Scheduling lists in GeMRTOS

Table of Contents

Scheduling lists in GeMRTOS #

In GeMRTOS, each task is assigned to a scheduling list and one or more processors may serve that scheduling list. Processors may serve one or more scheduling lists, thereby allowing the predictability of partitioned scheduling with a flexible balance of processor loads. Scheduling overhead is reduced using a timer-tickless scheduler. Floating scheduling reduces the overhead for the highest priority tasks and isolates the design of the system from the number of processors of the hardware architecture. The generic feature allows partial configuring, global and partitioned scheduling. Hybrid partition scheduling are based on scheduling lists which allow assigning tasks and processors to scheduling the different subsystems of the application.

Resources may be shared among tasks from several scheduling lists, as shown in the figure. Each scheduling list may implement a different scheduling priority and task may be migrated from one scheduling list to another scheduling list during runtime. Processors may be assigned to several scheduling lists, and their assignment may change at any time during runtime

Types of GeMRTOS scheduling lists #

The following are the type of tasks in GeMRTOS, but more can be defined:

GS_LCBType_FP


The GS_LCBType_FP scheduling list type implements the Fixed Priority (FP) discipline among the tasks assigned to the scheduling list. In the FP discipline, a priority is assigned to each task. Task priority may be modified during runtime. In order to maintain consistency, tasks that are assigned to an FP scheduled list should not be of the infinite-loop type without waiting for event suspension in order to avoid starving lower-priority tasks. Only the lowest-priority task could be implemented as an infinite-loop code. 


GS_LCBType_EDF


The GS_LCBType_EDF scheduling list type implements the Earliest Deadline First (EDF) discipline among the tasks assigned to the scheduling list. In the EDF discipline, the earliest deadline, the highest priority. In order to maintain consistency, tasks that are assigned to an EDF scheduled list should be of the periodic type, with the deadline taken into account starting from the release time.

Main scheduling list related functions #

Creating and initializing a scheduling list #

A scheduling list can easily be created using:

GS_LCB * gu_Get_LCB (INT32 lcbtype)

The gu_Get_LCB function creates a new scheduling list and it is defined according the parameter type passed.

The task is created with the following parameters:



lcbtypeIt is a valid scheduling list type. This parameter defines the scheduling discipline applied over all the tasks assigned to the scheduling list. 


Returns
The gu_Get_LCB function returns a pointer to the GS_LCB structure associate with the scheduling list created. This pointer should be used in all the references to the scheduling list.

Scheduling list / Processor association #

A processor is associated with a scheduling list with the following function:

INT32 gu_LCB_Associate_PCB (GS_LCB * plcb, INT32 CPUID, INT32 priority )

By using the gu_LCB_Associate_PCB function, a system processor can be associated with a scheduling list. Each association has a determined priority. In order of decreasing priority, the processor will search the scheduling lists for the first task ready state.

The processor is associated with the following parameters:



plcbIt is a pointer to the GS_LCB of the scheduling list 


CPUIDIt is the ID of the processor to be associated with the scheduling list. 


priorityIt is the priority for the assignment. Highest priority (lowest value) will be the first scheduling lists looking for the first task in ready state. 


Returns
The gu_LCB_Associate_PCB function return G_TRUE if successful, G_FALSE otherwise

Task / Scheduling list association #

A task is assigned to a scheduling list with the following function:

INT32 gu_SetTaskList (struct gs_tcb * ptcb, struct gs_lcb * plcb )

The gu_SetTaskList function assigns a task into a scheduling list. Consequently, the task will be schedule according the priority discipline of the scheduling list.

The task is assigned tothe scheduling list with the following parameters:



ptcbIt is a pointer to the GS_TCB structure of the task 


plcbIt is a pointer to the GS_LCB structure of the scheduling list 


Returns
The gu_SetTaskList function returns G_TRUE if successfull, G_FALSE otherwise