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doc: Edit nanokernel_fibers for doc structure with rst, grammar

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To enhance readability, add bold, lists in sentence form (as per
the style guide) and other parallel content from microkernels section.

Change-Id: Ida989ec7187bf7a035803644b5cc710571b66e3e
Signed-off-by: L.S. Cook <leonax.cook@intel.com>
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L.S. Cook 2016-03-16 11:35:30 -07:00 • committed by Gerrit Code Review
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112
doc/kernel/nanokernel/nanokernel_fibers.rst
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@ -6,46 +6,46 @@ Fiber Services
Concepts
********
A fiber is a lightweight, non-preemptible thread of execution that implements
a portion of an application's processing. It is is normally used when writing
device drivers and other performance critical work.
A :dfn:`fiber` is a lightweight, non-preemptible thread of execution that
implements a portion of an application's processing. Fibers are often
used in device drivers and for performance-critical work.
Fibers can be used by microkernel applications, as well as by nanokernel
applications. However, fibers can only interact with microkernel object types
to a very limited degree; for more information see
:ref:`Microkernel Fiber Services <microkernel_fibers>`.
applications. However, fibers can interact with microkernel object types
to only a limited degree; for more information see :ref:`microkernel_fibers`.
An application can use any number of fibers. Each fiber is anonymous, and
cannot be directly referenced by other fibers or tasks once it has started
executing. The properties that must be specified when a fiber is spawned
include:
* a memory region that is used for its stack and for other execution context
information,
* a function that is invoked when the fiber starts executing,
* a pair of arguments that are passed to that entry point function,
* a priority that is used by the nanokernel scheduler, and
* a set of options that apply to the fiber.
* A **memory region** to be used for stack and execution context information.
* A **function** to be invoked when the fiber starts executing.
* A **pair of arguments** to be passed to that entry point function.
* A **priority** to be used by the nanokernel scheduler.
* A **set of options** that will apply to the fiber.
The kernel may automatically spawn zero or more system fibers during system
initialization. The specific set of fibers spawned depends on the kernel
capabilities that have been configured by the application and by the
board configuration used to build the application image.
initialization. The specific set of fibers spawned depends upon both:
#. The kernel capabilities that have been configured by the application.
#. The board configuration used to build the application image.
Fiber Lifecycle
===============
A fiber can be spawned by another fiber, by a task, or by the kernel itself
during system initialization. A fiber typically becomes executable immediately;
however it is possible to delay the scheduling of a newly spawned fiber
for a specified time period---for example, to allow device hardware which
the fiber uses to become available. The kernel also supports a delayed start
cancellation capability, which prevents a newly spawned fiber from executing
if the fiber becomes unnecessary before its full delay period is reached.
however, it is possible to delay the scheduling of a newly-spawned fiber for a
specified time period. For example, scheduling can be delayed to allow device
hardware which the fiber uses to become available. The kernel also supports a
delayed start cancellation capability, which prevents a newly-spawned fiber from
executing if the fiber becomes unnecessary before its full delay period is reached.
Once a fiber is started it normally executes forever. A fiber may terminate
itself gracefully by simply returning from its entry point function. If it
does, it is the fiber's responsibility to release any system resources it may
itself gracefully by simply returning from its entry point function. When this
happens, it is the fiber's responsibility to release any system resources it may
own (such as a nanokernel semaphore being used in a mutex-like manner) prior
to returning, since the kernel does *not* attempt to reclaim them so they can
be reused.
@ -64,18 +64,18 @@ Fiber Scheduling
================
The nanokernel's scheduler selects which of the system's threads is allowed
to execute; this thread is known as the *current context*. The nanokernel's
scheduler permits threads to execute only when there is no ISR that needs
to execute, since ISR execution takes precedence.
to execute; this thread is known as the :dfn:`current context`. The nanokernel's
scheduler permits threads to execute only when no ISR needs to execute; execution
of ISRs take precedence.
When executing threads, the nanokernel's scheduler gives fiber execution
precedence over task execution. The scheduler preempts task execution
whenever a fiber needs to execute, but never preempts the execution of a fiber
to allow another fiber to execute---even if it is a higher priority fiber.
to allow another fiber to execute -- even if it is a higher priority fiber.
The kernel automatically takes care of saving an executing fiber's CPU register
values when it performs a context switch to a different fiber, a task, or
an ISR, and restores these values when the fiber later resumes execution.
The kernel automatically saves an executing fiber's CPU register values when
making a context switch to a different fiber, a task, or an ISR; these values
get restored when the fiber later resumes execution.
Fiber State
-----------
@ -84,9 +84,9 @@ A fiber has an implicit *state* that determines whether or not it can be
scheduled for execution. The state records all factors that can prevent
the fiber from executing, such as:
* the fiber has not been spawned
* the fiber is waiting for it needs (e.g. a semaphore, a timeout, ...)
* the fiber has terminated
* The fiber has not been spawned.
* The fiber is waiting for a kernel service, for example, a semaphore or a timer.
* The fiber has terminated.
A fiber whose state has no factors that prevent its execution is said to be
*executable*.
@ -104,17 +104,19 @@ spawned.
Fiber Scheduling Algorithm
--------------------------
The nanokernel's scheduler selects the highest priority executable fiber
to be the current context, if possible. If multiple executable fibers
of that priority are available the scheduler chooses the one that has been
Whenever possible, the nanokernel's scheduler selects the highest priority
executable fiber to be the current context. When multiple executable fibers
of that priority are available, the scheduler chooses the one that has been
waiting longest.
If no executable fibers exist the scheduler selects the current task
to be the current context. In a nanokernel application the current task is
the background task, while in a microkernel application it is the current task
selected by the microkernel's scheduler. The current task is always executable.
When no executable fibers exist, the scheduler selects the current task
to be the current context. The current task selected depends upon whether the
application is a nanokernel application or a microkernel application. In nanokernel
applications, the current task is always the background task. In microkernel
applications, the current task is the current task selected by the microkernel's
scheduler. The current task is always executable.
Once a fiber becomes the current context it remains scheduled for execution
Once a fiber becomes the current context, it remains scheduled for execution
by the nanokernel until one of the following occurs:
* The fiber is supplanted by another thread because it calls a kernel API
@ -126,11 +128,11 @@ by the nanokernel until one of the following occurs:
* The fiber aborts itself by performing an operation that causes a fatal error,
or by calling :cpp:func:`fiber_abort()`.
Once the current task becomes the current context it remains scheduled for
Once the current task becomes the current context, it remains scheduled for
execution by the nanokernel until is supplanted by a fiber.
.. note::
The current task is never directly supplanted by another task, since the
The current task is **never** directly supplanted by another task, since the
microkernel scheduler uses the microkernel server fiber to initiate a
change from one microkernel task to another.
@ -140,7 +142,7 @@ Cooperative Time Slicing
Due to the non-preemptive nature of the nanokernel's scheduler, a fiber that
performs lengthy computations may cause an unacceptable delay in the scheduling
of other fibers, including higher priority and equal priority ones. To overcome
such problems the fiber can choose to voluntarily relinquish the CPU from time
such problems, the fiber can choose to voluntarily relinquish the CPU from time
to time to permit other fibers to execute.
A fiber can relinquish the CPU in two ways:
@ -162,12 +164,12 @@ A fiber can relinquish the CPU in two ways:
Fiber Options
=============
The kernel supports several *fiber options* that inform the kernel about
special treatment the fiber requires.
The kernel supports several :dfn:`fiber options` that may be used to inform
the kernel of special treatment the fiber requires.
The set of kernel options associated with a fiber are specified when the fiber
is spawned. If the fiber uses multiple options they are separated using
:literal:`|`; i.e. the logical OR operator. A fiber that does not use any
is spawned. If the fiber uses multiple options, they are separated with
:literal:`|`, the logical ``OR`` operator. A fiber that does not use any
options is spawned using an options value of 0.
The fiber options listed below are pre-defined by the kernel.
@ -178,8 +180,8 @@ The fiber options listed below are pre-defined by the kernel.
:c:macro:`USE_SSE`
Instructs the kernel to save the fiber's SSE floating point context
information during context switches. (A fiber using this option
implicitly uses the :c:macro:`USE_FP` option too.)
information during context switches. A fiber with this option
implicitly uses the :c:macro:`USE_FP` option, as well.
Usage
*****
@ -275,20 +277,20 @@ each dedicated to processing data from a different communication channel.
APIs
****
The following APIs affecting the currently executing fiber are provided
APIs affecting the currently-executing fiber are provided
by :file:`microkernel.h` and by :file:`nanokernel.h`:
:cpp:func:`fiber_yield()`
Yields CPU to higher priority and equal priority fibers.
Yield the CPU to higher priority and equal priority fibers.
:cpp:func:`fiber_sleep()`
Yields CPU for a specified time period.
Yield the CPU for a specified time period.
:cpp:func:`fiber_abort()`
Terminates fiber execution.
Terminate fiber execution.
The following APIs affecting a specified fiber are provided
by :file:`microkernel.h` and by :file:`nanokernel.h`:
APIs affecting a specified fiber are provided by
:file:`microkernel.h` and by :file:`nanokernel.h`:
:cpp:func:`task_fiber_start()`, :cpp:func:`fiber_fiber_start()`,
:cpp:func:`fiber_start()`