95 lines
4.1 KiB
Diff
95 lines
4.1 KiB
Diff
From 5a595325d84fe48fcef921b0810073906ff9284b Mon Sep 17 00:00:00 2001
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From: Stefan Hajnoczi <stefanha@redhat.com>
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Date: Tue, 17 May 2022 09:27:45 +0100
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Subject: [PATCH] virtio-scsi: don't waste CPU polling the event virtqueue
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The virtio-scsi event virtqueue is not emptied by its handler function.
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This is typical for rx virtqueues where the device uses buffers when
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some event occurs (e.g. a packet is received, an error condition
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happens, etc).
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Polling non-empty virtqueues wastes CPU cycles. We are not waiting for
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new buffers to become available, we are waiting for an event to occur,
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so it's a misuse of CPU resources to poll for buffers.
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Introduce the new virtio_queue_aio_attach_host_notifier_no_poll() API,
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which is identical to virtio_queue_aio_attach_host_notifier() except
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that it does not poll the virtqueue.
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Before this patch the following command-line consumed 100% CPU in the
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IOThread polling and calling virtio_scsi_handle_event():
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$ qemu-system-x86_64 -M accel=kvm -m 1G -cpu host \
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--object iothread,id=iothread0 \
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--device virtio-scsi-pci,iothread=iothread0 \
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--blockdev file,filename=test.img,aio=native,cache.direct=on,node-name=drive0 \
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--device scsi-hd,drive=drive0
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After this patch CPU is no longer wasted.
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Reported-by: Nir Soffer <nsoffer@redhat.com>
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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Tested-by: Nir Soffer <nsoffer@redhat.com>
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Message-id: 20220427143541.119567-3-stefanha@redhat.com
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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(cherry picked from commit 38738f7dbbda90fbc161757b7f4be35b52205552)
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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---
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hw/scsi/virtio-scsi-dataplane.c | 2 +-
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hw/virtio/virtio.c | 13 +++++++++++++
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include/hw/virtio/virtio.h | 1 +
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3 files changed, 15 insertions(+), 1 deletion(-)
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diff --git a/hw/scsi/virtio-scsi-dataplane.c b/hw/scsi/virtio-scsi-dataplane.c
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index 29575cbaf6..8bb6e6acfc 100644
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--- a/hw/scsi/virtio-scsi-dataplane.c
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+++ b/hw/scsi/virtio-scsi-dataplane.c
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@@ -138,7 +138,7 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
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aio_context_acquire(s->ctx);
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virtio_queue_aio_attach_host_notifier(vs->ctrl_vq, s->ctx);
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- virtio_queue_aio_attach_host_notifier(vs->event_vq, s->ctx);
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+ virtio_queue_aio_attach_host_notifier_no_poll(vs->event_vq, s->ctx);
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for (i = 0; i < vs->conf.num_queues; i++) {
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virtio_queue_aio_attach_host_notifier(vs->cmd_vqs[i], s->ctx);
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diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
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index 9d637e043e..67a873f54a 100644
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--- a/hw/virtio/virtio.c
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+++ b/hw/virtio/virtio.c
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@@ -3534,6 +3534,19 @@ void virtio_queue_aio_attach_host_notifier(VirtQueue *vq, AioContext *ctx)
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virtio_queue_host_notifier_aio_poll_end);
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}
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+/*
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+ * Same as virtio_queue_aio_attach_host_notifier() but without polling. Use
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+ * this for rx virtqueues and similar cases where the virtqueue handler
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+ * function does not pop all elements. When the virtqueue is left non-empty
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+ * polling consumes CPU cycles and should not be used.
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+ */
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+void virtio_queue_aio_attach_host_notifier_no_poll(VirtQueue *vq, AioContext *ctx)
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+{
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+ aio_set_event_notifier(ctx, &vq->host_notifier, true,
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+ virtio_queue_host_notifier_read,
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+ NULL, NULL);
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+}
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+
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void virtio_queue_aio_detach_host_notifier(VirtQueue *vq, AioContext *ctx)
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{
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aio_set_event_notifier(ctx, &vq->host_notifier, true, NULL, NULL, NULL);
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diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
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index b31c4507f5..b62a35fdca 100644
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--- a/include/hw/virtio/virtio.h
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+++ b/include/hw/virtio/virtio.h
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@@ -317,6 +317,7 @@ EventNotifier *virtio_queue_get_host_notifier(VirtQueue *vq);
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void virtio_queue_set_host_notifier_enabled(VirtQueue *vq, bool enabled);
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void virtio_queue_host_notifier_read(EventNotifier *n);
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void virtio_queue_aio_attach_host_notifier(VirtQueue *vq, AioContext *ctx);
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+void virtio_queue_aio_attach_host_notifier_no_poll(VirtQueue *vq, AioContext *ctx);
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void virtio_queue_aio_detach_host_notifier(VirtQueue *vq, AioContext *ctx);
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VirtQueue *virtio_vector_first_queue(VirtIODevice *vdev, uint16_t vector);
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VirtQueue *virtio_vector_next_queue(VirtQueue *vq);
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--
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2.37.0.rc2
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