source: mainline/kernel/generic/src/proc/scheduler.c@ 07be3c4

/*
 * Copyright (c) 2001-2007 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.
 */

/** @addtogroup genericproc
 * @{
 */

/**
 * @file
 * @brief       Scheduler and load balancing.
 *
 * This file contains the scheduler and kcpulb kernel thread which
 * performs load-balancing of per-CPU run queues.
 */

#include <proc/scheduler.h>
#include <proc/thread.h>
#include <proc/task.h>
#include <mm/frame.h>
#include <mm/page.h>
#include <mm/as.h>
#include <time/timeout.h>
#include <time/delay.h>
#include <arch/asm.h>
#include <arch/faddr.h>
#include <arch/cycle.h>
#include <atomic.h>
#include <synch/spinlock.h>
#include <config.h>
#include <context.h>
#include <fpu_context.h>
#include <func.h>
#include <arch.h>
#include <adt/list.h>
#include <panic.h>
#include <cpu.h>
#include <print.h>
#include <debug.h>

static void before_task_runs(void);
static void before_thread_runs(void);
static void after_thread_ran(void);
static void scheduler_separated_stack(void);

atomic_t nrdy;  /**< Number of ready threads in the system. */

/** Carry out actions before new task runs. */
void before_task_runs(void)
{
        before_task_runs_arch();
}

/** Take actions before new thread runs.
 *
 * Perform actions that need to be
 * taken before the newly selected
 * tread is passed control.
 *
 * THREAD->lock is locked on entry
 *
 */
void before_thread_runs(void)
{
        before_thread_runs_arch();
#ifdef CONFIG_FPU_LAZY
        if(THREAD == CPU->fpu_owner) 
                fpu_enable();
        else
                fpu_disable(); 
#else
        fpu_enable();
        if (THREAD->fpu_context_exists)
                fpu_context_restore(THREAD->saved_fpu_context);
        else {
                fpu_init();
                THREAD->fpu_context_exists = 1;
        }
#endif
}

/** Take actions after THREAD had run.
 *
 * Perform actions that need to be
 * taken after the running thread
 * had been preempted by the scheduler.
 *
 * THREAD->lock is locked on entry
 *
 */
void after_thread_ran(void)
{
        after_thread_ran_arch();
}

#ifdef CONFIG_FPU_LAZY
void scheduler_fpu_lazy_request(void)
{
restart:
        fpu_enable();
        spinlock_lock(&CPU->lock);

        /* Save old context */
        if (CPU->fpu_owner != NULL) {  
                spinlock_lock(&CPU->fpu_owner->lock);
                fpu_context_save(CPU->fpu_owner->saved_fpu_context);
                /* don't prevent migration */
                CPU->fpu_owner->fpu_context_engaged = 0;
                spinlock_unlock(&CPU->fpu_owner->lock);
                CPU->fpu_owner = NULL;
        }

        spinlock_lock(&THREAD->lock);
        if (THREAD->fpu_context_exists) {
                fpu_context_restore(THREAD->saved_fpu_context);
        } else {
                /* Allocate FPU context */
                if (!THREAD->saved_fpu_context) {
                        /* Might sleep */
                        spinlock_unlock(&THREAD->lock);
                        spinlock_unlock(&CPU->lock);
                        THREAD->saved_fpu_context =
                            (fpu_context_t *) slab_alloc(fpu_context_slab, 0);
                        /* We may have switched CPUs during slab_alloc */
                        goto restart; 
                }
                fpu_init();
                THREAD->fpu_context_exists = 1;
        }
        CPU->fpu_owner = THREAD;
        THREAD->fpu_context_engaged = 1;
        spinlock_unlock(&THREAD->lock);

        spinlock_unlock(&CPU->lock);
}
#endif

/** 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.
 *
 */
static thread_t *find_best_thread(void)
{
        thread_t *t;
        runq_t *r;
        int i;

        ASSERT(CPU != NULL);

loop:
        interrupts_enable();
        
        if (atomic_get(&CPU->nrdy) == 0) {
                /*
                 * 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.
                 */

                /*
                 * An interrupt might occur right now and wake up a thread.
                 * In such case, the CPU will continue to go to sleep
                 * even though there is a runnable thread.
                 */

                 cpu_sleep();
                 goto loop;
        }

        interrupts_disable();
        
        for (i = 0; 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;
                }

                atomic_dec(&CPU->nrdy);
                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->priority = i;        /* correct rq index */

                /*
                 * Clear the THREAD_FLAG_STOLEN flag so that t can be migrated
                 * when load balancing needs emerge.
                 */
                t->flags &= ~THREAD_FLAG_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.
 *
 */
static 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.
 * Passes control directly to
 * scheduler_separated_stack().
 *
 */
void scheduler(void)
{
        volatile ipl_t ipl;

        ASSERT(CPU != NULL);

        ipl = interrupts_disable();

        if (atomic_get(&haltstate))
                halt();
        
        if (THREAD) {
                spinlock_lock(&THREAD->lock);
                
                /* Update thread accounting */
                THREAD->cycles += get_cycle() - THREAD->last_cycle;
                
#ifndef CONFIG_FPU_LAZY
                fpu_context_save(THREAD->saved_fpu_context);
#endif
                if (!context_save(&THREAD->saved_context)) {
                        /*
                         * This is the place where threads leave scheduler();
                         */
                        
                        /* Save current CPU cycle */
                        THREAD->last_cycle = get_cycle();
                        
                        spinlock_unlock(&THREAD->lock);
                        interrupts_restore(THREAD->saved_context.ipl);
                        
                        return;
                }

                /*
                 * Interrupt priority level of preempted thread is recorded
                 * here to facilitate scheduler() invocations from
                 * interrupts_disable()'d code (e.g. waitq_sleep_timeout()). 
                 */
                THREAD->saved_context.ipl = ipl;
        }

        /*
         * Through the 'THE' structure, we keep track of THREAD, TASK, CPU, VM
         * and preemption counter. At this point THE could be coming either
         * from THREAD's or CPU's stack.
         */
        the_copy(THE, (the_t *) CPU->stack);

        /*
         * 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),
            (uintptr_t) 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.
 *
 * Assume THREAD->lock is held.
 */
void scheduler_separated_stack(void)
{
        int priority;
        DEADLOCK_PROBE_INIT(p_joinwq);

        ASSERT(CPU != NULL);
        
        if (THREAD) {
                /* must be run after the switch to scheduler stack */
                after_thread_ran();

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

                case Exiting:
repeat:
                        if (THREAD->detached) {
                                thread_destroy(THREAD);
                        } else {
                                /*
                                 * The thread structure is kept allocated until
                                 * somebody calls thread_detach() on it.
                                 */
                                if (!spinlock_trylock(&THREAD->join_wq.lock)) {
                                        /*
                                         * Avoid deadlock.
                                         */
                                        spinlock_unlock(&THREAD->lock);
                                        delay(HZ);
                                        spinlock_lock(&THREAD->lock);
                                        DEADLOCK_PROBE(p_joinwq,
                                            DEADLOCK_THRESHOLD);
                                        goto repeat;
                                }
                                _waitq_wakeup_unsafe(&THREAD->join_wq,
                                    WAKEUP_FIRST);
                                spinlock_unlock(&THREAD->join_wq.lock);
                                
                                THREAD->state = JoinMe;
                                spinlock_unlock(&THREAD->lock);
                        }
                        break;
                        
                case Sleeping:
                        /*
                         * Prefer the thread after it's woken up.
                         */
                        THREAD->priority = -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%llu: unexpected state %s\n", THREAD->tid,
                            thread_states[THREAD->state]);
                        break;
                }

                THREAD = NULL;
        }

        THREAD = find_best_thread();
        
        spinlock_lock(&THREAD->lock);
        priority = THREAD->priority;
        spinlock_unlock(&THREAD->lock); 

        relink_rq(priority);            

        /*
         * If both the old and the new task are the same, lots of work is
         * avoided.
         */
        if (TASK != THREAD->task) {
                as_t *as1 = NULL;
                as_t *as2;

                if (TASK) {
                        spinlock_lock(&TASK->lock);
                        as1 = TASK->as;
                        spinlock_unlock(&TASK->lock);
                }

                spinlock_lock(&THREAD->task->lock);
                as2 = THREAD->task->as;
                spinlock_unlock(&THREAD->task->lock);
                
                /*
                 * Note that it is possible for two tasks to share one address
                 * space.
                 */
                if (as1 != as2) {
                        /*
                         * Both tasks and address spaces are different.
                         * Replace the old one with the new one.
                         */
                        as_switch(as1, as2);
                }
                TASK = THREAD->task;
                before_task_runs();
        }

        spinlock_lock(&THREAD->lock);   
        THREAD->state = Running;

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

        /*
         * Some architectures provide late kernel PA2KA(identity)
         * mapping in a page fault handler. However, the page fault
         * handler uses the kernel stack of the running thread and
         * therefore cannot be used to map it. The kernel stack, if
         * necessary, is to be mapped in before_thread_runs(). This
         * function must be executed before the switch to the new stack.
         */
        before_thread_runs();

        /*
         * Copy the knowledge of CPU, TASK, THREAD and preemption counter to
         * thread's stack.
         */
        the_copy(THE, (the_t *) THREAD->kstack);
        
        context_restore(&THREAD->saved_context);
        /* not reached */
}

#ifdef CONFIG_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, average, j, k = 0;
        unsigned int i;
        ipl_t ipl;

        /*
         * Detach kcpulb as nobody will call thread_join_timeout() on it.
         */
        thread_detach(THREAD);
        
loop:
        /*
         * Work in 1s intervals.
         */
        thread_sleep(1);

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.
         */
        average = atomic_get(&nrdy) / config.cpu_active + 1;
        count = average - atomic_get(&CPU->nrdy);

        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 has
                         * THREAD_FLAG_WIRED.
                         */
                        if (CPU == cpu)
                                continue;
                        if (atomic_get(&cpu->nrdy) <= average)
                                continue;

                        ipl = interrupts_disable();
                        r = &cpu->rq[j];
                        spinlock_lock(&r->lock);
                        if (r->n == 0) {
                                spinlock_unlock(&r->lock);
                                interrupts_restore(ipl);
                                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 & (THREAD_FLAG_WIRED |
                                    THREAD_FLAG_STOLEN))) &&
                                    (!(t->fpu_context_engaged))) {
                                        /*
                                         * Remove t from r.
                                         */
                                        spinlock_unlock(&t->lock);
                                        
                                        atomic_dec(&cpu->nrdy);
                                        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 %llu -> cpu%d, nrdy=%ld, "
                                    "avg=%nd\n", CPU->id, t->tid, CPU->id,
                                    atomic_get(&CPU->nrdy),
                                    atomic_get(&nrdy) / config.cpu_active);
#endif
                                t->flags |= THREAD_FLAG_STOLEN;
                                t->state = Entering;
                                spinlock_unlock(&t->lock);
        
                                thread_ready(t);

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

        if (atomic_get(&CPU->nrdy)) {
                /*
                 * Be a little bit light-weight and let migrated threads run.
                 */
                scheduler();
        } else {
                /*
                 * We failed to migrate a single thread.
                 * Give up this turn.
                 */
                goto loop;
        }
                
        goto not_satisfied;

satisfied:
        goto loop;
}

#endif /* CONFIG_SMP */


/** Print information about threads & scheduler queues */
void sched_print_list(void)
{
        ipl_t ipl;
        unsigned int cpu, i;
        runq_t *r;
        thread_t *t;
        link_t *cur;

        /* We are going to mess with scheduler structures,
         * let's not be interrupted */
        ipl = interrupts_disable();
        for (cpu = 0; cpu < config.cpu_count; cpu++) {

                if (!cpus[cpu].active)
                        continue;

                spinlock_lock(&cpus[cpu].lock);
                printf("cpu%d: address=%p, nrdy=%ld, needs_relink=%ld\n",
                    cpus[cpu].id, &cpus[cpu], atomic_get(&cpus[cpu].nrdy),
                    cpus[cpu].needs_relink);
                
                for (i = 0; i < RQ_COUNT; i++) {
                        r = &cpus[cpu].rq[i];
                        spinlock_lock(&r->lock);
                        if (!r->n) {
                                spinlock_unlock(&r->lock);
                                continue;
                        }
                        printf("\trq[%d]: ", i);
                        for (cur = r->rq_head.next; cur != &r->rq_head;
                                cur = cur->next) {
                                t = list_get_instance(cur, thread_t, rq_link);
                                printf("%llu(%s) ", t->tid,
                                    thread_states[t->state]);
                        }
                        printf("\n");
                        spinlock_unlock(&r->lock);
                }
                spinlock_unlock(&cpus[cpu].lock);
        }
        
        interrupts_restore(ipl);
}

/** @}
 */