is used to create a -deadline task). mkdir /dev/cpuset mount -t cgroup -o cpuset cpuset /dev/cpuset cd /dev/cpuset mkdir cpu0 echo 0 > cpu0/cpuset.cpus echo 0 > cpu0/cpuset.mems echo 1 > cpuset.cpu_exclusive echo 0 > cpuset.sched_load_balance echo 1 > cpu0/cpuset.cpu_exclusive echo 1 > cpu0/cpuset.mem_exclusive echo $$ > cpu0/tasks rt-app -t 100000:10000:d:0 -D5 (it is now actually superfluous to specify task affinity) 6. Future plans =============== Still missing: - refinements to deadline inheritance, especially regarding the possibility of retaining bandwidth isolation among non-interacting tasks. This is being studied from both theoretical and practical points of view, and hopefully we should be able to produce some demonstrative code soon; - (c)group based bandwidth management, and maybe scheduling; - access control for non-root users (and related security concerns to address), which is the best way to allow unprivileged use of the mechanisms and how to prevent non-root users "cheat" the system? As already discussed, we are planning also to merge this work with the EDF throttling patches [https://lkml.org/lkml/2010/2/23/239] but we still are in the preliminary phases of the merge and we really seek feedback that would help us decide on the direction it should take.