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scheduler.cc

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// scheduler.cc 
//      Routines to choose the next thread to run, and to dispatch to
//      that thread.
//
//      These routines assume that interrupts are already disabled.
//      If interrupts are disabled, we can assume mutual exclusion
//      (since we are on a uniprocessor).
//
//      NOTE: We can't use Locks to provide mutual exclusion here, since
//      if we needed to wait for a lock, and the lock was busy, we would 
//      end up calling FindNextToRun(), and that would put us in an 
//      infinite loop.
//
//      Very simple implementation -- no priorities, straight FIFO.
//      Might need to be improved in later assignments.
//
// Copyright (c) 1992-1993 The Regents of the University of California.
// All rights reserved.  See copyright.h for copyright notice and limitation 
// of liability and disclaimer of warranty provisions.

#include "copyright.h"
#include "scheduler.h"
#include "system.h"

//----------------------------------------------------------------------
// Scheduler::Scheduler
//      Initialize the list of ready but not running threads to empty.
//----------------------------------------------------------------------

Scheduler::Scheduler()
{ 
    readyList = new List; 
} 

//----------------------------------------------------------------------
// Scheduler::~Scheduler
//      De-allocate the list of ready threads.
//----------------------------------------------------------------------

Scheduler::~Scheduler()
{ 
    delete readyList; 
} 

//----------------------------------------------------------------------
// Scheduler::ReadyToRun
//      Mark a thread as ready, but not running.
//      Put it on the ready list, for later scheduling onto the CPU.
//
//      "thread" is the thread to be put on the ready list.
//----------------------------------------------------------------------

void
Scheduler::ReadyToRun (Thread *thread)
{
    DEBUG('t', "Putting thread %s on ready list.\n", thread->getName());

    thread->setStatus(READY);
    readyList->Append((void *)thread);
}

//----------------------------------------------------------------------
// Scheduler::FindNextToRun
//      Return the next thread to be scheduled onto the CPU.
//      If there are no ready threads, return NULL.
// Side effect:
//      Thread is removed from the ready list.
//----------------------------------------------------------------------

Thread *
Scheduler::FindNextToRun ()
{
    return (Thread *)readyList->Remove();
}

//----------------------------------------------------------------------
// Scheduler::Run
//      Dispatch the CPU to nextThread.  Save the state of the old thread,
//      and load the state of the new thread, by calling the machine
//      dependent context switch routine, SWITCH.
//
//      Note: we assume the state of the previously running thread has
//      already been changed from running to blocked or ready (depending).
// Side effect:
//      The global variable currentThread becomes nextThread.
//
//      "nextThread" is the thread to be put into the CPU.
//----------------------------------------------------------------------

void
Scheduler::Run (Thread *nextThread)
{
    Thread *oldThread = currentThread;
    
#ifdef USER_PROGRAM                     // ignore until running user programs 
    if (currentThread->space != NULL) { // if this thread is a user program,
        currentThread->SaveUserState(); // save the user's CPU registers
        currentThread->space->SaveState();
    }
#endif
    
    oldThread->CheckOverflow();             // check if the old thread
                                            // had an undetected stack overflow

    currentThread = nextThread;             // switch to the next thread
    currentThread->setStatus(RUNNING);      // nextThread is now running
    
    DEBUG('t', "Switching from thread \"%s\" to thread \"%s\"\n",
          oldThread->getName(), nextThread->getName());
    
    // This is a machine-dependent assembly language routine defined 
    // in switch.s.  You may have to think
    // a bit to figure out what happens after this, both from the point
    // of view of the thread and from the perspective of the "outside world".

    SWITCH(oldThread, nextThread);
    
    DEBUG('t', "Now in thread \"%s\"\n", currentThread->getName());

    // If the old thread gave up the processor because it was finishing,
    // we need to delete its carcass.  Note we cannot delete the thread
    // before now (for example, in Thread::Finish()), because up to this
    // point, we were still running on the old thread's stack!
    if (threadToBeDestroyed != NULL) {
        delete threadToBeDestroyed;
        threadToBeDestroyed = NULL;
    }
    
#ifdef USER_PROGRAM
    if (currentThread->space != NULL) {         // if there is an address space
        currentThread->RestoreUserState();     // to restore, do it.
        currentThread->space->RestoreState();
    }
#endif
}

//----------------------------------------------------------------------
// Scheduler::Print
//      Print the scheduler state -- in other words, the contents of
//      the ready list.  For debugging.
//----------------------------------------------------------------------
void
Scheduler::Print()
{
    printf("Ready list contents:\n");
    readyList->Mapcar((VoidFunctionPtr) ThreadPrint);
}

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