virtio: use defer_call() in virtio_irqfd_notify()

virtio-blk and virtio-scsi invoke virtio_irqfd_notify() to send Used
Buffer Notifications from an IOThread. This involves an eventfd
write(2) syscall. Calling this repeatedly when completing multiple I/O
requests in a row is wasteful.

Use the defer_call() API to batch together virtio_irqfd_notify() calls
made during thread pool (aio=threads), Linux AIO (aio=native), and
io_uring (aio=io_uring) completion processing.

Behavior is unchanged for emulated devices that do not use
defer_call_begin()/defer_call_end() since defer_call() immediately
invokes the callback when called outside a
defer_call_begin()/defer_call_end() region.

fio rw=randread bs=4k iodepth=64 numjobs=8 IOPS increases by ~9% with a
single IOThread and 8 vCPUs. iodepth=1 decreases by ~1% but this could
be noise. Detailed performance data and configuration specifics are
available here:
https://gitlab.com/stefanha/virt-playbooks/-/tree/blk_io_plug-irqfd

This duplicates the BH that virtio-blk uses for batching. The next
commit will remove it.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230913200045.1024233-4-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

2 files changed, 10 insertions, 0 deletions

diff --git a/block/io_uring.c b/block/io_uring.c
index 3a1e1f45b3..7cdd00e9f1 100644
--- a/block/io_uring.c
+++ b/block/io_uring.c
@@ -125,6 +125,9 @@ static void luring_process_completions(LuringState *s)
 {
     struct io_uring_cqe *cqes;
     int total_bytes;
+
+    defer_call_begin();
+
     /*
      * Request completion callbacks can run the nested event loop.
      * Schedule ourselves so the nested event loop will "see" remaining
@@ -217,7 +220,10 @@ end:
             aio_co_wake(luringcb->co);
         }
     }
+
     qemu_bh_cancel(s->completion_bh);
+
+    defer_call_end();
 }
 
 static int ioq_submit(LuringState *s)
diff --git a/block/linux-aio.c b/block/linux-aio.c
index a2670b3e46..ec05d946f3 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -205,6 +205,8 @@ static void qemu_laio_process_completions(LinuxAioState *s)
 {
     struct io_event *events;
 
+    defer_call_begin();
+
     /* Reschedule so nested event loops see currently pending completions */
     qemu_bh_schedule(s->completion_bh);
 
@@ -231,6 +233,8 @@ static void qemu_laio_process_completions(LinuxAioState *s)
      * own `for` loop.  If we are the last all counters dropped to zero. */
     s->event_max = 0;
     s->event_idx = 0;
+
+    defer_call_end();
 }
 
 static void qemu_laio_process_completions_and_submit(LinuxAioState *s)