source: rtems/doc/user/schedule.t @ 13fb305

@c
@c  COPYRIGHT (c) 1988-1998.
@c  On-Line Applications Research Corporation (OAR).
@c  All rights reserved.
@c
@c  $Id$
@c

@c
@c   This figure is not included:
@c      Figure 17-1  RTEMS Task State Transitions
@c

@chapter Scheduling Concepts

@section Introduction

The concept of scheduling in real-time systems
dictates the ability to provide immediate response to specific
external events, particularly the necessity of scheduling tasks
to run within a specified time limit after the occurrence of an
event.  For example, software embedded in life-support systems
used to monitor hospital patients must take instant action if a
change in the patient's status is detected.

The component of RTEMS responsible for providing this
capability is appropriately called the scheduler.  The
scheduler's sole purpose is to allocate the all important
resource of processor time to the various tasks competing for
attention.  The RTEMS scheduler allocates the processor using a
priority-based, preemptive algorithm augmented to provide
round-robin characteristics within individual priority groups.
The goal of this algorithm is to guarantee that the task which
is executing on the processor at any point in time is the one
with the highest priority among all tasks in the ready state.

There are two common methods of accomplishing the
mechanics of this algorithm.  Both ways involve a list or chain
of tasks in the ready state.  One method is to randomly place
tasks in the ready chain forcing the scheduler to scan the
entire chain to determine which task receives the processor.
The other method is to schedule the task by placing it in the
proper place on the ready chain based on the designated
scheduling criteria at the time it enters the ready state.
Thus, when the processor is free, the first task on the ready
chain is allocated the processor.  RTEMS schedules tasks using
the second method to guarantee faster response times to external
events.

@section Scheduling Mechanisms

RTEMS provides four mechanisms which allow the user
to impact the task scheduling process:

@itemize @bullet
@item user-selectable task priority level
@item task preemption control
@item task timeslicing control
@item manual round-robin selection
@end itemize

Each of these methods provides a powerful capability
to customize sets of tasks to satisfy the unique and particular
requirements encountered in custom real-time applications.
Although each mechanism operates independently, there is a
precedence relationship which governs the effects of scheduling
modifications.  The evaluation order for scheduling
characteristics is always priority, preemption mode, and
timeslicing.  When reading the descriptions of timeslicing and
manual round-robin it is important to keep in mind that
preemption (if enabled) of a task by higher priority tasks will
occur as required, overriding the other factors presented in the
description.

@subsection Task Priority and Scheduling

The most significant of these mechanisms is the
ability for the user to assign a priority level to each
individual task when it is created and to alter a task's
priority at run-time.  RTEMS provides 255 priority levels.
Level 255 is the lowest priority and level 1 is the highest.
When a task is added to the ready chain, it is placed behind all
other tasks of the same priority.  This rule provides a
round-robin within priority group scheduling characteristic.
This means that in a group of equal priority tasks, tasks will
execute in the order they become ready or FIFO order.  Even
though there are ways to manipulate and adjust task priorities,
the most important rule to remember is:

@itemize @code{ }
@item @b{The RTEMS scheduler will always select the highest
priority task that is ready to run when allocating the processor
to a task.}
@end itemize

@subsection Preemption

Another way the user can alter the basic scheduling
algorithm is by manipulating the preemption mode flag
(@code{@value{RPREFIX}PREEMPT_MASK}) of individual tasks.  If preemption is disabled
for a task (@code{@value{RPREFIX}NO_PREEMPT}), then the task will not relinquish
control of the processor until it terminates, blocks, or
re-enables preemption.  Even tasks which become ready to run and
possess higher priority levels will not be allowed to execute.
Note that the preemption setting has no effect on the manner in
which a task is scheduled.  It only applies once a task has
control of the processor.

@subsection Timeslicing

Timeslicing or round-robin scheduling is an
additional method which can be used to alter the basic
scheduling algorithm.  Like preemption, timeslicing is specified
on a task by task basis using the timeslicing mode flag
(@code{@value{RPREFIX}TIMESLICE_MASK}).  If timeslicing is enabled for a task
(@code{@value{RPREFIX}TIMESLICE}), then RTEMS will limit the amount of time the task
can execute before the processor is allocated to another task.
Each tick of the real-time clock reduces the currently running
task's timeslice.  When the execution time equals the timeslice,
RTEMS will dispatch another task of the same priority to
execute.  If there are no other tasks of the same priority ready
to execute, then the current task is allocated an additional
timeslice and continues to run.  Remember that a higher priority
task will preempt the task (unless preemption is disabled) as
soon as it is ready to run, even if the task has not used up its
entire timeslice.

@subsection Manual Round-Robin

The final mechanism for altering the RTEMS scheduling
algorithm is called manual round-robin.  Manual round-robin is
invoked by using the @code{@value{DIRPREFIX}task_wake_after}
directive with a time interval of @code{@value{RPREFIX}YIELD_PROCESSOR}.  
This allows a task to give up the
processor and be immediately returned to the ready chain at the
end of its priority group.  If no other tasks of the same
priority are ready to run, then the task does not lose control
of the processor.

@subsection Dispatching Tasks

The dispatcher is the RTEMS component responsible for
allocating the processor to a ready task.  In order to allocate
the processor to one task, it must be deallocated or retrieved
from the task currently using it.  This involves a concept
called a context switch.  To perform a context switch, the
dispatcher saves the context of the current task and restores
the context of the task which has been allocated to the
processor.  Saving and restoring a task's context is the
storing/loading of all the essential information about a task to
enable it to continue execution without any effects of the
interruption.  For example, the contents of a task's register
set must be the same when it is given the processor as they were
when it was taken away.  All of the information that must be
saved or restored for a context switch is located either in the
TCB or on the task's stacks.

Tasks that utilize a numeric coprocessor and are
created with the @code{@value{RPREFIX}FLOATING_POINT} attribute
require additional operations during a context switch.  These
additional operations
are necessary to save and restore the floating point context of
@code{@value{RPREFIX}FLOATING_POINT} tasks.  To avoid unnecessary save and restore
operations, the state of the numeric coprocessor is only saved
when a @code{@value{RPREFIX}FLOATING_POINT} task is dispatched and that task was not
the last task to utilize the coprocessor.

@section Task State Transitions

Tasks in an RTEMS system must always be in one of the
five allowable task states.  These states are: executing, ready,
blocked, dormant, and non-existent.

A task occupies the non-existent state before a
@code{@value{DIRPREFIX}task_create} has been
issued on its behalf.  A task enters the
non-existent state from any other state in the system when it is
deleted with the @code{@value{DIRPREFIX}task_delete}
directive.  While a task occupies
this state it does not have a TCB or a task ID assigned to it;
therefore, no other tasks in the system may reference this task.

When a task is created via the @code{@value{DIRPREFIX}task_create} directive
it enters the dormant state.  This state is not entered through
any other means.  Although the task exists in the system, it
cannot actively compete for system resources.  It will remain in
the dormant state until it is started via the @code{@value{DIRPREFIX}task_start}
directive, at which time it enters the ready state.  The task is
now permitted to be scheduled for the processor and to compete
for other system resources.

@ifset use-ascii
@example
@group
     +-------------------------------------------------------------+
     |                         Non-existent                        |
     |  +-------------------------------------------------------+  |
     |  |                                                       |  |
     |  |                                                       |  |
     |  |      Creating        +---------+     Deleting         |  |
     |  | -------------------> | Dormant | -------------------> |  |
     |  |                      +---------+                      |  |
     |  |                           |                           |  |
     |  |                  Starting |                           |  |
     |  |                           |                           |  |
     |  |                           V          Deleting         |  |
     |  |             +-------> +-------+ ------------------->  |  |
     |  |  Yielding  /   +----- | Ready | ------+               |  |
     |  |           /   /       +-------+ <--+   \              |  |
     |  |          /   /                      \   \ Blocking    |  |
     |  |         /   / Dispatching   Readying \   \            |  |
     |  |        /   V                          \   V           |  |
     |  |      +-----------+    Blocking     +---------+        |  |
     |  |      | Executing | --------------> | Blocked |        |  |
     |  |      +-----------+                 +---------+        |  |
     |  |                                                       |  |
     |  |                                                       |  |
     |  +-------------------------------------------------------+  |
     |                         Non-existent                        |
     +-------------------------------------------------------------+
@end group
@end example
@end ifset

@ifset use-tex
@c @page
@c @image{states,5in,4in}
@example
@group
     +-------------------------------------------------------------+
     |                         Non-existent                        |
     |  +-------------------------------------------------------+  |
     |  |                                                       |  |
     |  |                                                       |  |
     |  |      Creating        +---------+     Deleting         |  |
     |  | -------------------> | Dormant | -------------------> |  |
     |  |                      +---------+                      |  |
     |  |                           |                           |  |
     |  |                  Starting |                           |  |
     |  |                           |                           |  |
     |  |                           V          Deleting         |  |
     |  |             +-------> +-------+ ------------------->  |  |
     |  |  Yielding  /   +----- | Ready | ------+               |  |
     |  |           /   /       +-------+ <--+   \              |  |
     |  |          /   /                      \   \ Blocking    |  |
     |  |         /   / Dispatching   Readying \   \            |  |
     |  |        /   V                          \   V           |  |
     |  |      +-----------+    Blocking     +---------+        |  |
     |  |      | Executing | --------------> | Blocked |        |  |
     |  |      +-----------+                 +---------+        |  |
     |  |                                                       |  |
     |  |                                                       |  |
     |  +-------------------------------------------------------+  |
     |                         Non-existent                        |
     +-------------------------------------------------------------+
@end group
@end example
@end ifset

@ifset use-html
@html
<IMG SRC="states.png" WIDTH=550 HEIGHT=400 ALT="RTEMS Task States">
@end html
@end ifset

A task occupies the blocked state whenever it is
unable to be scheduled to run.  A running task may block itself
or be blocked by other tasks in the system.  The running task
blocks itself through voluntary operations that cause the task
to wait.  The only way a task can block a task other than itself
is with the @code{@value{DIRPREFIX}task_suspend} directive.  
A task enters the blocked state due to any of the following conditions:

@itemize @bullet
@item A task issues a @code{@value{DIRPREFIX}task_suspend} directive
which blocks either itself or another task in the system.

@item The running task issues a @code{@value{DIRPREFIX}message_queue_receive}
directive with the wait option and the message queue is empty.

@item The running task issues an @code{@value{DIRPREFIX}event_receive}
directive with the wait option and the currently pending events do not
satisfy the request.

@item The running task issues a @code{@value{DIRPREFIX}semaphore_obtain}
directive with the wait option and the requested semaphore is unavailable.

@item The running task issues a @code{@value{DIRPREFIX}task_wake_after}
directive which blocks the task for the given time interval.  If the time
interval specified is zero, the task yields the processor and
remains in the ready state.

@item The running task issues a @code{@value{DIRPREFIX}task_wake_when}
directive which blocks the task until the requested date and time arrives.

@item The running task issues a @code{@value{DIRPREFIX}region_get_segment}
directive with the wait option and there is not an available segment large
enough to satisfy the task's request.

@item The running task issues a @code{@value{DIRPREFIX}rate_monotonic_period}
directive and must wait for the specified rate monotonic period
to conclude.
@end itemize

A blocked task may also be suspended.  Therefore,
both the suspension and the blocking condition must be removed
before the task becomes ready to run again.

A task occupies the ready state when it is able to be
scheduled to run, but currently does not have control of the
processor.  Tasks of the same or higher priority will yield the
processor by either becoming blocked, completing their
timeslice, or being deleted.  All tasks with the same priority
will execute in FIFO order.  A task enters the ready state due
to any of the following conditions:

@itemize @bullet

@item A running task issues a @code{@value{DIRPREFIX}task_resume}
directive for a task that is suspended and the task is not blocked
waiting on any resource.

@item A running task issues a @code{@value{DIRPREFIX}message_queue_send},
@code{@value{DIRPREFIX}message_queue_broadcast}, or a
@code{@value{DIRPREFIX}message_queue_urgent} directive
which posts a message to the queue on which the blocked task is
waiting.

@item A running task issues an @code{@value{DIRPREFIX}event_send}
directive which sends an event condition to a task which is blocked
waiting on that event condition.

@item A running task issues a @code{@value{DIRPREFIX}semaphore_release}
directive which releases the semaphore on which the blocked task is
waiting.

@item A timeout interval expires for a task which was blocked
by a call to the @code{@value{DIRPREFIX}task_wake_after} directive.

@item A timeout period expires for a task which blocked by a
call to the @code{@value{DIRPREFIX}task_wake_when} directive.

@item A running task issues a @code{@value{DIRPREFIX}region_return_segment}
directive which releases a segment to the region on which the blocked task
is waiting and a resulting segment is large enough to satisfy
the task's request.

@item A rate monotonic period expires for a task which blocked
by a call to the @code{@value{DIRPREFIX}rate_monotonic_period} directive.

@item A timeout interval expires for a task which was blocked
waiting on a message, event, semaphore, or segment with a
timeout specified.

@item A running task issues a directive which deletes a
message queue, a semaphore, or a region on which the blocked
task is waiting.

@item A running task issues a @code{@value{DIRPREFIX}task_restart}
directive for the blocked task.

@item The running task, with its preemption mode enabled, may
be made ready by issuing any of the directives that may unblock
a task with a higher priority.  This directive may be issued
from the running task itself or from an ISR.

A ready task occupies the executing state when it has
control of the CPU.  A task enters the executing state due to
any of the following conditions:

@item The task is the highest priority ready task in the
system.

@item The running task blocks and the task is next in the
scheduling queue.  The task may be of equal priority as in
round-robin scheduling or the task may possess the highest
priority of the remaining ready tasks.

@item The running task may reenable its preemption mode and a
task exists in the ready queue that has a higher priority than
the running task.

@item The running task lowers its own priority and another
task is of higher priority as a result.

@item The running task raises the priority of a task above its
own and the running task is in preemption mode.

@end itemize