Similar to group scheduling under CFS, real-time processes can also be grouped for scheduling with CONFIG_RT_GROUP_SCHED set. For group scheduling to succeed, each group must be assigned a portion of CPU time, with a guarantee that the timeslice is enough to run the tasks under each entity, or it fails. So "run time" (a portion of how much time a CPU can spend running in a period) is allocated per group. The run time allocated to one group will not be used by another group. CPU time that is not allocated for real-time groups will be used by normal-priority tasks and any time unused by the real-time entities will also be picked by the normal tasks. FIFO and RR groups are represented by struct sched_rt_entity:

struct sched_rt_entity {
 struct list_head                run_list;
 unsigned long                   timeout;
  unsigned long                   watchdog_stamp;
   unsigned int                    time_slice;
       unsigned short                  on_rq;
    unsigned short                  on_list;

    struct sched_rt_entity          *back;
#ifdef CONFIG_RT_GROUP_SCHED
  struct sched_rt_entity          *parent;
  /* rq on which this entity is (to be) queued: */
  struct rt_rq                    *rt_rq;
   /* rq "owned" by this entity/group: */
    struct rt_rq                    *my_q;
#endif
};