Question

Normally we have to do like this to invoke a function from a function pointer:

int foo()
{
}

int main()
{
    int (*pFoo)() = foo; // pFoo points to function foo()
    foo();
    return 0;
}

In the Linux kernel code, sched_class has many function pointers:

struct sched_class {
        const struct sched_class *next;

        void (*enqueue_task) (struct rq *rq, struct task_struct *p, int flags);
        void (*dequeue_task) (struct rq *rq, struct task_struct *p, int flags);
        void (*yield_task) (struct rq *rq);
        bool (*yield_to_task) (struct rq *rq, struct task_struct *p, bool preempt);
        .....
}

In pick_next_task function, it defines a local instance of sched_class named class, and directly invoke the function in it without assigning to external functions with the same signature (start from for_each_class):

static inline struct task_struct *
pick_next_task(struct rq *rq)
{
        const struct sched_class *class;
        struct task_struct *p;
    /*
     * Optimization: we know that if all tasks are in
     * the fair class we can call that function directly:
     */
    if (likely(rq->nr_running == rq->cfs.h_nr_running)) {
            p = fair_sched_class.pick_next_task(rq);
            if (likely(p))
                    return p;
    }

    for_each_class(class) {
            p = class->pick_next_task(rq);
            if (p)
                    return p;
    }

    BUG(); /* the idle class will always have a runnable task */
}

Is this because each function pointer in the sched_class has the same name as the actual implemented function, so every time a call is made via a function pointer of sched_class, it will automatically find the matching symbol in the kernel address space?

Answer 1

The definition of for_each_class should clear it up for you

 #define for_each_class(class) \
       for (class = sched_class_highest; class; class = class->next)

If you go on tracing, sched_class_highest wil end up something like this

#define sched_class_highest (&stop_sched_class)
extern const struct sched_class stop_sched_class;

/*
* Simple, special scheduling class for the per-CPU stop tasks:
*/
const struct sched_class stop_sched_class = {
      .next                   = &rt_sched_class,

      .enqueue_task           = enqueue_task_stop,
      .dequeue_task           = dequeue_task_stop,
      .yield_task             = yield_task_stop,

      .check_preempt_curr     = check_preempt_curr_stop,

      .pick_next_task         = pick_next_task_stop,
      .put_prev_task          = put_prev_task_stop,

#ifdef CONFIG_SMP
      .select_task_rq         = select_task_rq_stop,
#endif

     .set_curr_task          = set_curr_task_stop,
     .task_tick              = task_tick_stop,

     .get_rr_interval        = get_rr_interval_stop,

     .prio_changed           = prio_changed_stop,
     .switched_to            = switched_to_stop,
};

Now are you happy? 🙂