4d8b60adca
Moves the Microkernel Object documentation to a new Microkernel Services section within the Kernel Primer. Labels, cross-references, figures, headings and filenames were updated to reflect the new structure. Change-Id: Ia2a91410a94caa8a97bb8211db5afc84b5dc0974 Signed-off-by: Rodrigo Caballero <rodrigo.caballero.abraham@intel.com>
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ReStructuredText
436 lines
16 KiB
ReStructuredText
.. _tasks:
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Task Services
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#############
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Properties of Tasks
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*******************
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A task is an execution thread that implements part or all
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of the application functionality using the objects described in detail by the
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Nanokernel Objects and Microkernel Objects sections. Tasks are cooperatively
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scheduled, and will run until they explicitly yield, call a blocking interface
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or are preempted by a higher priority task.
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Task Groups
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***********
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TBD (how they are used; maximum of 32 groups; mention pre-defined task groups)
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Task Behavior
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*************
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When a task calls an API to operate on a kernel object, it passes
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an abstract object identifier called objectID. A task shall always
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manipulate kernel data structures through the APIs and shall not
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directly access the internals of any object, for example, the internals
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of a semaphore or a FIFO.
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Task Implementation
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*******************
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Use kernel objects and routine calls to interface a task with
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other tasks running in the system. For example, achieve cooperation
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between tasks by using synchronization objects, such as resources and
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semaphores, or by passing parameters from one task to another using a
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data-passing object.
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Task Stack
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==========
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The compiler uses the task stack to store local task variables and to
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implement parameter-passing between functions. Static and global
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variables do not use memory from the stack. For more information about
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defining memory segments, and the defaults used for different variable
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types, consult the documentation for your compiler.
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Task States
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===========
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Each task has a task state that the scheduler uses to determine whether
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it is ready to run. This figure shows the possible task states and the
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possible transitions. The most usual transitions are green,
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bidirectional transitions are blue and uncommon transitions are marked
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orange.
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.. figure:: figures/microkernel_tasks_states.svg
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:scale: 75 %
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:alt: Possible Task States
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Shows the possible states that a task might have and their transitions.
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Starting and Stopping Tasks
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---------------------------
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Tasks are started in one of three ways:
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+ Automatically at boot time if it is assigned to the EXE task group.
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+ Another task issues a :c:func:`task_start()` for the task.
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+ Another task issues a :c:func:`task_group_start()` for any task
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group the task belongs to..
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The scheduler manages the execution of a task once it is running. If the
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task performs a return from the routine that started it, the task
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terminates and its stack can be reused. This ensures that the task
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terminates safely and cleanly.
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Automatically Starting Tasks
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----------------------------
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Starting tasks automatically at boot utilizes the Task Grouping concept.
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The EXE group at boot time will put all tasks belonging to the group in
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a runnable state immediately after the kernel boots up.
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Tasks Starting Other Tasks
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--------------------------
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.. todo:: Add details on how to start a task from within another task.
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Tasks Scheduling Model
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**********************
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Once started, a task is scheduled for execution by the microkernel until
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one of the following occurs:
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* A higher-priority task becomes ready to run.
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* The task completes.
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* The task's time slice expires and another runnable task of equal
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priority exists.
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* The task becomes non-runnable.
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Task Completion
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===============
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.. todo:: Add details on how tasks complete.
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Task Priorities
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===============
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The kernel offers a configurable number of task priority levels. The
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number ranges from 0 to :literal:`NUM_TASK_PRIORITIES-1`. The lowest
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priority level ( :literal:`NUM_TASK_PRIORITIES-1` is reserved for use
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by the microkernel's idle task. The priority of tasks is assigned
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during the build process based upon the task definition in the project
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file. The priority can be changed at any time, by either the task
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itself or by another task calling :c:func:`task_priority_set()`.
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If a task of higher priority becomes runnable, the kernel saves the
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current tasks context and runs the higher-priority task. It is also
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possible for a tasks priority to be temporarily changed to prevent a
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condition known as priority inversion.
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Priority Preemption
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-------------------
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The microkernel uses a priority-based preemptive scheduling algorithm
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where the highest-priority task that is ready to run, runs. When a task
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with a higher priority becomes runnable, the running task is
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unscheduled and the task of higher priority is started. This is the
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principle of preemption.
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Suspended Tasks
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===============
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Tasks can suspend other tasks, or themselves, using
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:c:func:`task_suspend()`. The task stays suspended until
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:c:func:`task_resume()` or :c:func:`task_abort()` is called by another
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task. Use :c:func:`task_abort()` and :c:func:`task_group_abort()` with
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care, as none of the affected tasks may own or be using kernel objects
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when they are called. The safest abort practice is for a task to abort
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only itself.
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Aborting a Task
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---------------
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Tasks can have an abort handler, C routines that run as a critical
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section when a task is aborted. Since the routine runs as critical, it
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cannot be preempted or unscheduled allowing the task to properly clean
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up. Because of this, abort handlers cannot make kernel API calls.
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To install an abort handler function use
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:c:func:`task_abort_handler_set()`. This will bind the routine for
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execution when :c:func:`task_abort()` is called, and run the abort
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handler function immediately.
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Task Time-Slicing
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=================
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Time-slicing, enabled through the :c:func:`sys_scheduler_time_slice_set()`
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function, can share a processor between multiple tasks with the same
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priority. When enabled, the kernel preempts a task that has run for a
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certain amount of time, the time slice, and schedules another runnable
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task with the same priority. The sorting of tasks of equal priority
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order is a fundamental microkernel scheduling concept and is not
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limited to cases involving :c:func:`task_yield()`.
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The same effect as time-slicing can be achieved using
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:c:func:`task_yield()`. When this call is made, the current task
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relinquishes the processor if another task of the same priority is
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ready to run. The calling task returns to the queue of runnable tasks.
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If no other task of the same priority is runnable, the task that called
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:c:func:`task_yield()` continues running.
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.. note::
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:c:func:`task_yield()` sorts the tasks in FIFO order.
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Task Context Switches
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=====================
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When a task swap occurs, the kernel saves the context of the task
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that is swapped out and restores the context of the task that is
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swapped in.
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Defining a Task
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***************
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Inside MDEF files
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=================
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The following parameters must be defined:
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*name*
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This specifies a unique name for the task.
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*priority*
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This specifies the scheduling priority of the task. A smaller
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integer value indicates higher priority, with zero indicating
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highest priority.
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*entry_point*
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This specifies the name of the task's entry point function,
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which should have the following form:
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.. code-block:: c
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void <entry_point>(void)".
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{
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/* task mainline processing */
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...
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/* (optional) normal task termination */
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return;
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}
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*stack_size*
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This specifies the size of the task's stack, in bytes.
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*groups*
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This specifies the task groups the task belongs to. It consists
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of a comma-separated list of task group names enclosed in square
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brackets, with no embedded spaces. If a task does not belong
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to any groups an empty list must be specified; i.e. :literal:`[]`.
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Add an entry for a task in the project .MDEF file using the
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following syntax:
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.. code-block:: console
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TASK %name %priority %entry_point %stack_size %groups
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For example, the file :file:`projName.mdef` defines a system comprised
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of six tasks as follows:
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.. code-block:: console
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% TASK NAME PRIO ENTRY STACK GROUPS
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% ===================================================================
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TASK MAIN_TASK 6 keypad_main 1024 [KEYPAD_TASKS,EXE]
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TASK PROBE_TASK 2 probe_main 400 []
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TASK SCREEN1_TASK 8 screen_1_main 4096 [VIDEO_TASKS]
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TASK SCREEN2_TASK 8 screen_2_main 4096 [VIDEO_TASKS]
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TASK SPEAKER1_TASK 10 speaker_1_main 1024 [AUDIO_TASKS]
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TASK SPEAKER2_TASK 10 speaker_2_main 1024 [AUDIO_TASKS]
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Inside Source Code
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==================
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In addition to defining tasks in MDEF file, it is also possible to
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define tasks inside code. The macro ``DEFINE_TASK(...)`` can be
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used for this purpose.
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For example, the following code can be used to define a global task
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``PRIV_TASK``.
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.. code-block:: c
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DEFINE_TASK(PRIV_TASK, priority, entry, stack_size, groups);
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where the parameters are the same as tasks defined in MDEF file.
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The task ``PRIV_TASK`` can be used in the same style as those
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defined in MDEF file.
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It is possible to utilize this task in another source file, simply
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add:
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.. code-block:: c
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extern const ktask_t PRIV_TASK;
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to that file. The task ``PRIV_TASK`` can be then used there.
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Defining a Task Group
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*************************
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The following parameters must be defined:
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*name*
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This specifies a unique name for the task group.
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Add an entry for a task in the project .MDEF file using the
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following syntax:
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.. code-block:: console
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TASKGROUP %name
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For example, the file :file:`projName.mdef` defines three new
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task groups as follows:
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.. code-block:: console
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% TASKGROUP NAME
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% ========================
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TASKGROUP VIDEO_TASKS
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TASKGROUP AUDIO_TASKS
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TASKGROUP KEYPAD_TASKS
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Example: Starting a Task from a Different Task
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==============================================
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This code shows how the currently executing task can start another task.
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.. code-block:: c
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void keypad_main(void)
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{
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/* begin system initialization */
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...
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/* start task to monitor temperature */
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task_start(PROBE_TASK);
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/* continue to bring up and operate system */
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...
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}
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Example: Suspending and Resuming a Set of Tasks
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===============================================
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This code shows how the currently executing task can temporarily suspend
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the execution of all tasks belonging to the designated task groups.
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.. code-block:: c
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void probe_main(void)
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{
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int was_overheated = 0;
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/* continuously monitor temperature */
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while (1) {
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now_overheated = overheating_update();
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/* suspend non-essential tasks when overheating is detected */
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if (now_overheated && !was_overheated) {
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task_group_suspend(VIDEO_TASKS | AUDIO_TASKS);
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was_overheated = 1;
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}
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/* resume non-essential tasks when overheating abates */
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if (!now_overheated && was_overheated) {
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task_group_resume(VIDEO_TASKS | AUDIO_TASKS);
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was_overheated = 0;
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}
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/* wait 10 ticks of system clock before checking again */
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task_sleep(10);
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}
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}
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APIs
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****
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The following APIs affecting the currently executing task
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are provided by :file:`microkernel.h`.
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+-------------------------------------+-----------------------------------------+
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| Call | Description |
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+=====================================+=========================================+
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| :c:func:`task_id_get()` | Gets the task's ID. |
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+-------------------------------------+-----------------------------------------+
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| :c:func:`isr_task_id_get()` | Gets the task's ID from an ISR. |
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+-------------------------------------+-----------------------------------------+
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| :c:func:`task_priority_get()` | Gets the task's priority. |
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+-------------------------------------+-----------------------------------------+
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| :c:func:`isr_task_priority_get()` | Gets the task's priority from an ISR. |
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+-------------------------------------+-----------------------------------------+
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| :c:func:`task_group_mask_get()` | Gets the task's group memberships. |
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+-------------------------------------+-----------------------------------------+
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| :c:func:`isr_task_group_mask_get()` | Gets the task's group memberships from |
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| | an ISR. |
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+-------------------------------------+-----------------------------------------+
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| :c:func:`task_yield()` | Yields CPU to equal-priority tasks. |
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+-------------------------------------+-----------------------------------------+
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| :c:func:`task_sleep()` | Yields CPU for a specified time period. |
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+-------------------------------------+-----------------------------------------+
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| :c:func:`task_abort_handler_set()` | Installs the task's abort handler. |
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+-------------------------------------+-----------------------------------------+
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The following APIs affecting a specified task
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are provided by :file:`microkernel.h`.
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+-------------------------------------------+----------------------------------+
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| Call | Description |
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+===========================================+==================================+
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| :c:func:`task_priority_set()` | Sets a task's priority. |
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+-------------------------------------------+----------------------------------+
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| :c:func:`task_entry_set()` | Sets a task's entry point. |
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+-------------------------------------------+----------------------------------+
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| :c:func:`task_start()` | Starts execution of a task. |
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+-------------------------------------------+----------------------------------+
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| :c:func:`task_suspend()` | Suspends execution of a task. |
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+-------------------------------------------+----------------------------------+
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| :c:func:`task_resume()` | Resumes execution of a task. |
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+-------------------------------------------+----------------------------------+
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| :c:func:`task_abort()` | Aborts execution of a task. |
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+-------------------------------------------+----------------------------------+
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| :c:func:`task_group_join()` | Adds a task to the specified |
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| | task group(s). |
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+-------------------------------------------+----------------------------------+
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| :c:func:`task_group_leave()` | Removes a task from the |
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| | specified task group(s). |
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+-------------------------------------------+----------------------------------+
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The following APIs affecting multiple tasks
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are provided by :file:`microkernel.h`.
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+-------------------------------------------+---------------------------------+
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| Call | Description |
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+===========================================+=================================+
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| :c:func:`sys_scheduler_time_slice_set()` | Sets the time slice period used |
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| | in round-robin task scheduling. |
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+-------------------------------------------+---------------------------------+
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| :c:func:`task_group_start()` | Starts execution of all tasks |
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| | in the specified task groups. |
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+-------------------------------------------+---------------------------------+
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| :c:func:`task_group_suspend()` | Suspends execution of all tasks |
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| | in the specified task groups. |
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+-------------------------------------------+---------------------------------+
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| :c:func:`task_group_resume()` | Resumes execution of all tasks |
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| | in the specified task groups. |
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+-------------------------------------------+---------------------------------+
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| :c:func:`task_group_abort()` | Aborts execution of all tasks |
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| | in the specified task groups. |
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+-------------------------------------------+---------------------------------+ |