Fiber Services

Concepts

A 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 interact with microkernel object types to only a limited degree; for more information see Fiber Services.

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 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 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, 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. 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.

A fiber may also terminate non-gracefully by aborting. The kernel automatically aborts a fiber when it generates a fatal error condition, such as dereferencing a null pointer. A fiber can also explicitly abort itself using fiber_abort(). As with graceful fiber termination, the kernel does not attempt to reclaim system resources owned by the fiber.

Note

The kernel does not currently make any claims regarding an application’s ability to restart a terminated fiber.

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 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.

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

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 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.

Fiber Priorities

The kernel supports a virtually unlimited number of fiber priority levels, ranging from 0 (highest priority) to 2^31-1 (lowest priority). Negative priority levels must not be used.

A fiber’s original priority cannot be altered up or down after it has been spawned.

Fiber Scheduling Algorithm

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.

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 by the nanokernel until one of the following occurs:

The fiber is supplanted by another thread because it calls a kernel API that blocks its own execution. (For example, the fiber attempts to take a nanokernel semaphore that is unavailable.)
The fiber terminates itself by returning from its entry point function.
The fiber aborts itself by performing an operation that causes a fatal error, or by calling fiber_abort().

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 microkernel scheduler uses the microkernel server fiber to initiate a change from one microkernel task to another.

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 to time to permit other fibers to execute.

A fiber can relinquish the CPU in two ways:

Calling fiber_yield() places the fiber back in the nanokernel scheduler’s list of executable fibers and then invokes the scheduler. All executable fibers whose priority is higher or equal to that of the yielding fiber are then allowed to execute before the yielding fiber is rescheduled. If no such executable fibers exist, the scheduler immediately reschedules the yielding fiber without context switching.
Calling fiber_sleep() blocks the execution of the fiber for a specified time period. Executable fibers of all priorities are then allowed to execute, although there is no guarantee that fibers whose priority is lower than that of the sleeping task will actually be scheduled before the time period expires and the sleeping task becomes executable once again.

Fiber Options

The kernel supports several 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 with |, 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.

USE_FP: Instructs the kernel to save the fiber’s x87 FPU and MMX floating point context information during context switches.
USE_SSE: Instructs the kernel to save the fiber’s SSE floating point context information during context switches. A fiber with this option implicitly uses the USE_FP option, as well.

Usage

Defining a Fiber

The following properties must be defined when spawning a fiber:

stack_name
This specifies the memory region used for the fiber’s stack and for other execution context information. To ensure proper memory alignment, it should have the following form:
char __stack <stack_name>[<stack_size>];
stack_size

This specifies the size of the stack_name memory region, in bytes.

entry_point
This specifies the name of the fiber’s entry point function, which should have the following form:
void <entry_point>(int arg1, int arg2)
{
    /* fiber mainline processing */
    ...
    /* (optional) normal fiber termination */
    return;
}
arguments

This specifies the two arguments passed to entry_point when the fiber begins executing. Non-integer arguments can be passed in by casting to an integer type.

priority

This specifies the scheduling priority of the fiber.

options

This specifies the fiber’s options.

Example: Spawning a Fiber from a Task

This code shows how the currently executing task can spawn multiple fibers, each dedicated to processing data from a different communication channel.

#define COMM_STACK_SIZE    512
#define NUM_COMM_CHANNELS  8

struct descriptor {
    ...;
};

char __stack comm_stack[NUM_COMM_CHANNELS][COMM_STACK_SIZE];
struct descriptor comm_desc[NUM_COMM_CHANNELS] = { ... };

...

void comm_fiber(int desc_arg, int unused);
{
    ARG_UNUSED(unused);

    struct descriptor  *desc = (struct descriptor *) desc_arg;

    while (1) {
        /* process packet of data from comm channel */

        ...
    }
}

void comm_main(void)
{
    ...

    for (int i = 0; i < NUM_COMM_CHANNELS; i++) {
        task_fiber_start(&comm_stack[i][0], COMM_STACK_SIZE,
                         comm_fiber, (int) &comm_desc[i], 0,
                         10, 0);
    }

    ...
}

APIs

APIs affecting the currently-executing fiber are provided by microkernel.h and by nanokernel.h:

fiber_yield()

Yield the CPU to higher priority and equal priority fibers.

fiber_sleep()

Yield the CPU for a specified time period.

fiber_abort()

Terminate fiber execution.

APIs affecting a specified fiber are provided by microkernel.h and by nanokernel.h:

task_fiber_start(), fiber_fiber_start(), fiber_start()

Spawn a new fiber.

task_fiber_delayed_start(), fiber_fiber_delayed_start(), fiber_delayed_start()

Spawn a new fiber after a specified time period.

task_fiber_delayed_start_cancel(), fiber_fiber_delayed_start_cancel(), fiber_delayed_start_cancel()

Cancel spawning of a new fiber, if not already started.