source: mainline/src/proc/scheduler.c@ 18e0a6c

/*
 * Copyright (C) 2001-2004 Jakub Jermar
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * - Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 * - The name of the author may not be used to endorse or promote products
 *   derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include <proc/scheduler.h>
#include <proc/thread.h>
#include <proc/task.h>
#include <cpu.h>
#include <mm/vm.h>
#include <config.h>
#include <context.h>
#include <func.h>
#include <arch.h>
#include <arch/asm.h>
#include <list.h>
#include <panic.h>
#include <typedefs.h>
#include <mm/page.h>
#include <synch/spinlock.h>
#include <arch/faddr.h>
#include <arch/atomic.h>

volatile int nrdy;


/** Initialize context switching
 *
 * Initialize context switching and lazy FPU
 * context switching.
 *
 */
void before_thread_runs(void)
{
        before_thread_runs_arch(); 
        fpu_context_restore(&(THREAD->saved_fpu_context));
}


/** Initialize scheduler
 *
 * Initialize kernel scheduler.
 *
 */
void scheduler_init(void)
{
}


/** Get thread to be scheduled
 *
 * Get the optimal thread to be scheduled
 * according to thread accounting and scheduler
 * policy.
 *
 * @return Thread to be scheduled.
 *
 */
struct thread *find_best_thread(void)
{
        thread_t *t;
        runq_t *r;
        int i, n;

loop:
        cpu_priority_high();

        spinlock_lock(&CPU->lock);
        n = CPU->nrdy;
        spinlock_unlock(&CPU->lock);

        cpu_priority_low();
        
        if (n == 0) {
                #ifdef __SMP__
                /*
                 * If the load balancing thread is not running, wake it up and
                 * set CPU-private flag that the kcpulb has been started.
                 */
                if (test_and_set(&CPU->kcpulbstarted) == 0) {
                        waitq_wakeup(&CPU->kcpulb_wq, 0);
                        goto loop;
                }
                #endif /* __SMP__ */
                
                /*
                 * For there was nothing to run, the CPU goes to sleep
                 * until a hardware interrupt or an IPI comes.
                 * This improves energy saving and hyperthreading.
                 * On the other hand, several hardware interrupts can be ignored.
                 */
                 cpu_sleep();
                 goto loop;
        }

        cpu_priority_high();
        
        i = 0;
retry:
        for (; i<RQ_COUNT; i++) {
                r = &CPU->rq[i];
                spinlock_lock(&r->lock);
                if (r->n == 0) {
                        /*
                         * If this queue is empty, try a lower-priority queue.
                         */
                        spinlock_unlock(&r->lock);
                        continue;
                }
        
                /* avoid deadlock with relink_rq() */
                if (!spinlock_trylock(&CPU->lock)) {
                        /*
                         * Unlock r and try again.
                         */
                        spinlock_unlock(&r->lock);
                        goto retry;
                }
                CPU->nrdy--;
                spinlock_unlock(&CPU->lock);

                atomic_dec(&nrdy);
                r->n--;

                /*
                 * Take the first thread from the queue.
                 */
                t = list_get_instance(r->rq_head.next, thread_t, rq_link);
                list_remove(&t->rq_link);

                spinlock_unlock(&r->lock);

                spinlock_lock(&t->lock);
                t->cpu = CPU;

                t->ticks = us2ticks((i+1)*10000);
                t->pri = i;     /* eventually correct rq index */

                /*
                 * Clear the X_STOLEN flag so that t can be migrated when load balancing needs emerge.
                 */
                t->flags &= ~X_STOLEN;
                spinlock_unlock(&t->lock);

                return t;
        }
        goto loop;

}


/** Prevent rq starvation
 *
 * Prevent low priority threads from starving in rq's.
 *
 * When the function decides to relink rq's, it reconnects
 * respective pointers so that in result threads with 'pri'
 * greater or equal 'start' are moved to a higher-priority queue.
 *
 * @param start Threshold priority.
 *
 */
void relink_rq(int start)
{
        link_t head;
        runq_t *r;
        int i, n;

        list_initialize(&head);
        spinlock_lock(&CPU->lock);
        if (CPU->needs_relink > NEEDS_RELINK_MAX) {
                for (i = start; i<RQ_COUNT-1; i++) {
                        /* remember and empty rq[i + 1] */
                        r = &CPU->rq[i + 1];
                        spinlock_lock(&r->lock);
                        list_concat(&head, &r->rq_head);
                        n = r->n;
                        r->n = 0;
                        spinlock_unlock(&r->lock);
                
                        /* append rq[i + 1] to rq[i] */
                        r = &CPU->rq[i];
                        spinlock_lock(&r->lock);
                        list_concat(&r->rq_head, &head);
                        r->n += n;
                        spinlock_unlock(&r->lock);
                }
                CPU->needs_relink = 0;
        }
        spinlock_unlock(&CPU->lock);                            

}


/** The scheduler
 *
 * The thread scheduling procedure.
 *
 */
void scheduler(void)
{
        volatile pri_t pri;

        pri = cpu_priority_high();

        if (haltstate)
                halt();

        if (THREAD) {
                spinlock_lock(&THREAD->lock);
                fpu_context_save(&(THREAD->saved_fpu_context));
                if (!context_save(&THREAD->saved_context)) {
                        /*
                         * This is the place where threads leave scheduler();
                         */
                        before_thread_runs();
                        spinlock_unlock(&THREAD->lock);
                        cpu_priority_restore(THREAD->saved_context.pri);
                        return;
                }
                THREAD->saved_context.pri = pri;
        }

        /*
         * We may not keep the old stack.
         * Reason: If we kept the old stack and got blocked, for instance, in
         * find_best_thread(), the old thread could get rescheduled by another
         * CPU and overwrite the part of its own stack that was also used by
         * the scheduler on this CPU.
         *
         * Moreover, we have to bypass the compiler-generated POP sequence
         * which is fooled by SP being set to the very top of the stack.
         * Therefore the scheduler() function continues in
         * scheduler_separated_stack().
         */
        context_save(&CPU->saved_context);
        context_set(&CPU->saved_context, FADDR(scheduler_separated_stack), CPU->stack, CPU_STACK_SIZE);
        context_restore(&CPU->saved_context);
        /* not reached */
}


/** Scheduler stack switch wrapper
 *
 * Second part of the scheduler() function
 * using new stack. Handling the actual context
 * switch to a new thread.
 *
 */
void scheduler_separated_stack(void)
{
        int priority;

        if (THREAD) {
                switch (THREAD->state) {
                    case Running:
                            THREAD->state = Ready;
                            spinlock_unlock(&THREAD->lock);
                            thread_ready(THREAD);
                            break;

                    case Exiting:
                            frame_free((__address) THREAD->kstack);
                            if (THREAD->ustack) {
                                    frame_free((__address) THREAD->ustack);
                            }
                            
                            /*
                             * Detach from the containing task.
                             */
                            spinlock_lock(&TASK->lock);
                            list_remove(&THREAD->th_link);
                            spinlock_unlock(&TASK->lock);

                            spinlock_unlock(&THREAD->lock);
                            
                            spinlock_lock(&threads_lock);
                            list_remove(&THREAD->threads_link);
                            spinlock_unlock(&threads_lock);

                            spinlock_lock(&CPU->lock);
                            if(CPU->fpu_owner==THREAD) CPU->fpu_owner=NULL;
                            spinlock_unlock(&CPU->lock);

                            
                            free(THREAD);
                            
                            break;
                            
                    case Sleeping:
                            /*
                             * Prefer the thread after it's woken up.
                             */
                            THREAD->pri = -1;

                            /*
                             * We need to release wq->lock which we locked in waitq_sleep().
                             * Address of wq->lock is kept in THREAD->sleep_queue.
                             */
                            spinlock_unlock(&THREAD->sleep_queue->lock);

                            /*
                             * Check for possible requests for out-of-context invocation.
                             */
                            if (THREAD->call_me) {
                                    THREAD->call_me(THREAD->call_me_with);
                                    THREAD->call_me = NULL;
                                    THREAD->call_me_with = NULL;
                            }

                            spinlock_unlock(&THREAD->lock);
                            
                            break;

                    default:
                            /*
                             * Entering state is unexpected.
                             */
                            panic("tid%d: unexpected state %s\n", THREAD->tid, thread_states[THREAD->state]);
                            break;
                }
                THREAD = NULL;
        }
    
        THREAD = find_best_thread();
        
        spinlock_lock(&THREAD->lock);
        priority = THREAD->pri;
        spinlock_unlock(&THREAD->lock); 
        
        relink_rq(priority);            

        spinlock_lock(&THREAD->lock);   

        /*
         * If both the old and the new task are the same, lots of work is avoided.
         */
        if (TASK != THREAD->task) {
                vm_t *m1 = NULL;
                vm_t *m2;

                if (TASK) {
                        spinlock_lock(&TASK->lock);
                        m1 = TASK->vm;
                        spinlock_unlock(&TASK->lock);
                }

                spinlock_lock(&THREAD->task->lock);
                m2 = THREAD->task->vm;
                spinlock_unlock(&THREAD->task->lock);
                
                /*
                 * Note that it is possible for two tasks to share one vm mapping.
                 */
                if (m1 != m2) {
                        /*
                         * Both tasks and vm mappings are different.
                         * Replace the old one with the new one.
                         */
                        if (m1) {
                                vm_uninstall(m1);
                        }
                        vm_install(m2);
                }
                TASK = THREAD->task;    
        }

        THREAD->state = Running;

        #ifdef SCHEDULER_VERBOSE
        printf("cpu%d: tid %d (pri=%d,ticks=%d,nrdy=%d)\n", CPU->id, THREAD->tid, THREAD->pri, THREAD->ticks, CPU->nrdy);
        #endif  

        context_restore(&THREAD->saved_context);
        /* not reached */
}


#ifdef __SMP__
/** Load balancing thread
 *
 * SMP load balancing thread, supervising thread supplies
 * for the CPU it's wired to.
 *
 * @param arg Generic thread argument (unused).
 *
 */
void kcpulb(void *arg)
{
        thread_t *t;
        int count, i, j, k = 0;
        pri_t pri;

loop:
        /*
         * Sleep until there's some work to do.
         */
        waitq_sleep(&CPU->kcpulb_wq);

not_satisfied:
        /*
         * Calculate the number of threads that will be migrated/stolen from
         * other CPU's. Note that situation can have changed between two
         * passes. Each time get the most up to date counts.
         */
        pri = cpu_priority_high();
        spinlock_lock(&CPU->lock);
        count = nrdy / config.cpu_active;
        count -= CPU->nrdy;
        spinlock_unlock(&CPU->lock);
        cpu_priority_restore(pri);

        if (count <= 0)
                goto satisfied;

        /*
         * Searching least priority queues on all CPU's first and most priority queues on all CPU's last.
         */
        for (j=RQ_COUNT-1; j >= 0; j--) {
                for (i=0; i < config.cpu_active; i++) {
                        link_t *l;
                        runq_t *r;
                        cpu_t *cpu;

                        cpu = &cpus[(i + k) % config.cpu_active];

                        /*
                         * Not interested in ourselves.
                         * Doesn't require interrupt disabling for kcpulb is X_WIRED.
                         */
                        if (CPU == cpu)
                                continue;                               

restart:                pri = cpu_priority_high();
                        r = &cpu->rq[j];
                        spinlock_lock(&r->lock);
                        if (r->n == 0) {
                                spinlock_unlock(&r->lock);
                                cpu_priority_restore(pri);
                                continue;
                        }
                
                        t = NULL;
                        l = r->rq_head.prev;    /* search rq from the back */
                        while (l != &r->rq_head) {
                                t = list_get_instance(l, thread_t, rq_link);
                                /*
                                 * We don't want to steal CPU-wired threads neither threads already stolen.
                                 * The latter prevents threads from migrating between CPU's without ever being run.
                                 * We don't want to steal threads whose FPU context is still in CPU.
                                 */
                                spinlock_lock(&t->lock);
                                if ( (!(t->flags & (X_WIRED | X_STOLEN))) && (!(t->fpu_context_engaged)) ) {
                                
                                        /*
                                         * Remove t from r.
                                         */

                                        spinlock_unlock(&t->lock);
                                        
                                        /*
                                         * Here we have to avoid deadlock with relink_rq(),
                                         * because it locks cpu and r in a different order than we do.
                                         */
                                        if (!spinlock_trylock(&cpu->lock)) {
                                                /* Release all locks and try again. */ 
                                                spinlock_unlock(&r->lock);
                                                cpu_priority_restore(pri);
                                                goto restart;
                                        }
                                        cpu->nrdy--;
                                        spinlock_unlock(&cpu->lock);

                                        atomic_dec(&nrdy);

                                        r->n--;
                                        list_remove(&t->rq_link);

                                        break;
                                }
                                spinlock_unlock(&t->lock);
                                l = l->prev;
                                t = NULL;
                        }
                        spinlock_unlock(&r->lock);

                        if (t) {
                                /*
                                 * Ready t on local CPU
                                 */
                                spinlock_lock(&t->lock);
                                #ifdef KCPULB_VERBOSE
                                printf("kcpulb%d: TID %d -> cpu%d, nrdy=%d, avg=%d\n", CPU->id, t->tid, CPU->id, CPU->nrdy, nrdy / config.cpu_active);
                                #endif
                                t->flags |= X_STOLEN;
                                spinlock_unlock(&t->lock);
        
                                thread_ready(t);

                                cpu_priority_restore(pri);
        
                                if (--count == 0)
                                        goto satisfied;
                                        
                                /*
                                 * We are not satisfied yet, focus on another CPU next time.
                                 */
                                k++;
                                
                                continue;
                        }
                        cpu_priority_restore(pri);
                }
        }

        if (CPU->nrdy) {
                /*
                 * Be a little bit light-weight and let migrated threads run.
                 */
                scheduler();
        } 
        else {
                /*
                 * We failed to migrate a single thread.
                 * Something more sophisticated should be done.
                 */
                scheduler();
        }
                
        goto not_satisfied;
    
satisfied:
        /*
         * Tell find_best_thread() to wake us up later again.
         */
        CPU->kcpulbstarted = 0;
        goto loop;
}

#endif /* __SMP__ */