## Step 5: Adding P-EDF scheduling logic

Step 4 covered adding necessary state variables to our plugin, so we’re now ready to define how the DEMO plugin selects the task that should be scheduled next. Since we’re implementing a P-EDF scheduler, we will use the edf_preemption_needed() function from litmus/edf_common.h to determine when the previous task should be preempted.

### More headers related to job parameters and budgets

Add the following two includes to the list at the start of sched_demo.c:

#include <litmus/jobs.h>
#include <litmus/budget.h>


We will need these headers when managing job- and budget- related information in the scheduler.

Before diving into the scheduling function, we are going to define two helper functions that will aid in preventing the schedule() implementation from becoming too convoluted.

The first helper, demo_job_completion(), is called to process a job when it the job completes. In this simple example, it simply delegates all of the work to the common helper, prepare_for_next_period(), declared in litmus/jobs.h. In more complicated scheduling policies, additional housekeeping code to be run on job completions may be put here.

/* This helper is called when task prev exhausted its budget or when
*it signaled a job completion. */
static void demo_job_completion(struct task_struct *prev, int budget_exhausted)
{
/* Call common helper code to compute the next release time, deadline,
* etc. */
prepare_for_next_period(prev);
}


Next, we add the demo_requeue() helper function. It is used to place a task on the appropriate queue. If the task has a pending job, it is placed in the (core-local) ready queue. Otherwise, if the next job’s earliest release time is in the future, the task will be placed in the (also core-local) release queue.

/* Add the task tsk to the appropriate queue. Assumes the caller holds the ready lock.
*/
static void demo_requeue(struct task_struct *tsk, struct demo_cpu_state *cpu_state)
{
if (is_released(tsk, litmus_clock())) {
* hold the ready lock. */
} else {
/* Uses add_release() because we DON'T have the release lock. */
}
}


Note that demo_requeue() uses __add_ready(), but not __add_release(). This is because demo_requeue() will be called only from contexts where the calling thread already holds the lock for the ready queue.

Finally, we can define the P-EDF scheduling logic. Conceptually, it follows these three steps:

1. Checks what state the previously scheduled real-time task is in (if any)
2. Checks whether a higher-priority job is waiting in the ready queue (i.e., if a preemption is required)
3. Returns the next task to be scheduled (which may be NULL if there is no real-time workload pending)

The scheduler is serialized (with respect to each core) using the ready queue lock local_state->local_queues.ready_lock. Reusing the ready queue lock to serialize scheduling decisions is a common idiom in LITMUSRT. Alternatively, we could also define an additional spinlock inside struct demo_cpu_state.

Modify the demo_schedule function to contain the following content:

static struct task_struct* demo_schedule(struct task_struct * prev)
{
struct demo_cpu_state *local_state = local_cpu_state();

/* next == NULL means "schedule background work". */

int exists, out_of_time, job_completed, self_suspends, preempt, resched;

BUG_ON(local_state->scheduled && local_state->scheduled != prev);
BUG_ON(local_state->scheduled && !is_realtime(prev));

exists = local_state->scheduled != NULL;
self_suspends = exists && !is_current_running();
out_of_time = exists && budget_enforced(prev) && budget_exhausted(prev);
job_completed = exists && is_completed(prev);

/* preempt is true if task prev has lower priority than something on
preempt = edf_preemption_needed(&local_state->local_queues, prev);

/* check all conditions that make us reschedule */
resched = preempt;

/* if prev suspends, it CANNOT be scheduled anymore => reschedule */
if (self_suspends) {
resched = 1;
}

/* also check for (in-)voluntary job completions */
if (out_of_time || job_completed) {
demo_job_completion(prev, out_of_time);
resched = 1;
}

if (resched) {
/* First check if the previous task goes back onto the ready
* queue, which it does if it did not self_suspend.
*/
if (exists && !self_suspends) {
demo_requeue(prev, local_state);
}
} else {
/* No preemption is required. */
next = local_state->scheduled;
}

local_state->scheduled = next;
if (exists && prev != next) {
}
if (next) {
}

/* This mandatory. It triggers a transition in the LITMUS^RT remote
* preemption state machine. Call this AFTER the plugin has made a local
* scheduling decision.
*/

return next;
}


The scheduler is serialized by obtaining the obtaining the ready queue lock: raw_spin_lock(&local_state->local_queues.ready_lock). Note that interrupts are already disabled when a scheduler plugin’s schedule callback is invoked, so we do not have to worry about local interrupts within demo_schedule().

The BUG_ON lines assert that the following invariant holds: when a real-time task is scheduled on the local core, then it is pointed to by local_state->scheduled, and if no real-time task is scheduled, then local_state->scheduled is NULL. Note that prev may refer to a non-real-time task.

The next lines establish the state of prev:

• exists is true if the previous task is a real-time task.
• self_suspends is true if prev cannot be scheduled any longer.
• out_of_time is true if the current job overran its budget.
• job_completed is true if the current job signaled completion via syscall.

The call to edf_preemption_needed checks whether higher-priority work is pending (i.e., jobs with earlier deadlines) on the local ready queue.

The next few lines determine whether a preemption / scheduling decision is required. No change in scheduling is required if the previous task does not self-suspend or complete and if no higher-priority work is pending. Note that this involves calling the demo_job_completion() helper function, which is simply a wrapper around prepare_for_next_period, as described above.

The final lines carry out the actual scheduling decision. If prev needs to be preempted (in the if (resched) block), then the previous task is requeued (if required) using demo_requeue and a new task is taken from the ready queue. Otherwise, next is simply the locally scheduled task, local_state->scheduled, which may be NULL. The “local state invariant” is maintained by the local_state->scheduled = next assignment.

### Testing

With these changes in place, the kernel should compile and boot without problems. However, scheduling is still not possible because all tasks are still being rejected. Before tasks can be accepted, however, we need to add support for task state changes (i.e., self-suspensions).

### Source code

The full code for this step of the tutorial is available here.