Step 8: Admitting real-time tasks

In the final step before the plugin can be used to schedule real-time tasks, we are going to change to demo_admit_task() callback to allow tasks, provided that they are located on the right core.

To this end, change demo_admit_task() to look as follows:

static long demo_admit_task(struct task_struct *tsk)
{
        if (task_cpu(tsk) == get_partition(tsk)) {
                TRACE_TASK(tsk, "accepted by demo plugin.\n");
                return 0;
        }
        return -EINVAL;
}

Since DEMO implements a partitioned scheduler, we require that real-time tasks have migrated to the appropriated core before they become real-time tasks. This constraint simplifies the implementation of the plugin greatly and is not difficult to support in userspace (liblitmus contains a helper function be_migrate_to_domain() that can take care of this requirement).

Note that task_cpu(tsk) is Linux’s notion of where the task currently is, whereas get_partition(tsk) is LITMUS^RT’s notion of where the task is logically assigned to.

The plugin is now fully functional by itself. However, liblitmus contains code to make migrating tasks to the right processors (or clusters of processors) easier, and this support code requires additional information from the plugin. In the next step, the plugin is changed to export the required information.

Source code

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

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