[PATCH 1/2] virtio: use blk_io_plug_call() in virtio_irqfd_notify()

[Stefan Hajnoczi <stefanha@redhat.com>]

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 blk_io_plug_call() API to batch together virtio_irqfd_notify()
calls made during Linux AIO (aio=native) or io_uring (aio=io_uring)
completion processing. Do not modify the thread pool (aio=threads) to
avoid introducing a dependency from util/ onto the block layer.

Behavior is unchanged for emulated devices that do not use blk_io_plug()
since blk_io_plug_call() immediately invokes the callback when called
outside a blk_io_plug()/blk_io_unplug() 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.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/io_uring.c   |  6 ++++++
 block/linux-aio.c  |  4 ++++
 hw/virtio/virtio.c | 10 +++++++++-
 3 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/block/io_uring.c b/block/io_uring.c
index 69d9820928..749cf83934 100644
--- a/block/io_uring.c
+++ b/block/io_uring.c
@@ -124,6 +124,9 @@ static void luring_process_completions(LuringState *s)
 {
     struct io_uring_cqe *cqes;
     int total_bytes;
+
+    blk_io_plug();
+
     /*
      * Request completion callbacks can run the nested event loop.
      * Schedule ourselves so the nested event loop will "see" remaining
@@ -216,7 +219,10 @@ end:
             aio_co_wake(luringcb->co);
         }
     }
+
     qemu_bh_cancel(s->completion_bh);
+
+    blk_io_unplug();
 }
 
 static int ioq_submit(LuringState *s)
diff --git a/block/linux-aio.c b/block/linux-aio.c
index 561c71a9ae..cef3d6b1c7 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -204,6 +204,8 @@ static void qemu_laio_process_completions(LinuxAioState *s)
 {
     struct io_event *events;
 
+    blk_io_plug();
+
     /* Reschedule so nested event loops see currently pending completions */
     qemu_bh_schedule(s->completion_bh);
 
@@ -230,6 +232,8 @@ static void qemu_laio_process_completions(LinuxAioState *s)
      * own `for` loop.  If we are the last all counters droped to zero. */
     s->event_max = 0;
     s->event_idx = 0;
+
+    blk_io_unplug();
 }
 
 static void qemu_laio_process_completions_and_submit(LinuxAioState *s)
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 309038fd46..a691e8526b 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -28,6 +28,7 @@
 #include "hw/virtio/virtio-bus.h"
 #include "hw/qdev-properties.h"
 #include "hw/virtio/virtio-access.h"
+#include "sysemu/block-backend.h"
 #include "sysemu/dma.h"
 #include "sysemu/runstate.h"
 #include "virtio-qmp.h"
@@ -2426,6 +2427,13 @@ static bool virtio_should_notify(VirtIODevice *vdev, VirtQueue *vq)
     }
 }
 
+/* Batch irqs while inside a blk_io_plug()/blk_io_unplug() section */
+static void virtio_notify_irqfd_unplug_fn(void *opaque)
+{
+    EventNotifier *notifier = opaque;
+    event_notifier_set(notifier);
+}
+
 void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
 {
     WITH_RCU_READ_LOCK_GUARD() {
@@ -2452,7 +2460,7 @@ void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
      * to an atomic operation.
      */
     virtio_set_isr(vq->vdev, 0x1);
-    event_notifier_set(&vq->guest_notifier);
+    blk_io_plug_call(virtio_notify_irqfd_unplug_fn, &vq->guest_notifier);
 }
 
 static void virtio_irq(VirtQueue *vq)
-- 
2.41.0