Realtime System Fundamentals : Scheduling and Priority-based scheduling B. Ramamurthy Amrita-UB-MSES-2013-7 5/11/2013

Realtime scheduling We will realtime system scheduling as in: Earliest deadline scheduling (EDS) Starting deadline Completion deadline Dynamic priority scheduling Rate monotonic scheduling (RMS) Periodic tasks are prioritized by the frequency of repetition (high priority to tasks with shorter periods) Preemptive scheduling Fixed priority scheduling Schedulability according to RMS Σ(Ci/Ti) <= n(21/n-1) Cyclic executives (pre-scheduled) Concepts of cycle, slot and frame Repeated execution times Amrita-UB-MSES-2013-7 5/11/2013

Task State Diagram New Ready Blocked Run 5/11/2013 Task admitted Resources allocated Dispatched; cpu allocated Event occurred Task exit Blocked Run Waiting for event Amrita-UB-MSES-2013-7 5/11/2013

Deadline driven scheduling Parameters: ready time, starting deadline, completion deadline, processing time, resource requirement, priority, preemptive or non-preemptive Amrita-UB-MSES-2013-7 5/11/2013

Deadline Scheduling Process Arrival Time Execution Time Ending Deadline A(1) 0 10 20 A(2) 20 10 40 A(3) 40 10 60 A(4) 60 10 80 A(5) 80 10 100 • • • • B(1) 0 25 50 B(2) 50 25 100 Amrita-UB-MSES-2013-7 5/11/2013

Fixed-priority scheduling; A has priority B has priority deadline A1 B1 A2 B1 A3 B2 A4 B2 A5 B2 A1 A2 B1 A3 A4 A5, B2 (missed) A1 A2 A3 A4 A5, B2 B1 A2 A3 B2 A5 A1 B1 A2 B1 A3 B2 A4 B2 A5 Fixed-priority scheduling; A has priority B has priority Earliest-deadline scheduling using completion deadlines B1 Amrita-UB-MSES-2013-7 5/11/2013

Aperiodic Task set Arrival Time Execution Time Starting Deadline A 10 20 110 B 20 20 20 C 40 20 50 D 50 20 90 E 60 20 70 Use earliest deadline with unforced idle time Amrita-UB-MSES-2013-7 5/11/2013

Rate-monotonic scheduling First proposed by Liu. For RMS, the highest-priority task is the one with the shortest period, the second highest-priority task is the one with the second shortest period, and so on. Schedulability according to RMS Σ(Ci/Ti) <= n(21/n-1) Amrita-UB-MSES-2013-7 5/11/2013

Resources & Critical Resources Shared resources: need mutual exclusion Tasks cooperating to complete a job Tasks contending to access a resource Tasks synchronizing Critical resources and critical region A important synchronization and mutual exclusion primitive / resource is “semaphore” Amrita-UB-MSES-2013-7 5/11/2013

Critical sections and Semaphores When multiples tasks are executing there may be sections where only one task could execute at a given time: critical region or critical section There may be resources which can be accessed only be one of the processes: critical resource Semaphores can be used to ensure mutual exclusion to critical sections and critical resources Amrita-UB-MSES-2013-7 5/11/2013

Semaphores See semaphore.h of xinu Amrita-UB-MSES-2013-7 5/11/2013

Semaphore: wait() ppcb->sem = sem; /* record semaphore id in pcb */ enqueue(currpid, psem->queue); resched(); /* place in wait queue and reschedule */ } restore(ps); /* restore interrupts */ return OK; Amrita-UB-MSES-2013-7 5/11/2013

Semaphore: signal() /*signal - signal a semaphore, releasing one waiting process, and block * @param sem id of semaphore to signal * @return OK on success, SYSERR on failure */ syscall signal(semaphore sem) { irqmask ps; register struct sentry *psem; ps = disable(); /* disable interrupts */ if ( isbadsem(sem) ) /* safety check */ restore(ps); return SYSERR; } psem = &semtab[sem]; /* retrieve semaphore entry */ if ( (psem->count++) < 0 ) /* release one process from wait queue */ { ready(dequeue(psem->queue), RESCHED_YES); } restore(ps); /* restore interrupts */ return OK; Amrita-UB-MSES-2013-7 5/11/2013

Semaphore: usage Problem 1: Problem 2: Create 3 tasks that each sleep for a random time and update a counter. Counter is the critical resources shared among the processes. Only one task can update the counter at a time so that counter value is correct. Problem 2: Create 3 tasks; task 1 updates the counter by 1 and then signal task 2 that updates the counter by 2 and then signals task 3 to update the counter by 3. Amrita-UB-MSES-2013-7 5/11/2013

Problem 1 #include <..> //declare semaphore semaphore mutex1 = newsem(1); int counter = 0; //declare functions: proc1,proc1, proc3 ready(create((void *)proc1, INITSTK, INITPRIO, “PROC1",, 2, 0, NULL), RESCHED_NO); ready(create((void *)proc2, INITSTK, INITPRIO, “PROC2",, 2, 0, NULL), RESCHED_NO); ready(create((void *)proc3, INITSTK, INITPRIO, “PROC3",, 2, 0, NULL), RESCHED_NO); Amrita-UB-MSES-2013-7 5/11/2013

Problem 1: multi-tasks void proc1() { while (1) { sleep (rand()%10); wait(mutex1); counter++; signal(mutex1); } } void proc2() //similarly proc3 Amrita-UB-MSES-2013-7 5/11/2013

Problem 1 Task 1 Task 2 Counter1 Task 3 5/11/2013 Amrita-UB-MSES-2013-7 5/11/2013

Problem 2 semaphore synch12 = newsem(0); semaphore synch23 = newsem(0); semaphore synch31 = newsem(0); ready(create((void *)proc1, INITSTK, INITPRIO, “PROC1",, 2, 0, NULL), RESCHED_NO); ready(create((void *)proc2, INITSTK, INITPRIO, “PROC2",, 2, 0, NULL), RESCHED_NO); ready(create((void *)proc3, INITSTK, INITPRIO, “PROC3",, 2, 0, NULL), RESCHED_NO); signal(synch31); Amrita-UB-MSES-2013-7 5/11/2013

Task flow void proc1() void proc2() void proc3() 5/11/2013 { while (1) { sleep (rand()%10); wait(synch31); counter++; signal(synch12); } } void proc2() wait(synch12); signal(synch23); void proc3() sleep(rand()%10); wait(synch23); signal(synch31); } } Amrita-UB-MSES-2013-7 5/11/2013

Priority Inversion When we allow concurrent task to execute and with semaphore and mailboxes and other synchronization primitives, it is possible that a low priority task may come to block a high priority task. This situation is known as priority inversion. What happened on Mars? Amrita-UB-MSES-2013-7 5/11/2013

Priority inversion (Priority: t1>t2>t3) Critical section task1 time 0 1 2 3 4 5 6 7 8 9 10 blocked task2 task3 Amrita-UB-MSES-2013-7 5/11/2013

Problem: Priority inversion Solution1: Priority Inheritance task1 blocked Task 2 delayed task2 Priority of t1 inherited Critical section Priority reverted To t3 task3 0 1 2 3 4 5 6 7 8 9 10 time Amrita-UB-MSES-2013-7 5/11/2013

Solution2:Priority Ceiling Protocol CS Used by Priority Ceiling S1 t1,t2 P(t1) S2 t1,t2,t3 S3 t3 P(t3) Solution2:Priority Ceiling Protocol Acquire S1 Release S1 task1 Attempt to Acquire S1 Acquire S1 Acquire S2 No way task2 Acquire S2 Release S2 Critical section task3 0 1 2 3 4 5 6 7 8 9 10 time Amrita-UB-MSES-2013-7 5/11/2013