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RTEMS
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Files | |
| file | taskbuild.c |
| RTEMS Task Create. | |
| file | taskconstruct.c |
| RTEMS Task Create from Config. | |
| file | taskdelete.c |
| RTEMS Delete Task. | |
| file | taskgetaffinity.c |
| RTEMS Task Get Affinity. | |
| file | taskinitusers.c |
| _RTEMS_tasks_Initialize_user_tasks_body | |
| file | taskissuspended.c |
| rtems_task_is_suspended | |
| file | taskrestart.c |
| RTEMS Task Restart. | |
| file | taskresume.c |
| RTEMS Resume Task. | |
| file | tasks.c |
| RTEMS Task API Extensions. | |
| file | taskself.c |
| RTEMS Get Self Task Id. | |
| file | tasksetaffinity.c |
| RTEMS Task Set Affinity. | |
| file | tasksetpriority.c |
| RTEMS Set Task Priority. | |
| file | taskstart.c |
| RTEMS Start Task. | |
| file | tasksuspend.c |
| RTEMS Suspend Task. | |
| file | taskwakeafter.c |
| RTEMS Task Wake After. | |
Many definitions of a task have been proposed in computer literature. Unfortunately, none of these definitions encompasses all facets of the concept in a manner which is operating system independent. Several of the more common definitions are provided to enable each user to select a definition which best matches their own experience and understanding of the task concept:
From RTEMS' perspective, a task is the smallest thread of execution which can compete on its own for system resources. A task is manifested by the existence of a task control block (TCB).
The Task Control Block (TCB) is an RTEMS defined data structure which contains all the information that is pertinent to the execution of a task. During system initialization, RTEMS reserves a TCB for each task configured. A TCB is allocated upon creation of the task and is returned to the TCB free list upon deletion of the task.
The TCB's elements are modified as a result of system calls made by the application in response to external and internal stimuli. TCBs are the only RTEMS internal data structure that can be accessed by an application via user extension routines. The TCB contains a task's name, ID, current priority, current and starting states, execution mode, TCB user extension pointer, scheduling control structures, as well as data required by a blocked task.
A task's context is stored in the TCB when a task switch occurs. When the task regains control of the processor, its context is restored from the TCB. When a task is restarted, the initial state of the task is restored from the starting context area in the task's TCB.
A task may exist in one of the following five states:
An active task may occupy the executing, ready, blocked or dormant state, otherwise the task is considered non-existent. One or more tasks may be active in the system simultaneously. Multiple tasks communicate, synchronize, and compete for system resources with each other via system calls. The multiple tasks appear to execute in parallel, but actually each is dispatched to the CPU for periods of time determined by the RTEMS scheduling algorithm. The scheduling of a task is based on its current state and priority.
A task's priority determines its importance in relation to the other tasks executing on the same processor. RTEMS supports 255 levels of priority ranging from 1 to 255. The data type rtems_task_priority() is used to store task priorities.
Tasks of numerically smaller priority values are more important tasks than tasks of numerically larger priority values. For example, a task at priority level 5 is of higher privilege than a task at priority level 10. There is no limit to the number of tasks assigned to the same priority.
Each task has a priority associated with it at all times. The initial value of this priority is assigned at task creation time. The priority of a task may be changed at any subsequent time.
Priorities are used by the scheduler to determine which ready task will be allowed to execute. In general, the higher the logical priority of a task, the more likely it is to receive processor execution time.
A task's execution mode is a combination of the following four components:
It is used to modify RTEMS' scheduling process and to alter the execution environment of the task. The data type rtems_task_mode() is used to manage the task execution mode.
The preemption component allows a task to determine when control of the processor is relinquished. If preemption is disabled (RTEMS_NO_PREEMPT), the task will retain control of the processor as long as it is in the executing state – even if a higher priority task is made ready. If preemption is enabled (RTEMS_PREEMPT) and a higher priority task is made ready, then the processor will be taken away from the current task immediately and given to the higher priority task.
The timeslicing component is used by the RTEMS scheduler to determine how the processor is allocated to tasks of equal priority. If timeslicing is enabled (RTEMS_TIMESLICE), then RTEMS will limit the amount of time the task can execute before the processor is allocated to another ready task of equal priority. The length of the timeslice is application dependent and specified in the Configuration Table. If timeslicing is disabled (RTEMS_NO_TIMESLICE), then the task will be allowed to execute until a task of higher priority is made ready. If RTEMS_NO_PREEMPT is selected, then the timeslicing component is ignored by the scheduler.
The asynchronous signal processing component is used to determine when received signals are to be processed by the task. If signal processing is enabled (RTEMS_ASR), then signals sent to the task will be processed the next time the task executes. If signal processing is disabled (RTEMS_NO_ASR), then all signals received by the task will remain posted until signal processing is enabled. This component affects only tasks which have established a routine to process asynchronous signals.
The interrupt level component is used to determine which interrupts will be enabled when the task is executing. RTEMS_INTERRUPT_LEVEL(n) specifies that the task will execute at interrupt level n.
The set of default modes may be selected by specifying the RTEMS_DEFAULT_MODES constant.
All RTEMS tasks are invoked with a single argument which is specified when they are started or restarted. The argument is commonly used to communicate startup information to the task. The simplest manner in which to define a task which accesses it argument is:
Application tasks requiring more information may view this single argument as an index into an array of parameter blocks.
Creating a task with the RTEMS_FLOATING_POINT attribute flag results in additional memory being allocated for the TCB to store the state of the numeric coprocessor during task switches. This additional memory is NOT allocated for RTEMS_NO_FLOATING_POINT tasks. Saving and restoring the context of a RTEMS_FLOATING_POINT task takes longer than that of a RTEMS_NO_FLOATING_POINT task because of the relatively large amount of time required for the numeric coprocessor to save or restore its computational state.
Since RTEMS was designed specifically for embedded military applications which are floating point intensive, the executive is optimized to avoid unnecessarily saving and restoring the state of the numeric coprocessor. The state of the numeric coprocessor is only saved when a RTEMS_FLOATING_POINT task is dispatched and that task was not the last task to utilize the coprocessor. In a system with only one RTEMS_FLOATING_POINT task, the state of the numeric coprocessor will never be saved or restored.
Although the overhead imposed by RTEMS_FLOATING_POINT tasks is minimal, some applications may wish to completely avoid the overhead associated with RTEMS_FLOATING_POINT tasks and still utilize a numeric coprocessor. By preventing a task from being preempted while performing a sequence of floating point operations, a RTEMS_NO_FLOATING_POINT task can utilize the numeric coprocessor without incurring the overhead of a RTEMS_FLOATING_POINT context switch. This approach also avoids the allocation of a floating point context area. However, if this approach is taken by the application designer, NO tasks should be created as RTEMS_FLOATING_POINT tasks. Otherwise, the floating point context will not be correctly maintained because RTEMS assumes that the state of the numeric coprocessor will not be altered by RTEMS_NO_FLOATING_POINT tasks.
If the supported processor type does not have hardware floating capabilities or a standard numeric coprocessor, RTEMS will not provide built-in support for hardware floating point on that processor. In this case, all tasks are considered RTEMS_NO_FLOATING_POINT whether created as RTEMS_FLOATING_POINT or RTEMS_NO_FLOATING_POINT tasks. A floating point emulation software library must be utilized for floating point operations.
On some processors, it is possible to disable the floating point unit dynamically. If this capability is supported by the target processor, then RTEMS will utilize this capability to enable the floating point unit only for tasks which are created with the RTEMS_FLOATING_POINT attribute. The consequence of a RTEMS_NO_FLOATING_POINT task attempting to access the floating point unit is CPU dependent but will generally result in an exception condition.
Per task variables are no longer available. In particular the rtems_task_variable_add(), rtems_task_variable_get() and rtems_task_variable_delete() functions are neither declared nor defined anymore. Use thread local storage or POSIX Keys instead.
In general, an attribute set is built by a bitwise OR of the desired components. The set of valid task attribute components is listed below:
Attribute values are specifically designed to be mutually exclusive, therefore bitwise OR and addition operations are equivalent as long as each attribute appears exactly once in the component list. A component listed as a default is not required to appear in the component list, although it is a good programming practice to specify default components. If all defaults are desired, then RTEMS_DEFAULT_ATTRIBUTES should be used. This example demonstrates the attribute_set parameter needed to create a local task which utilizes the numeric coprocessor. The attribute_set parameter could be RTEMS_FLOATING_POINT or RTEMS_LOCAL | RTEMS_FLOATING_POINT. The attribute_set parameter can be set to RTEMS_FLOATING_POINT because RTEMS_LOCAL is the default for all created tasks. If the task were global and used the numeric coprocessor, then the attribute_set parameter would be RTEMS_GLOBAL | RTEMS_FLOATING_POINT.
In general, a mode and its corresponding mask is built by a bitwise OR of the desired components. The set of valid mode constants and each mode's corresponding mask constant is listed below:
| Mode Constant | Mask Constant | Description |
|---|---|---|
| RTEMS_PREEMPT | RTEMS_PREEMPT_MASK | enables preemption |
| RTEMS_NO_PREEMPT | RTEMS_PREEMPT_MASK | disables preemption |
| RTEMS_NO_TIMESLICE | RTEMS_TIMESLICE_MASK | disables timeslicing |
| RTEMS_TIMESLICE | RTEMS_TIMESLICE_MASK | enables timeslicing |
| RTEMS_ASR | RTEMS_ASR_MASK | enables ASR processing |
| RTEMS_NO_ASR | RTEMS_ASR_MASK | disables ASR processing |
| RTEMS_INTERRUPT_LEVEL(0) | RTEMS_INTERRUPT_MASK | enables all interrupts |
| RTEMS_INTERRUPT_LEVEL(n) | RTEMS_INTERRUPT_MASK | sets interrupts level n |
Mode values are specifically designed to be mutually exclusive, therefore bitwise OR and addition operations are equivalent as long as each mode appears exactly once in the component list. A mode component listed as a default is not required to appear in the mode component list, although it is a good programming practice to specify default components. If all defaults are desired, the mode RTEMS_DEFAULT_MODES and the mask RTEMS_ALL_MODE_MASKS should be used.
The following example demonstrates the mode and mask parameters used with the rtems_task_mode() directive to place a task at interrupt level 3 and make it non-preemptible. The mode should be set to RTEMS_INTERRUPT_LEVEL(3) | RTEMS_NO_PREEMPT to indicate the desired preemption mode and interrupt level, while the mask parameter should be set to RTEMS_INTERRUPT_MASK | RTEMS_PREEMPT_MASK to indicate that the calling task's interrupt level and preemption mode are being altered.
1.8.14