Realtime API

Scheduling Policies

Scheduling policies determine the scheduler’s way of assigning CPUs …

• SCHED_OTHER: the fair world (runnable processes are only added to pool of all runnables)

• SCHED_FIFO (First-in first-out)

  • Process gets CPU immediately assigned

  • Remains on CPU until he relinquishes

  • … or a higher prio process wants CPU

• SCHED_RR (Round Robin)

  • Like SCHED_FIFO

  • Equal prio processes: short timeslices in round robin order

Scheduling Priorities

• 0 … Reserved for good old fair processes (SCHED_OTHER)

• 1 … lowest realtime priority

• …

• 99 … highest realtime priority

What’s Runnning On A System?

Scheduling: System Calls

Manipulating scheduling attributes of a process:

• man 2 sched_setscheduler

  int sched_setscheduler(pid_t pid, int policy, const struct sched_param *param);
  int sched_getscheduler(pid_t pid);
  
  struct sched_param {
      int sched_priority;
  };
  

Scheduling: Threads (1)

Manipulating scheduling attributes of an existing thread:

• man 3 pthread_setschedparam

  pthread_setschedparam(
      pthread_t thread, int policy,
      const struct sched_param *param);
  pthread_getschedparam(
      pthread_t thread, int *policy,
      struct sched_param *param);
  

Scheduling: Threads (2)

Start a new thread with predefinied scheduling attributes:

• man 3 pthread_attr_setschedparam

  int pthread_attr_setschedparam(
      pthread_attr_t *attr,
      const struct sched_param *param);
  

• man 3 pthread_attr_setschedpolicy

  int pthread_attr_setschedpolicy(
      pthread_attr_t *attr, int policy);
  

Priority Inversion

• Wikipedia

• FreeRTOS

Priority Inversion: Mutex Protocols (1)

Solution, in spoken words: at the time that C wants the mutex, A has to carry on ⟶ protocol between both, communicated via the mutex

⟶ Mutex Attribute

• man 3 pthread_mutexattr_setprotocol

  int pthread_mutexattr_setprotocol(
      pthread_mutexattr_t *attr,
      int protocol);
  

Protocols

• PTHREAD_PRIO_INHERIT: A’s priority is temporarily* (until mutex is acquired) boosted to B’s

• PTHREAD_PRIO_PROTECT: A’s priority is temporarily risen to a fixed limit ⟶ man 3 pthread_mutexattr_setprioceiling