Hi,
I'm trying to understand the RTAI scheduler (rt_schedule()). Could anyone help
?
RT_TASK *task, *new_task;
RTIME intr_time, now;
int prio, delay, preempt;
task = new_task = &rt_linux_task;
// SETTING DEFAULT PRIORITY TO LINUX PRIORITY ?
prio = RT_LINUX_PRIORITY;
//ONESHOT mode
if (oneshot_timer) {
// GETTING ACTUAL TIME + SIGMA
rt_time_h = rdtsc() + rt_half_tick;
while ((task = task->next)) {
if ((task->state & DELAYED) && task->resume_time <= rt_time_h) {
// IF TASK IS WAITING FOR A RESSOURCE (SEMAPHORE..) DROP IT
// FROM THE TASKS LIST.
if (task->state & (SEMAPHORE | SEND | RPC)) {
(task->queue.prev)->next = task->queue.next;
(task->queue.next)->prev = task->queue.prev;
}
// DROP ALL STATE VARIABLES TO 0
task->state &= ~(DELAYED | SEMAPHORE | RECEIVE |
SEND | RPC | RETURN);
}
// SELECT THE HIGHTEST PRIORITY TASK THAT IS READY.
if (task->state == READY && task->priority < prio) {
new_task = task;
prio = task->priority;
}
}
Ok, so far, everything is fine except maybe from the
// DROP ALL STATE VARIABLES TO 0
task->state &= ~(DELAYED | SEMAPHORE | RECEIVE |
SEND | RPC | RETURN);
Why do you want to drop this bit field. If a task is waiting for a semaphore,
we don't want to give it access to the CPU, right ?? So, why drop the
SEMAPHORE bit to 0 ? If its priority is the highest, it will access the CPU
and do nothing...
preempt = 0;
task = &rt_linux_task;
intr_time = shot_fired ? rt_times.intr_time :
rt_times.intr_time + rt_times.linux_tick;
Ok, there, what does shot_fired stand for ? And what intr_time is supposed to
mean ? When does a preemption will occur ? I understand that the following
lines will check, for each task resume_time E [rt_time_h; intr_time] if the
task state is DELAYED and see if its priority is higher than the current one
we have but why do we check this time interval, what does it represent ?
while ((task = task->next)) {
if ((task->state & DELAYED) && task->priority <= prio &&
task->resume_time < intr_time)
{
intr_time = task->resume_time;
preempt = 1;
}
}
if (preempt || (!shot_fired && (prio == RT_LINUX_PRIORITY))) {
shot_fired = 1;
if (preempt) {
rt_times.intr_time = intr_time;
}
delay = (int)(rt_times.intr_time - (now = rdtsc())) - tuned.latency;
if (delay >= tuned.setup_time_TIMER_CPUNIT) {
delay = imuldiv(delay, TIMER_FREQ, tuned.cpu_freq);
}
else {
delay = tuned.setup_time_TIMER_UNIT;
rt_times.intr_time = now + (tuned.setup_time_TIMER_CPUNIT);
}
outb(delay & 0xFF, 0x40);
outb(delay >> 8, 0x40);
}
}
Finally, for the ONESHOT mode, what does the delay stands for ?
//PERIODIC
else {
while ((task = task->next)) {
if (task->state == READY && task->priority < prio) {
new_task = task;
prio = task->priority;
}
}
}
Ok, this one should be easy to reply to. I understand that in periodic mode,
each task is running at the minimal frequency or is a multiple of it but when
a task is created and resumed, it will call the rt_schedule function and the
current task will be preempted if the new task priority is higher. So, what's
the difference between the periodic and one-shot mode ? Each can preempt the
current task, each calls rt_schedule for the same reason (end of task, new
task...). So ?
Thanks
Alex.
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