Library prosa.results.prm.edf.floating_nonpreemptive

RTA for EDF Scheduling with Floating Non-Preemptive Regions on Uniprocessors under the Periodic Resource Model

In the following, we derive a response-time analysis for EDF schedulers, assuming a workload of sporadic real-time tasks with floating non-preemptive regions, characterized by arbitrary arrival curves, executing upon a uniprocessor under the periodic resource model (see "Periodic Resource Model for Compositional Real-Time Guarantees" by Shin & Lee, RTSS 2003). To this end, we instantiate the sequential variant of abstract Restricted-Supply Analysis (aRSA) as provided in the prosa.analysis.abstract.restricted_supply module.

Section RTAforFloatingEDFModelwithArrivalCurves.

Defining the System Model

Before any formal claims can be stated, an initial setup is needed to define the system model under consideration. To this end, we next introduce and define the following notions using Prosa's standard definitions and behavioral semantics:

tasks, jobs, and their parameters,
the task set and the task under analysis,
the processor model,
the sequence of job arrivals,
the absence of self-suspensions,
an arbitrary schedule of the task set, and finally,
the resource-supply model.

Tasks and Jobs

Consider tasks characterized by a WCET task_cost, relative deadline task_deadline, an arrival curve max_arrivals, and a bound on the task's longest non-preemptive segment task_max_nonpreemptive_segment, ...

Context {Task : TaskType} `{TaskCost Task} `{TaskDeadline Task}
`{MaxArrivals Task} `{TaskMaxNonpreemptiveSegment Task}.

... and their associated jobs, where each job has a corresponding task job_task, an execution time job_cost, an arrival time job_arrival, and a list of preemption points job_preemptive_points.

Context {Job : JobType} `{JobTask Job Task} `{JobCost Job} `{JobArrival Job}
`{JobPreemptionPoints Job}.

We assume that jobs are limited-preemptive.

#[local] Existing Instance limited_preemptive_job_model.

The Task Set and the Task Under Analysis

Consider an arbitrary task set ts, and ...

Variable ts : seq Task.

... let tsk be any task in ts that is to be analyzed.

Variable tsk : Task.
Hypothesis H_tsk_in_ts : tsk \in ts.

Processor Model

Consider any kind of fully-supply-consuming, unit-supply processor state model.

  Context `{PState : ProcessorState Job}.
  Hypothesis H_uniprocessor_proc_model : uniprocessor_model PState.
  Hypothesis H_unit_supply_proc_model : unit_supply_proc_model PState.
  Hypothesis H_consumed_supply_proc_model : fully_consuming_proc_model PState.

The Job Arrival Sequence

Allow for any possible arrival sequence arr_seq consistent with the parameters of the task set ts. That is, arr_seq is a valid arrival sequence in which each job's cost is upper-bounded by its task's WCET, every job stems from a task in ts, and the number of arrivals respects each task's max_arrivals bound.

Variable arr_seq : arrival_sequence Job.
Hypothesis H_valid_task_arrival_sequence : valid_task_arrival_sequence ts arr_seq.

Assume a model with floating non-preemptive regions. I.e., for each task only the length of the maximal non-preemptive segment is known and each job is divided into a number of non-preemptive segments by inserting preemption points.

Hypothesis H_valid_task_model_with_floating_nonpreemptive_regions :
valid_model_with_floating_nonpreemptive_regions arr_seq.

Absence of Self-Suspensions

We assume the classic (i.e., Liu & Layland) model of readiness without jitter or self-suspensions, wherein pending jobs are always ready.

#[local] Existing Instance basic_ready_instance.

The Schedule

Consider a work-conserving, valid uniprocessor schedule with limited preemptions that corresponds to the given arrival sequence arr_seq (and hence the given task set ts).

  Variable sched : schedule PState.
  Hypothesis H_valid_schedule : valid_schedule sched arr_seq.
  Hypothesis H_work_conserving : work_conserving arr_seq sched.
  Hypothesis H_schedule_with_limited_preemptions :
    schedule_respects_preemption_model arr_seq sched.

We assume that the schedule respects the given EDF scheduling policy.

Hypothesis H_respects_policy :
respects_JLFP_policy_at_preemption_point arr_seq sched (EDF Job).

Periodic Resource Model

Assume that the processor supply follows the *periodic resource model* as defined in "Periodic Resource Model for Compositional Real-Time Guarantees" by Shin & Lee (RTSS 2003). Under this model, the supply is distributed periodically with a resource period Π and a resource allocation time γ: in every interval [Π⋅k, Π⋅(k+1)), the processor provides at least γ units of supply. Furthermore, let prm_sbf Π γ denote the corresponding SBF defined in the paper, which, as proven in the same paper and verified in prosa.analysis.facts.model.sbf.periodic, is a valid SBF.

Variable Π γ : duration.
Hypothesis H_periodic_resource_model : periodic_resource_model Π γ sched.

Maximum Length of a Busy Interval

In order to apply aRSA, we require a bound on the maximum busy-window length. To this end, let L be any positive solution of the busy-interval "recurrences" (i.e., a set of inequalities) prm_sbf Π γ L ≥ total_request_bound_function ts L and prm_sbf Π γ L ≥ longest_busy_interval_with_pi ts tsk, as defined below.

As the lemma busy_intervals_are_bounded_rs_edf shows, under EDF scheduling, this condition is sufficient to guarantee that the maximum busy-window length is at most L, i.e., the length of any busy interval is bounded by L.

  Definition busy_window_recurrence_solution (L : duration) :=
    L > 0
    ∧ prm_sbf Π γ L ≥ total_request_bound_function ts L
    ∧ prm_sbf Π γ L ≥ longest_busy_interval_with_pi ts tsk.

Response-Time Bound

Having established all necessary preliminaries, it is finally time to state the claimed response-time bound R.

A value R is a response-time bound for task tsk if, for any given offset A in the search space (w.r.t. the busy-window bound L), the response-time bound "recurrence" (i.e., inequality) has a solution F not exceeding A + R.

  Definition rta_recurrence_solution L R :=
    ∀ (A : duration),
      is_in_search_space ts tsk L A →
      ∃ (F : duration ),
        prm_sbf Π γ F ≥ blocking_bound ts tsk A
                        + task_request_bound_function tsk (A + ε)
                        + bound_on_athep_workload ts tsk A F
        ∧ A + R ≥ F.

Finally, using the sequential variant of abstract restricted-supply analysis, we establish that, given a bound on the maximum busy-window length L, any such R is indeed a sound response-time bound for task tsk under EDF scheduling with floating non-preemptive regions on a unit-speed uniprocessor under the periodic resource model.

  Theorem uniprocessor_response_time_bound_floating_edf :
    ∀ (L : duration),
      busy_window_recurrence_solution L →
      ∀ (R : duration),
        rta_recurrence_solution L R →
        task_response_time_bound arr_seq sched tsk R.

End RTAforFloatingEDFModelwithArrivalCurves.