3/1/2016Page 1 Realtime System Fundamentals : Scheduling B. Ramamurthy

3/1/2016Page 2 Realtime scheduling We discussed realtime system scheduling: –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(C i /T i ) <= n(2 1/n -1) –Cyclic executives (pre-scheduled) (later: next class?) Concepts of cycle, slot and frame Repeated execution times

3/1/2016Page 3 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

3/1/2016Page 4 Semaphores See semaphore.h of xinu

Semaphores in exinu #include #include /**< queue.h must define # of sem queues */ /* Semaphore state definitions */ #define SFREE 0x01 /**< this semaphore is free */ #define SUSED 0x02 /**< this semaphore is used */ /* type definition of "semaphore" */ typedef ulong semaphore; /* Semaphore table entry */ struct sentry { char state; /**< the state SFREE or SUSED */ short count; /**< count for this semaphore */ queue queue; /**< requires q.h. */ }; 5

Semaphores in exinu (contd.) extern struct sentry semtab[]; /** * isbadsem - check validity of reqested semaphore id and state s id number to test; NSEM is declared to be 100 in kernel.h A system typically has a predetermined limited number of semaphores */ #define isbadsem(s) (((ushort)(s) >= NSEM) || (SFREE == semtab[s].state)) /* Semaphore function declarations */ syscall wait(semaphore); syscall signal(semaphore); syscall signaln(semaphore, short); semaphore newsem(short); syscall freesem(semaphore); syscall scount(semaphore); 6

Definition of Semaphores functions static semaphore allocsem(void); /** * newsem - allocate and initialize a new semaphore. count - number of resources available without waiting. * example: count = 1 for mutual exclusion lock new semaphore id on success, SYSERR on failure */ semaphore newsem(short count) { irqmask ps; semaphore sem; ps = disable(); /* disable interrupts */ sem = allocsem(); /* request new semaphore */ if ( sem != SYSERR && count >= 0 ) /* safety check */ { semtab[sem].count = count; /* initialize count */ restore(ps); /* restore interrupts */ return sem; /* return semaphore id */ } restore(ps); } 7

Semaphore: newsem contd. /** * allocsem - allocate an unused semaphore and return its index. * Scan the global semaphore table for a free entry, mark the entry * used, and return the new semaphore available semaphore id on success, SYSERR on failure */ static semaphore allocsem(void) { int i = 0; while(i < NSEM) /* loop through semaphore table */ { /* to find SFREE semaphore */ if( semtab[i].state == SFREE ) { semtab[i].state = SUSED; return i; } i++; } return SYSERR; } 8

Semaphore: wait(…) /** * wait - make current process wait on a semaphore sem semaphore for which to wait OK on success, SYSERR on failure */ syscall wait(semaphore sem) { irqmask ps; struct sentry *psem; pcb *ppcb; ps = disable(); /* disable interrupts */ if ( isbadsem(sem) ) /* safety check */ { restore(ps); return SYSERR; } ppcb = &proctab[currpid]; /* retrieve pcb from process table */ psem = &semtab[sem]; /* retrieve semaphore entry */ if( --(psem->count) < 0 ) /* if requested resource is unavailable */ { ppcb->state = PRWAIT; /* set process state to PRWAIT*/ 9

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; } 10

Semaphore: signal() /*signal - signal a semaphore, releasing one waiting process, and block sem id of semaphore to signal 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; } 11

Semaphore: usage Problem 1: – 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. 12

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); 13

Problem 1: multi-tasks void proc1() { while (1) { sleep (rand()%10); wait(mutex1); counter++; signal(mutex1); } void proc2() { while (1) { sleep (rand()%10); wait(mutex1); counter++; signal(mutex1); } //similarly proc3 Simulation of this; 14

Problem 1 15 Task 1Task 2 Task 3 Counter1

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); 16

Task flow void proc1() { while (1) { sleep (rand()%10); wait(synch31); counter++; signal(synch12); } void proc2() { while (1) { sleep (rand()%10); wait(synch12); counter++; signal(synch23); } void proc3() { while (1) { sleep(rand()%10); wait(synch23); counter++; signal(synch31); } } 17

3/1/2016Page 18 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.

3/1/2016Page 19 Priority inversion (Priority: t1>t2>t3) task3 task2 Critical section task1 time blocked

3/1/2016Page 20 Problem: Priority inversion Solution1: Priority Inheritance task3 task2 Critical section task1 time blocked Priority of t1 inherited Task 2 delayed Priority reverted To t3

3/1/2016Page 21 Solution2:Priority Ceiling Protocol Critical section task1 time task2 task3 Acquire S2 Attempt to Acquire S1 Acquire S2 CSUsed by Priority Ceiling S1t1,t2P(t1) S2t1,t2,t3P(t1) S3t3P(t3) Release S1 Release S2 No way