source: mainline/kernel/generic/src/proc/scheduler.c@ 4e7d3dd

/*
 * Copyright (c) 2010 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 scheduler_separated_stack(void);

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

/** Carry out actions before new task runs. */
static 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
 *
 */
static 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
 *
 */
static void after_thread_ran(void)
{
        after_thread_ran_arch();
}

#ifdef CONFIG_FPU_LAZY
void scheduler_fpu_lazy_request(void)
{
restart:
        fpu_enable();
        irq_spinlock_lock(&CPU->lock, false);
        
        /* Save old context */
        if (CPU->fpu_owner != NULL) {
                irq_spinlock_lock(&CPU->fpu_owner->lock, false);
                fpu_context_save(CPU->fpu_owner->saved_fpu_context);
                
                /* Don't prevent migration */
                CPU->fpu_owner->fpu_context_engaged = 0;
                irq_spinlock_unlock(&CPU->fpu_owner->lock, false);
                CPU->fpu_owner = NULL;
        }
        
        irq_spinlock_lock(&THREAD->lock, false);
        if (THREAD->fpu_context_exists) {
                fpu_context_restore(THREAD->saved_fpu_context);
        } else {
                /* Allocate FPU context */
                if (!THREAD->saved_fpu_context) {
                        /* Might sleep */
                        irq_spinlock_unlock(&THREAD->lock, false);
                        irq_spinlock_unlock(&CPU->lock, false);
                        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;
        irq_spinlock_unlock(&THREAD->lock, false);
        
        irq_spinlock_unlock(&CPU->lock, false);
}
#endif /* CONFIG_FPU_LAZY */

/** 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)
{
        ASSERT(CPU != NULL);
        
loop:
        
        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.
                 */
                irq_spinlock_lock(&CPU->lock, false);
                CPU->idle = true;
                irq_spinlock_unlock(&CPU->lock, false);
                interrupts_enable();
                
                /*
                 * 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();
                interrupts_disable();
                goto loop;
        }
        
        unsigned int i;
        for (i = 0; i < RQ_COUNT; i++) {
                irq_spinlock_lock(&(CPU->rq[i].lock), false);
                if (CPU->rq[i].n == 0) {
                        /*
                         * If this queue is empty, try a lower-priority queue.
                         */
                        irq_spinlock_unlock(&(CPU->rq[i].lock), false);
                        continue;
                }
                
                atomic_dec(&CPU->nrdy);
                atomic_dec(&nrdy);
                CPU->rq[i].n--;
                
                /*
                 * Take the first thread from the queue.
                 */
                thread_t *thread =
                    list_get_instance(CPU->rq[i].rq_head.next, thread_t, rq_link);
                list_remove(&thread->rq_link);
                
                irq_spinlock_pass(&(CPU->rq[i].lock), &thread->lock);
                
                thread->cpu = CPU;
                thread->ticks = us2ticks((i + 1) * 10000);
                thread->priority = i;  /* Correct rq index */
                
                /*
                 * Clear the THREAD_FLAG_STOLEN flag so that t can be migrated
                 * when load balancing needs emerge.
                 */
                thread->flags &= ~THREAD_FLAG_STOLEN;
                irq_spinlock_unlock(&thread->lock, false);
                
                return thread;
        }
        
        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;
        
        list_initialize(&head);
        irq_spinlock_lock(&CPU->lock, false);
        
        if (CPU->needs_relink > NEEDS_RELINK_MAX) {
                int i;
                for (i = start; i < RQ_COUNT - 1; i++) {
                        /* Remember and empty rq[i + 1] */
                        
                        irq_spinlock_lock(&CPU->rq[i + 1].lock, false);
                        list_concat(&head, &CPU->rq[i + 1].rq_head);
                        size_t n = CPU->rq[i + 1].n;
                        CPU->rq[i + 1].n = 0;
                        irq_spinlock_unlock(&CPU->rq[i + 1].lock, false);
                        
                        /* Append rq[i + 1] to rq[i] */
                        
                        irq_spinlock_lock(&CPU->rq[i].lock, false);
                        list_concat(&CPU->rq[i].rq_head, &head);
                        CPU->rq[i].n += n;
                        irq_spinlock_unlock(&CPU->rq[i].lock, false);
                }
                
                CPU->needs_relink = 0;
        }
        
        irq_spinlock_unlock(&CPU->lock, false);
}

/** 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) {
                irq_spinlock_lock(&THREAD->lock, false);
                
                /* Update thread kernel accounting */
                THREAD->kcycles += 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();
                        
                        irq_spinlock_unlock(&THREAD->lock, false);
                        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.
 *
 */
void scheduler_separated_stack(void)
{
        DEADLOCK_PROBE_INIT(p_joinwq);
        task_t *old_task = TASK;
        as_t *old_as = AS;
        
        ASSERT((!THREAD) || (irq_spinlock_locked(&THREAD->lock)));
        ASSERT(CPU != NULL);
        
        /*
         * Hold the current task and the address space to prevent their
         * possible destruction should thread_destroy() be called on this or any
         * other processor while the scheduler is still using them.
         */
        if (old_task)
                task_hold(old_task);
        
        if (old_as)
                as_hold(old_as);
        
        if (THREAD) {
                /* Must be run after the switch to scheduler stack */
                after_thread_ran();
                
                switch (THREAD->state) {
                case Running:
                        irq_spinlock_unlock(&THREAD->lock, false);
                        thread_ready(THREAD);
                        break;
                
                case Exiting:
repeat:
                        if (THREAD->detached) {
                                thread_destroy(THREAD, false);
                        } else {
                                /*
                                 * The thread structure is kept allocated until
                                 * somebody calls thread_detach() on it.
                                 */
                                if (!irq_spinlock_trylock(&THREAD->join_wq.lock)) {
                                        /*
                                         * Avoid deadlock.
                                         */
                                        irq_spinlock_unlock(&THREAD->lock, false);
                                        delay(HZ);
                                        irq_spinlock_lock(&THREAD->lock, false);
                                        DEADLOCK_PROBE(p_joinwq,
                                            DEADLOCK_THRESHOLD);
                                        goto repeat;
                                }
                                _waitq_wakeup_unsafe(&THREAD->join_wq,
                                    WAKEUP_FIRST);
                                irq_spinlock_unlock(&THREAD->join_wq.lock, false);
                                
                                THREAD->state = Lingering;
                                irq_spinlock_unlock(&THREAD->lock, false);
                        }
                        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.
                         */
                        irq_spinlock_unlock(&THREAD->sleep_queue->lock, false);
                        
                        irq_spinlock_unlock(&THREAD->lock, false);
                        break;
                
                default:
                        /*
                         * Entering state is unexpected.
                         */
                        panic("tid%" PRIu64 ": unexpected state %s.",
                            THREAD->tid, thread_states[THREAD->state]);
                        break;
                }
                
                THREAD = NULL;
        }
        
        THREAD = find_best_thread();
        
        irq_spinlock_lock(&THREAD->lock, false);
        int priority = THREAD->priority;
        irq_spinlock_unlock(&THREAD->lock, false);
        
        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 *new_as = THREAD->task->as;
                
                /*
                 * Note that it is possible for two tasks
                 * to share one address space.
                 */
                if (old_as != new_as) {
                        /*
                         * Both tasks and address spaces are different.
                         * Replace the old one with the new one.
                         */
                        as_switch(old_as, new_as);
                }
                
                TASK = THREAD->task;
                before_task_runs();
        }
        
        if (old_task)
                task_release(old_task);
        
        if (old_as)
                as_release(old_as);
        
        irq_spinlock_lock(&THREAD->lock, false);
        THREAD->state = Running;
        
#ifdef SCHEDULER_VERBOSE
        printf("cpu%u: tid %" PRIu64 " (priority=%d, ticks=%" PRIu64 
            ", 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)
{
        atomic_count_t average;
        atomic_count_t rdy;
        
        /*
         * 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;
        rdy = atomic_get(&CPU->nrdy);
        
        if (average <= rdy)
                goto satisfied;
        
        atomic_count_t count = average - rdy;
        
        /*
         * Searching least priority queues on all CPU's first and most priority
         * queues on all CPU's last.
         *
         */
        size_t acpu;
        size_t acpu_bias = 0;
        int rq;
        
        for (rq = RQ_COUNT - 1; rq >= 0; rq--) {
                for (acpu = 0; acpu < config.cpu_active; acpu++) {
                        cpu_t *cpu = &cpus[(acpu + acpu_bias) % 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;
                        
                        irq_spinlock_lock(&(cpu->rq[rq].lock), true);
                        if (cpu->rq[rq].n == 0) {
                                irq_spinlock_unlock(&(cpu->rq[rq].lock), true);
                                continue;
                        }
                        
                        thread_t *thread = NULL;
                        
                        /* Search rq from the back */
                        link_t *link = cpu->rq[rq].rq_head.prev;
                        
                        while (link != &(cpu->rq[rq].rq_head)) {
                                thread = (thread_t *) list_get_instance(link, 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.
                                 *
                                 */
                                irq_spinlock_lock(&thread->lock, false);
                                
                                if ((!(thread->flags & (THREAD_FLAG_WIRED | THREAD_FLAG_STOLEN)))
                                    && (!(thread->fpu_context_engaged))) {
                                        /*
                                         * Remove thread from ready queue.
                                         */
                                        irq_spinlock_unlock(&thread->lock, false);
                                        
                                        atomic_dec(&cpu->nrdy);
                                        atomic_dec(&nrdy);
                                        
                                        cpu->rq[rq].n--;
                                        list_remove(&thread->rq_link);
                                        
                                        break;
                                }
                                
                                irq_spinlock_unlock(&thread->lock, false);
                                
                                link = link->prev;
                                thread = NULL;
                        }
                        
                        if (thread) {
                                /*
                                 * Ready thread on local CPU
                                 *
                                 */
                                
                                irq_spinlock_pass(&(cpu->rq[rq].lock), &thread->lock);
                                
#ifdef KCPULB_VERBOSE
                                printf("kcpulb%u: TID %" PRIu64 " -> cpu%u, "
                                    "nrdy=%ld, avg=%ld\n", CPU->id, t->tid,
                                    CPU->id, atomic_get(&CPU->nrdy),
                                    atomic_get(&nrdy) / config.cpu_active);
#endif
                                
                                thread->flags |= THREAD_FLAG_STOLEN;
                                thread->state = Entering;
                                
                                irq_spinlock_unlock(&thread->lock, true);
                                thread_ready(thread);
                                
                                if (--count == 0)
                                        goto satisfied;
                                
                                /*
                                 * We are not satisfied yet, focus on another
                                 * CPU next time.
                                 *
                                 */
                                acpu_bias++;
                                
                                continue;
                        } else
                                irq_spinlock_unlock(&(cpu->rq[rq].lock), true);
                        
                }
        }
        
        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)
{
        size_t cpu;
        for (cpu = 0; cpu < config.cpu_count; cpu++) {
                if (!cpus[cpu].active)
                        continue;
                
                irq_spinlock_lock(&cpus[cpu].lock, true);
                
                printf("cpu%u: address=%p, nrdy=%" PRIua ", needs_relink=%zu\n",
                    cpus[cpu].id, &cpus[cpu], atomic_get(&cpus[cpu].nrdy),
                    cpus[cpu].needs_relink);
                
                unsigned int i;
                for (i = 0; i < RQ_COUNT; i++) {
                        irq_spinlock_lock(&(cpus[cpu].rq[i].lock), false);
                        if (cpus[cpu].rq[i].n == 0) {
                                irq_spinlock_unlock(&(cpus[cpu].rq[i].lock), false);
                                continue;
                        }
                        
                        printf("\trq[%u]: ", i);
                        link_t *cur;
                        for (cur = cpus[cpu].rq[i].rq_head.next;
                            cur != &(cpus[cpu].rq[i].rq_head);
                            cur = cur->next) {
                                thread_t *thread = list_get_instance(cur, thread_t, rq_link);
                                printf("%" PRIu64 "(%s) ", thread->tid,
                                    thread_states[thread->state]);
                        }
                        printf("\n");
                        
                        irq_spinlock_unlock(&(cpus[cpu].rq[i].lock), false);
                }
                
                irq_spinlock_unlock(&cpus[cpu].lock, true);
        }
}

/** @}
 */