Research On Scheduling Algorithms For Mixed Criticality Systems To Improve Low-Criticality Tasks Schedulability

Mixed-critical system combine high security tasks and non-safety-critical tasks on the same platform to meet the current development requirements of real-time system hardware platforms and software functions,which has become one of the important subject in the field of embedded real-time systems.Traditional real-time systems do not distinguish tasks of different importance levels,and the execution mode remains constant during the system running.And the scheduling strategy only needs to ensure all tasks complete within the deadline,and cannot adapt to change of system execution mode and task time attributes in a time.The execution mode of mixed-critical level system is more complicated,and it is necessary to consider the mutual influence of tasks of different importance levels in the execution process,which brings new challenges to the task scheduling research.The current research on mixed-critical system has following problems: When the system execution mode changes from low to high,the scheduling strategy usually only guarantees the correct execution of the tasks with high importance level,discards low critical tasks directly,and ignores the condition which those tasks can continue to execute.which lead to the destruction of data integrity and waste of resources;in addition,now there is little research on the change of the system's critical level from high to low,usually waiting for the processor to be idle before the security degradation,and the system maintain in high-critical mode for long time,resulting in the low schudulable rate of low critical level tasks.Aiming at the above problems,this dissertation research the scheme of improving the scheduling ratio of low-critical tasks from the aspects of system level promotion and fall-back,to meet the scheduling balance of different important level tasks and make full use of processor resources.The research work and main contribution as follows:In the first part,study the scheme of low-critical tasks executed in the weak constraint mode during the critical-level promotion phase of the system based on the response time analysis Establish a mixed multi-critical task model,extend the existing weakly constrainedexecution mode to multi-critical system,and adjust the weakly constrained execution parameters based on the difference between the tasks' own critical level and the system critical level.Two methods are used to calculate the task response time,and two weakly constrained scheduling algorithms AMCwe-x and AMCwemax-x are proposed corresponding to OPA(optimal priority assignment),assign priorities to tasks offline and estimate the schedulable rate of the task sets.Comparing the schedulable ratio of the task set through experiments with classic scheduling strategies,the results show that the scheduling strategy proposed in the dissertation can achieve the scheduling of low-critical tasks positively and adjust service levels by setting weak constraint execution parameters.In the second aspect,a mixed dual-critical mission model is established to study the system's critical level security fallback protocol DL.Three system execution modes are defined: initial mode,compensation mode,and recovery mode.The compensation value bp is introduced as an indicator for switching between modes to control the change of the system execution mode.The schedulable budget time of the task is set through sensitivity analysis offline,and in runtime use a two-layer scheduling mode,meanwhile adjust the budget time is dynamically.Find the task with the smallest budget time in the pending task queue,and increase budget time of this task.The performance of the DL protocol is analyzed using three indicators: HDM(high critical task missed deadline ratio),NIH(number of times the system entered high critical level),and JNE(low critical task discard ratio).We compare the DL protocol with the BPS and BPSG protocols through experiments,the results show that while the DL protocol satisfies the correct execution of high-critical tasks,meanwhile reduces the drop rate of low-critical tasks compared to the BPS and BPSG protocols,and improves the system security degradation performance.