How to change the preemption rate in xv6? - operating-system

Question

I need to achieve the following:
Process preemption should be done in every time quantum (measured in clock ticks) instead of every clock tick
In order to achieve this, I have modified the yield() function in proc.c
void
yield(void)
{
struct proc *p = myproc();
acquire(&ptable.lock); //DOC: yieldlock
p->state = RUNNABLE;
p->timeslice--;
if(!p->timeslice)
sched();
release(&ptable.lock);
}
I have added the field int timeslice in struct proc. I timeslice is initialized in scheduler(), every time a process is scheduled
void
scheduler(void)
{
struct proc *p;
struct cpu *c = mycpu();
c->proc = 0;
for(;;){
// Enable interrupts on this processor.
sti();
// Loop over process table looking for process to run.
acquire(&ptable.lock);
for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
if(p->state != RUNNABLE)
continue;
// Switch to chosen process. It is the process's job
// to release ptable.lock and then reacquire it
// before jumping back to us.
c->proc = p;
switchuvm(p);
p->state = RUNNING;
p->timeslice = QUANTA;
swtch(&(c->scheduler), p->context);
switchkvm();
// Process is done running for now.
// It should have changed its p->state before coming back.
c->proc = 0;
}
release(&ptable.lock);
}
}
But I am getting the following error in xv6 shell
xv6...
cpu1: starting 1
cpu0: starting 0
sb: size 1000 nblocks 941 ninodes 200 nlog 30 logstart 2 inodestart 32 bmap start 58
unexpected trap 14 from cpu 1 eip 801059d2 (cr2=0x3920)
lapicid 1: panic: trap
80105cdf 801059c9 0 0 0 0 0 0 0 0
Please help me fix the error. Thanks in advance ...

Answer 1

You should not change the process state if you don't call sched().
void
yield(void)
{
acquire(&ptable.lock);
struct proc *p = myproc();
p->timeslice--;
if(!p->timeslice ) {
acquire(&ptable.lock);
p->state = RUNNABLE;
sched();
}
release(&ptable.lock);
}
Note:
In code above, you might acquire and release the process table in the if body, but I'm not sure that it's save to change p->timeslice if process table is not locked.