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>
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@ -125,6 +125,9 @@ static void luring_process_completions(LuringState *s)
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{
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struct io_uring_cqe *cqes;
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int total_bytes;
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defer_call_begin();
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/*
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* Request completion callbacks can run the nested event loop.
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* Schedule ourselves so the nested event loop will "see" remaining
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@ -217,7 +220,10 @@ end:
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aio_co_wake(luringcb->co);
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}
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}
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qemu_bh_cancel(s->completion_bh);
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defer_call_end();
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}
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static int ioq_submit(LuringState *s)
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@ -205,6 +205,8 @@ static void qemu_laio_process_completions(LinuxAioState *s)
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{
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struct io_event *events;
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defer_call_begin();
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/* Reschedule so nested event loops see currently pending completions */
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qemu_bh_schedule(s->completion_bh);
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@ -231,6 +233,8 @@ static void qemu_laio_process_completions(LinuxAioState *s)
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* own `for` loop. If we are the last all counters dropped to zero. */
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s->event_max = 0;
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s->event_idx = 0;
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defer_call_end();
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}
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static void qemu_laio_process_completions_and_submit(LinuxAioState *s)
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@ -73,6 +73,7 @@ virtqueue_fill(void *vq, const void *elem, unsigned int len, unsigned int idx) "
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virtqueue_flush(void *vq, unsigned int count) "vq %p count %u"
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virtqueue_pop(void *vq, void *elem, unsigned int in_num, unsigned int out_num) "vq %p elem %p in_num %u out_num %u"
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virtio_queue_notify(void *vdev, int n, void *vq) "vdev %p n %d vq %p"
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virtio_notify_irqfd_deferred_fn(void *vdev, void *vq) "vdev %p vq %p"
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virtio_notify_irqfd(void *vdev, void *vq) "vdev %p vq %p"
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virtio_notify(void *vdev, void *vq) "vdev %p vq %p"
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virtio_set_status(void *vdev, uint8_t val) "vdev %p val %u"
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@ -15,6 +15,7 @@
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#include "qapi/error.h"
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#include "qapi/qapi-commands-virtio.h"
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#include "trace.h"
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#include "qemu/defer-call.h"
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#include "qemu/error-report.h"
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#include "qemu/log.h"
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#include "qemu/main-loop.h"
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@ -2445,6 +2446,16 @@ static bool virtio_should_notify(VirtIODevice *vdev, VirtQueue *vq)
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}
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}
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/* Batch irqs while inside a defer_call_begin()/defer_call_end() section */
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static void virtio_notify_irqfd_deferred_fn(void *opaque)
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{
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EventNotifier *notifier = opaque;
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VirtQueue *vq = container_of(notifier, VirtQueue, guest_notifier);
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trace_virtio_notify_irqfd_deferred_fn(vq->vdev, vq);
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event_notifier_set(notifier);
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}
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void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
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{
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WITH_RCU_READ_LOCK_GUARD() {
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@ -2471,7 +2482,7 @@ void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
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* to an atomic operation.
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*/
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virtio_set_isr(vq->vdev, 0x1);
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event_notifier_set(&vq->guest_notifier);
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defer_call(virtio_notify_irqfd_deferred_fn, &vq->guest_notifier);
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}
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static void virtio_irq(VirtQueue *vq)
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@ -15,6 +15,7 @@
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* GNU GPL, version 2 or (at your option) any later version.
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*/
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#include "qemu/osdep.h"
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#include "qemu/defer-call.h"
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#include "qemu/queue.h"
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#include "qemu/thread.h"
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#include "qemu/coroutine.h"
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@ -175,6 +176,8 @@ static void thread_pool_completion_bh(void *opaque)
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ThreadPool *pool = opaque;
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ThreadPoolElement *elem, *next;
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defer_call_begin(); /* cb() may use defer_call() to coalesce work */
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restart:
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QLIST_FOREACH_SAFE(elem, &pool->head, all, next) {
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if (elem->state != THREAD_DONE) {
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@ -208,6 +211,8 @@ restart:
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qemu_aio_unref(elem);
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}
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}
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defer_call_end();
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}
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static void thread_pool_cancel(BlockAIOCB *acb)
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