[Qemu-devel] [RFC 1/1] linux-aio: consume events in userspace instead of calling io_getevents

[Roman Pen]

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.

My VM configuration is the following:

VCPU=8, MQ=8, 1 iothread

FIO results:

io_getevents in kernel:

   READ: io=56479MB, aggrb=1882.5MB/s, minb=1882.5MB/s, maxb=1882.5MB/s, 
mint=30003msec, maxt=30003msec
  WRITE: io=56371MB, aggrb=1878.9MB/s, minb=1878.9MB/s, maxb=1878.9MB/s, 
mint=30003msec, maxt=30003msec

io_getevents in userspace:

   READ: io=57576MB, aggrb=1919.2MB/s, minb=1919.2MB/s, maxb=1919.2MB/s, 
mint=30003msec, maxt=30003msec
  WRITE: io=57492MB, aggrb=1916.3MB/s, minb=1916.3MB/s, maxb=1916.3MB/s, 
mint=30003msec, maxt=30003msec

The gain is only ~2%, but the price is almost nothing.

Signed-off-by: Roman Pen <address@hidden>
Cc: Stefan Hajnoczi <address@hidden>
Cc: Paolo Bonzini <address@hidden>
Cc: address@hidden
---
 block/linux-aio.c | 86 ++++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 70 insertions(+), 16 deletions(-)

diff --git a/block/linux-aio.c b/block/linux-aio.c
index fe7cece..4ab09c0 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -58,7 +58,7 @@ struct LinuxAioState {
 
     /* I/O completion processing */
     QEMUBH *completion_bh;
-    struct io_event events[MAX_EVENTS];
+    struct io_event *events;
     int event_idx;
     int event_max;
 };
@@ -101,6 +101,60 @@ static void qemu_laio_process_completion(struct 
qemu_laiocb *laiocb)
     }
 }
 
+/**
+ * aio_ring buffer which is shared between userspace and kernel.
+ */
+struct aio_ring {
+       unsigned        id;     /* kernel internal index number */
+       unsigned        nr;     /* number of io_events */
+       unsigned        head;   /* Written to by userland or under ring_lock
+                                * mutex by aio_read_events_ring(). */
+       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() - 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 int io_getevents_peek(io_context_t aio_ctx, unsigned int max,
+                             struct io_event **events)
+{
+       struct aio_ring *ring = (struct aio_ring*)aio_ctx;
+       unsigned head = ring->head, tail = ring->tail;
+       unsigned 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() - advances head of a ring buffer and returns
+ *                         1 if there are some events left in a ring.
+ */
+static int io_getevents_commit(io_context_t aio_ctx, unsigned int nr)
+{
+       struct aio_ring *ring = (struct aio_ring*)aio_ctx;
+
+       ring->head = (ring->head + nr) % ring->nr;
+       return (ring->head != ring->tail);
+}
+
 /* The completion BH fetches completed I/O requests and invokes their
  * callbacks.
  *
@@ -118,32 +172,32 @@ static void qemu_laio_completion_bh(void *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);
-
+more:
+        s->event_max = io_getevents_peek(s->ctx, MAX_EVENTS, &s->events);
         s->event_idx = 0;
-        if (s->event_max <= 0) {
-            s->event_max = 0;
+        if (s->event_max == 0)
             return; /* no more 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 =
+    if (s->event_idx < s->event_max) {
+        unsigned nr = s->event_max - s->event_idx;
+
+        while (s->event_idx < s->event_max) {
+            struct iocb *iocb = s->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(&s->events[s->event_idx]);
+            s->event_idx++;
 
-        qemu_laio_process_completion(laiocb);
+            qemu_laio_process_completion(laiocb);
+        }
+        if (io_getevents_commit(s->ctx, nr))
+            goto more;
     }
 
     if (!s->io_q.plugged && !QSIMPLEQ_EMPTY(&s->io_q.pending)) {
-- 
2.8.2