Archipelago » qemu

/*

 * QEMU block layer thread pool

 *

 * Copyright IBM, Corp. 2008

 * Copyright Red Hat, Inc. 2012

 *

 * Authors:

 *  Anthony Liguori   <aliguori@us.ibm.com>

 *  Paolo Bonzini     <pbonzini@redhat.com>

 *

 * This work is licensed under the terms of the GNU GPL, version 2.  See

 * the COPYING file in the top-level directory.

 *

 * Contributions after 2012-01-13 are licensed under the terms of the

 * GNU GPL, version 2 or (at your option) any later version.

 */

#include "qemu-common.h"

#include "qemu-queue.h"

#include "qemu-thread.h"

#include "osdep.h"

#include "block/coroutine.h"

#include "trace.h"

#include "block/block_int.h"

#include "event_notifier.h"

#include "block/thread-pool.h"

static void do_spawn_thread(void);

typedef struct ThreadPoolElement ThreadPoolElement;

enum ThreadState {

    THREAD_QUEUED,

    THREAD_ACTIVE,

    THREAD_DONE,

    THREAD_CANCELED,

};

struct ThreadPoolElement {

    BlockDriverAIOCB common;

    ThreadPoolFunc *func;

    void *arg;

    /* Moving state out of THREAD_QUEUED is protected by lock.  After

     * that, only the worker thread can write to it.  Reads and writes

     * of state and ret are ordered with memory barriers.

     */

    enum ThreadState state;

    int ret;

    /* Access to this list is protected by lock.  */

    QTAILQ_ENTRY(ThreadPoolElement) reqs;

    /* Access to this list is protected by the global mutex.  */

    QLIST_ENTRY(ThreadPoolElement) all;

};

static EventNotifier notifier;

static QemuMutex lock;

static QemuCond check_cancel;

static QemuSemaphore sem;

static int max_threads = 64;

static QEMUBH *new_thread_bh;

/* The following variables are protected by the global mutex.  */

static QLIST_HEAD(, ThreadPoolElement) head;

/* The following variables are protected by lock.  */

static QTAILQ_HEAD(, ThreadPoolElement) request_list;

static int cur_threads;

static int idle_threads;

static int new_threads;     /* backlog of threads we need to create */

static int pending_threads; /* threads created but not running yet */

static int pending_cancellations; /* whether we need a cond_broadcast */

static void *worker_thread(void *unused)

{

    qemu_mutex_lock(&lock);

    pending_threads--;

    do_spawn_thread();

    while (1) {

        ThreadPoolElement *req;

        int ret;

        do {

            idle_threads++;

            qemu_mutex_unlock(&lock);

            ret = qemu_sem_timedwait(&sem, 10000);

            qemu_mutex_lock(&lock);

            idle_threads--;

        } while (ret == -1 && !QTAILQ_EMPTY(&request_list));

        if (ret == -1) {

            break;

        }

        req = QTAILQ_FIRST(&request_list);

        QTAILQ_REMOVE(&request_list, req, reqs);

        req->state = THREAD_ACTIVE;

        qemu_mutex_unlock(&lock);

        ret = req->func(req->arg);

        req->ret = ret;

        /* Write ret before state.  */

        smp_wmb();

        req->state = THREAD_DONE;

        qemu_mutex_lock(&lock);

        if (pending_cancellations) {

            qemu_cond_broadcast(&check_cancel);

        }

        event_notifier_set(&notifier);

    }

    cur_threads--;

    qemu_mutex_unlock(&lock);

    return NULL;

}

static void do_spawn_thread(void)

{

    QemuThread t;

    /* Runs with lock taken.  */

    if (!new_threads) {

        return;

    }

    new_threads--;

    pending_threads++;

    qemu_thread_create(&t, worker_thread, NULL, QEMU_THREAD_DETACHED);

}

static void spawn_thread_bh_fn(void *opaque)

{

    qemu_mutex_lock(&lock);

    do_spawn_thread();

    qemu_mutex_unlock(&lock);

}

static void spawn_thread(void)

{

    cur_threads++;

    new_threads++;

    /* If there are threads being created, they will spawn new workers, so

     * we don't spend time creating many threads in a loop holding a mutex or

     * starving the current vcpu.

     *

     * If there are no idle threads, ask the main thread to create one, so we

     * inherit the correct affinity instead of the vcpu affinity.

     */

    if (!pending_threads) {

        qemu_bh_schedule(new_thread_bh);

    }

}

static void event_notifier_ready(EventNotifier *notifier)

{

    ThreadPoolElement *elem, *next;

    event_notifier_test_and_clear(notifier);

restart:

    QLIST_FOREACH_SAFE(elem, &head, all, next) {

        if (elem->state != THREAD_CANCELED && elem->state != THREAD_DONE) {

            continue;

        }

        if (elem->state == THREAD_DONE) {

            trace_thread_pool_complete(elem, elem->common.opaque, elem->ret);

        }

        if (elem->state == THREAD_DONE && elem->common.cb) {

            QLIST_REMOVE(elem, all);

            /* Read state before ret.  */

            smp_rmb();

            elem->common.cb(elem->common.opaque, elem->ret);

            qemu_aio_release(elem);

            goto restart;

        } else {

            /* remove the request */

            QLIST_REMOVE(elem, all);

            qemu_aio_release(elem);

        }

    }

}

static int thread_pool_active(EventNotifier *notifier)

{

    return !QLIST_EMPTY(&head);

}

static void thread_pool_cancel(BlockDriverAIOCB *acb)

{

    ThreadPoolElement *elem = (ThreadPoolElement *)acb;

    trace_thread_pool_cancel(elem, elem->common.opaque);

    qemu_mutex_lock(&lock);

    if (elem->state == THREAD_QUEUED &&

        /* No thread has yet started working on elem. we can try to "steal"

         * the item from the worker if we can get a signal from the

         * semaphore.  Because this is non-blocking, we can do it with

         * the lock taken and ensure that elem will remain THREAD_QUEUED.

         */

        qemu_sem_timedwait(&sem, 0) == 0) {

        QTAILQ_REMOVE(&request_list, elem, reqs);

        elem->state = THREAD_CANCELED;

        event_notifier_set(&notifier);

    } else {

        pending_cancellations++;

        while (elem->state != THREAD_CANCELED && elem->state != THREAD_DONE) {

            qemu_cond_wait(&check_cancel, &lock);

        }

        pending_cancellations--;

    }

    qemu_mutex_unlock(&lock);

}

static const AIOCBInfo thread_pool_aiocb_info = {

    .aiocb_size         = sizeof(ThreadPoolElement),

    .cancel             = thread_pool_cancel,

};

BlockDriverAIOCB *thread_pool_submit_aio(ThreadPoolFunc *func, void *arg,

        BlockDriverCompletionFunc *cb, void *opaque)

{

    ThreadPoolElement *req;

    req = qemu_aio_get(&thread_pool_aiocb_info, NULL, cb, opaque);

    req->func = func;

    req->arg = arg;

    req->state = THREAD_QUEUED;

    QLIST_INSERT_HEAD(&head, req, all);

    trace_thread_pool_submit(req, arg);

    qemu_mutex_lock(&lock);

    if (idle_threads == 0 && cur_threads < max_threads) {

        spawn_thread();

    }

    QTAILQ_INSERT_TAIL(&request_list, req, reqs);

    qemu_mutex_unlock(&lock);

    qemu_sem_post(&sem);

    return &req->common;

}

typedef struct ThreadPoolCo {

    Coroutine *co;

    int ret;

} ThreadPoolCo;

static void thread_pool_co_cb(void *opaque, int ret)

{

    ThreadPoolCo *co = opaque;

    co->ret = ret;

    qemu_coroutine_enter(co->co, NULL);

}

int coroutine_fn thread_pool_submit_co(ThreadPoolFunc *func, void *arg)

{

    ThreadPoolCo tpc = { .co = qemu_coroutine_self(), .ret = -EINPROGRESS };

    assert(qemu_in_coroutine());

    thread_pool_submit_aio(func, arg, thread_pool_co_cb, &tpc);

    qemu_coroutine_yield();

    return tpc.ret;

}

void thread_pool_submit(ThreadPoolFunc *func, void *arg)

{

    thread_pool_submit_aio(func, arg, NULL, NULL);

}

static void thread_pool_init(void)

{

    QLIST_INIT(&head);

    event_notifier_init(&notifier, false);

    qemu_mutex_init(&lock);

    qemu_cond_init(&check_cancel);

    qemu_sem_init(&sem, 0);

    qemu_aio_set_event_notifier(&notifier, event_notifier_ready,

                                thread_pool_active);

    QTAILQ_INIT(&request_list);

    new_thread_bh = qemu_bh_new(spawn_thread_bh_fn, NULL);

}

block_init(thread_pool_init)