LITMUS^RT: Linux Testbed for Multiprocessor Scheduling in Real-Time Systems

Step 9: Exporting plugin topology in /proc

The userspace library liblitmus offers several functions that allow tasks to migrate to the appropriate CPUs under partitioned and clustered schedulers. This code needs to know which CPUs a particular plugin considers to form a “partition” or a “cluster”. To enable liblitmus to work as intended, a plugin must hence export the required topology hints via the /proc/ filesystem, for which LITMUSRT provides a wrapper API.

Using the LITMUSRT API to provide domain information

To access the LITMUSRT topology hints API, the plugin needs to include litmus/litmus_proc.h. Add the following to the list of includes in sched_demo.c:

#include <litmus/litmus_proc.h>

Secondly, allocate a struct of type struct domain_proc_info, which will hold the required topology information. Add the following line near the beginning of sched_demo.c:

static struct domain_proc_info demo_domain_proc_info;

To communicate with the /proc wrapper, the plugin must define another callback function, which provides a pointer to a struct proc_domain_info instance:

static long demo_get_domain_proc_info(struct domain_proc_info **ret)
        *ret = &demo_domain_proc_info;
        return 0;

We’ll later update the plugin’s struct sched_plugin instance to include the demo_get_domain_proc_info callback, but first we will add code to initialize demo_domain_proc_info.

Initializing the domain info structure

When the plugin is activated, the current topology must be stored in demo_domain_proc_info. In short, in a simple partitioned plugin such as the DEMO plugin, each processor forms its own “scheduling domain”. The initialization code shown here iterates over all online CPUs and creates an entry for the corresponding “scheduling domain”. Add this function to sched_demo.c, somewhere before the demo_activate_plugin function:

static void demo_setup_domain_proc(void)
        int i, cpu;
        int num_rt_cpus = num_online_cpus();

        struct cd_mapping *cpu_map, *domain_map;

        memset(&demo_domain_proc_info, 0, sizeof(demo_domain_proc_info));
        init_domain_proc_info(&demo_domain_proc_info, num_rt_cpus, num_rt_cpus);
        demo_domain_proc_info.num_cpus = num_rt_cpus;
        demo_domain_proc_info.num_domains = num_rt_cpus;

        i = 0;
        for_each_online_cpu(cpu) {
                cpu_map = &demo_domain_proc_info.cpu_to_domains[i];
                domain_map = &demo_domain_proc_info.domain_to_cpus[i];

                cpu_map->id = cpu;
                domain_map->id = i;
                cpumask_set_cpu(i, cpu_map->mask);
                cpumask_set_cpu(cpu, domain_map->mask);

We’ll next add a call to the demo_setup_domain_proc helper function in the demo_activate_plugin function. Modify demo_activate_plugin to contain the call to demo_setup_domain_proc:

static long demo_activate_plugin(void)
        int cpu;
        struct demo_cpu_state *state;

        for_each_online_cpu(cpu) {
                TRACE("Initializing CPU%d...\n", cpu);

                state = cpu_state_for(cpu);

                state->cpu = cpu;
                state->scheduled = NULL;
                edf_domain_init(&state->local_queues, demo_check_for_preemption_on_release, NULL);

        return 0;

Adding a cleanup function

LITMUSRT supports another callback function that is invoked whenever a plugin is unloaded. We’ll use this callback to tell LITMUSRT to clean up any state used for this plugin’s /proc interface. Add the following function to sched_demo.c:

static long demo_deactivate_plugin(void)
        return 0;

Registering the new callback functions

Finally, we’ll need to add the demo_deactivate_plugin and demo_setup_domain_proc functions to the demo_plugin struct:

static struct sched_plugin demo_plugin = {
        .plugin_name            = "DEMO",
        .schedule               = demo_schedule,
        .task_wake_up           = demo_task_resume,
        .admit_task             = demo_admit_task,
        .task_new               = demo_task_new,
        .task_exit              = demo_task_exit,
        .get_domain_proc_info   = demo_get_domain_proc_info,
        .activate_plugin        = demo_activate_plugin,
        .deactivate_plugin      = demo_deactivate_plugin,


With these changes in place, our P-EDF scheduler should be complete and ready for use by liblitmus. To test the changes:

  1. Re-compile the LITMUSRT kernel and reboot.
  2. Activate the DEMO plugin using setsched.
  3. Launch some real-time tasks (as a test, you can use rtspin or rt_launch from liblitmus).
cd liblitmus

# Activate the demo plugin
sudo ./setsched DEMO

# Run a real-time task. This rtspin instance should run on CPU 1, have a WCET of 10 ms,
# a period of 100ms, and run for 5 seconds.
sudo ./rtspin -p 1 10 100 5

The rtspin instance in the above sample should terminate after 5 seconds and produce no output. The exact behavior of the plugin can be observed using the sched_trace infrastructure described in the tracing tutorial.

Source code

The full code for this step of the tutorial (the full working plugin) is available here.