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virtio: use defer_call() in virtio_irqfd_notify()

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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>
This commit is contained in:
Stefan Hajnoczi 2023-09-13 16:00:44 -04:00 • committed by Kevin Wolf
parent 433fcea40c
commit 84d61e5f36
5 changed files with 28 additions and 1 deletions
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13
hw/virtio/virtio.c
View file

@ -15,6 +15,7 @@
#include "qapi/error.h"
#include "qapi/qapi-commands-virtio.h"
#include "trace.h"
#include "qemu/defer-call.h"
#include "qemu/error-report.h"
#include "qemu/log.h"
#include "qemu/main-loop.h"
@ -2445,6 +2446,16 @@ static bool virtio_should_notify(VirtIODevice *vdev, VirtQueue *vq)
}
}
/* Batch irqs while inside a defer_call_begin()/defer_call_end() section */
static void virtio_notify_irqfd_deferred_fn(void *opaque)
{
EventNotifier *notifier = opaque;
VirtQueue *vq = container_of(notifier, VirtQueue, guest_notifier);
trace_virtio_notify_irqfd_deferred_fn(vq->vdev, vq);
event_notifier_set(notifier);
}
void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
{
WITH_RCU_READ_LOCK_GUARD() {
@ -2471,7 +2482,7 @@ void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
* to an atomic operation.
*/
virtio_set_isr(vq->vdev, 0x1);
event_notifier_set(&vq->guest_notifier);
defer_call(virtio_notify_irqfd_deferred_fn, &vq->guest_notifier);
}
static void virtio_irq(VirtQueue *vq)