linux-aio: consume events in userspace instead of calling io_getevents

AIO context in userspace is represented as a simple ring buffer, which
can be consumed directly without entering the kernel, which obviously
can bring some performance gain.  QEMU does not use timeout value for
waiting for events completions, so we can consume all events from
userspace.

Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Message-id: 1468931263-32667-2-git-send-email-roman.penyaev@profitbricks.com
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/linux-aio.c

@@ -59,7 +59,6 @@ struct LinuxAioState {
 
     /* I/O completion processing */
     QEMUBH *completion_bh;
-    struct io_event events[MAX_EVENTS];
     int event_idx;
     int event_max;
 };
@@ -102,6 +101,85 @@ static void qemu_laio_process_completion(struct qemu_laiocb *laiocb)
     }
 }
 
+/**
+ * aio_ring buffer which is shared between userspace and kernel.
+ *
+ * This copied from linux/fs/aio.c, common header does not exist
+ * but AIO exists for ages so we assume ABI is stable.
+ */
+struct aio_ring {
+    unsigned    id;    /* kernel internal index number */
+    unsigned    nr;    /* number of io_events */
+    unsigned    head;  /* Written to by userland or by kernel. */
+    unsigned    tail;
+
+    unsigned    magic;
+    unsigned    compat_features;
+    unsigned    incompat_features;
+    unsigned    header_length;  /* size of aio_ring */
+
+    struct io_event io_events[0];
+};
+
+/**
+ * io_getevents_peek:
+ * @ctx: AIO context
+ * @events: pointer on events array, output value
+
+ * Returns the number of completed events and sets a pointer
+ * on events array.  This function does not update the internal
+ * ring buffer, only reads head and tail.  When @events has been
+ * processed io_getevents_commit() must be called.
+ */
+static inline unsigned int io_getevents_peek(io_context_t ctx,
+                                             struct io_event **events)
+{
+    struct aio_ring *ring = (struct aio_ring *)ctx;
+    unsigned int head = ring->head, tail = ring->tail;
+    unsigned int nr;
+
+    nr = tail >= head ? tail - head : ring->nr - head;
+    *events = ring->io_events + head;
+    /* To avoid speculative loads of s->events[i] before observing tail.
+       Paired with smp_wmb() inside linux/fs/aio.c: aio_complete(). */
+    smp_rmb();
+
+    return nr;
+}
+
+/**
+ * io_getevents_commit:
+ * @ctx: AIO context
+ * @nr: the number of events on which head should be advanced
+ *
+ * Advances head of a ring buffer.
+ */
+static inline void io_getevents_commit(io_context_t ctx, unsigned int nr)
+{
+    struct aio_ring *ring = (struct aio_ring *)ctx;
+
+    if (nr) {
+        ring->head = (ring->head + nr) % ring->nr;
+    }
+}
+
+/**
+ * io_getevents_advance_and_peek:
+ * @ctx: AIO context
+ * @events: pointer on events array, output value
+ * @nr: the number of events on which head should be advanced
+ *
+ * Advances head of a ring buffer and returns number of elements left.
+ */
+static inline unsigned int
+io_getevents_advance_and_peek(io_context_t ctx,
+                              struct io_event **events,
+                              unsigned int nr)
+{
+    io_getevents_commit(ctx, nr);
+    return io_getevents_peek(ctx, events);
+}
+
 /* The completion BH fetches completed I/O requests and invokes their
  * callbacks.
  *
@@ -116,43 +194,38 @@ static void qemu_laio_process_completion(struct qemu_laiocb *laiocb)
 static void qemu_laio_completion_bh(void *opaque)
 {
     LinuxAioState *s = opaque;
-
-    /* Fetch more completion events when empty */
-    if (s->event_idx == s->event_max) {
-        do {
-            struct timespec ts = { 0 };
-            s->event_max = io_getevents(s->ctx, MAX_EVENTS, MAX_EVENTS,
-                                        s->events, &ts);
-        } while (s->event_max == -EINTR);
-
-        s->event_idx = 0;
-        if (s->event_max <= 0) {
-            s->event_max = 0;
-            return; /* no more events */
-        }
-        s->io_q.in_flight -= s->event_max;
-    }
+    struct io_event *events;
 
     /* Reschedule so nested event loops see currently pending completions */
     qemu_bh_schedule(s->completion_bh);
 
-    /* Process completion events */
-    while (s->event_idx < s->event_max) {
-        struct iocb *iocb = s->events[s->event_idx].obj;
-        struct qemu_laiocb *laiocb =
+    while ((s->event_max = io_getevents_advance_and_peek(s->ctx, &events,
+                                                         s->event_idx))) {
+        for (s->event_idx = 0; s->event_idx < s->event_max; ) {
+            struct iocb *iocb = events[s->event_idx].obj;
+            struct qemu_laiocb *laiocb =
                 container_of(iocb, struct qemu_laiocb, iocb);
 
-        laiocb->ret = io_event_ret(&s->events[s->event_idx]);
-        s->event_idx++;
+            laiocb->ret = io_event_ret(&events[s->event_idx]);
 
-        qemu_laio_process_completion(laiocb);
+            /* Change counters one-by-one because we can be nested. */
+            s->io_q.in_flight--;
+            s->event_idx++;
+            qemu_laio_process_completion(laiocb);
+        }
     }
 
+    qemu_bh_cancel(s->completion_bh);
+
+    /* If we are nested we have to notify the level above that we are done
+     * by setting event_max to zero, upper level will then jump out of it's
+     * own `for` loop.  If we are the last all counters droped to zero. */
+    s->event_max = 0;
+    s->event_idx = 0;
+
     if (!s->io_q.plugged && !QSIMPLEQ_EMPTY(&s->io_q.pending)) {
         ioq_submit(s);
     }
-
-    qemu_bh_cancel(s->completion_bh);
 }
 
 static void qemu_laio_completion_cb(EventNotifier *e)