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diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt new file mode 100644 index 00000000000..18adc92a6b3 --- /dev/null +++ b/Documentation/scheduler/sched-deadline.txt @@ -0,0 +1,281 @@ + 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 is guaranteed to 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 + smallest scheduling deadline is selected for execution). Notice that this + guaranteed is respected if a proper "admission control" strategy (see Section + "4. Bandwidth management") is used. + + 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 smallest scheduling deadline as the one + to be executed first. Thanks to this feature, also 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 "current runtime". These two parameters are initially set to 0; + + - When a SCHED_DEADLINE task wakes up (becomes ready for execution), + the scheduler checks if + + current 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 + current budget are re-initialised as + + scheduling deadline = current time + deadline + current runtime = runtime + + otherwise, the scheduling deadline and the current runtime are + left unchanged; + + - When a SCHED_DEADLINE task executes for an amount of time t, its + current runtime is decreased as + + current runtime = current runtime - t + + (technically, the runtime is decreased at every tick, or when the + task is descheduled / preempted); + + - When the current 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 current runtime are + updated as + + scheduling deadline = scheduling deadline + period + current runtime = current 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. + + 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]). + + 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://xoomer.virgilio.it/lucabe72/pubs/tr-98-01.ps + +4. Bandwidth management +======================= + + In order for the -deadline scheduling to be effective and useful, it is + important to have some method to keep the allocation of the available CPU + bandwidth to the tasks under control. + This is usually called "admission control" and if it is not performed at all, + no guarantee can be given on the actual scheduling of the -deadline tasks. + + Since when RT-throttling has been introduced each task group has a bandwidth + associated, calculated as a certain amount of runtime over a period. + Moreover, to make it possible to manipulate such bandwidth, readable/writable + controls have been added to both procfs (for system wide settings) and cgroupfs + (for per-group settings). + Therefore, the same interface is being used for controlling the bandwidth + distrubution to -deadline tasks. + + However, more discussion is needed in order to figure out how we want to manage + SCHED_DEADLINE bandwidth at the task group level. Therefore, SCHED_DEADLINE + uses (for now) a less sophisticated, but actually very sensible, mechanism to + ensure that a certain utilization cap is not overcome per each root_domain. + + Another 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 an higher level throttling mechanism to enforce the + desired bandwidth. + +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 dl 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. + + 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. |