aio: add generic thread-pool facility

Add a generic thread-pool.  The code is roughly based on posix-aio-compat.c,
with some changes, especially the following:

- use QemuSemaphore instead of QemuCond;

- separate the state of the thread from the return code of the worker
function.  The return code is totally opaque for the thread pool;

- do not busy wait when doing cancellation.

A more generic threadpool (but still specific to I/O so that in the future
it can use special scheduling classes or PI mutexes) can have many uses:
it allows more flexibility in raw-posix.c and can more easily be extended
to Win32, and it will also be used to do an msync of the persistent bitmap.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 282 insertions, 0 deletions

diff --git a/thread-pool.c b/thread-pool.c
new file mode 100644
index 0000000000..80749b77e0
--- /dev/null
+++ b/thread-pool.c
@@ -0,0 +1,282 @@
+/*
+ * 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 "qemu-coroutine.h"
+#include "trace.h"
+#include "block_int.h"
+#include "event_notifier.h"
+#include "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;
+    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);
+
+        qemu_mutex_lock(&lock);
+        req->state = THREAD_DONE;
+        req->ret = ret;
+        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) {
+            qemu_mutex_lock(&lock);
+            int ret = elem->ret;
+            qemu_mutex_unlock(&lock);
+            QLIST_REMOVE(elem, all);
+            elem->common.cb(elem->common.opaque, 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 AIOPool thread_pool_cb_pool = {
+    .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_cb_pool, 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)