xv6 scheduler kernel trap

Question

I am writing a simple scheduler in which we iterate the process array and find all runnable processes and store it another array for lookup. I have a srand function call that will generate a number between 0 and the runnable processes. Using that index acquire that process from the runnable process array, run it then repeat. However I keep getting a panic kernel trap error if anyone can help.

void
scheduler(void)
{
  struct proc *p;
  struct cpu *c = mycpu();
  struct proc *arr[NPROC]; 
  c->proc = 0;
  
  for(;;){
    // Avoid deadlock by ensuring that devices can interrupt.
    intr_on();  
    
    int i=0;
    int runnableProc=0;
      for(p = proc; p < &proc[NPROC]; p++) {
      acquire(&p->lock);
      if(p->state == RUNNABLE) {   
        // Switch to chosen process.  It is the process's job
        // to release its lock and then reacquire it
        // before jumping back to us.
       arr[i]=p;
       runnableProc++;
       i++;
       }       
       release(&p->lock);
       }
        int randomNum=sys_srand()%runnableProc;
        acquire(&arr[randomNum]->lock);
  arr[randomNum]->state=RUNNING;
        c->proc = arr[randomNum];     
        swtch(&c->context, &arr[randomNum]->context);

        // Process is done running for now.
        // It should have changed its p->state before coming back.
        c->proc = 0;
        release(&arr[randomNum]->lock);
       
        }
}

Answer 1

I guess that sys_srand is the problem, but I can’t find it in xv6 repos.

Given its name, sys_srand seems to be a system call, declared as taking no parameter (xxx sys_srand(void)) but I believe that it’s a syscall implementation of srand which takes an integer as parameter.

So the problems are:

• You call sys_xxx from the kernel instead of directly calling xxx function. (or what is called in sys_srand())
• You mixed srand and rand functions.
  • srand is to initialize the random generator seed (it should be called once per process) and
  • rand which return a pseudo random number.

In sort:

• Call srand once as your scheduler starts
• Call rand to pick a process to run.