From 38738f7dbbda90fbc161757b7f4be35b52205552 Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Wed, 27 Apr 2022 15:35:37 +0100 Subject: virtio-scsi: don't waste CPU polling the event virtqueue The virtio-scsi event virtqueue is not emptied by its handler function. This is typical for rx virtqueues where the device uses buffers when some event occurs (e.g. a packet is received, an error condition happens, etc). Polling non-empty virtqueues wastes CPU cycles. We are not waiting for new buffers to become available, we are waiting for an event to occur, so it's a misuse of CPU resources to poll for buffers. Introduce the new virtio_queue_aio_attach_host_notifier_no_poll() API, which is identical to virtio_queue_aio_attach_host_notifier() except that it does not poll the virtqueue. Before this patch the following command-line consumed 100% CPU in the IOThread polling and calling virtio_scsi_handle_event(): $ qemu-system-x86_64 -M accel=kvm -m 1G -cpu host \ --object iothread,id=iothread0 \ --device virtio-scsi-pci,iothread=iothread0 \ --blockdev file,filename=test.img,aio=native,cache.direct=on,node-name=drive0 \ --device scsi-hd,drive=drive0 After this patch CPU is no longer wasted. Reported-by: Nir Soffer Signed-off-by: Stefan Hajnoczi Tested-by: Nir Soffer Message-id: 20220427143541.119567-3-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi --- hw/virtio/virtio.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) (limited to 'hw/virtio/virtio.c') diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c index 9d637e043e..67a873f54a 100644 --- a/hw/virtio/virtio.c +++ b/hw/virtio/virtio.c @@ -3534,6 +3534,19 @@ void virtio_queue_aio_attach_host_notifier(VirtQueue *vq, AioContext *ctx) virtio_queue_host_notifier_aio_poll_end); } +/* + * Same as virtio_queue_aio_attach_host_notifier() but without polling. Use + * this for rx virtqueues and similar cases where the virtqueue handler + * function does not pop all elements. When the virtqueue is left non-empty + * polling consumes CPU cycles and should not be used. + */ +void virtio_queue_aio_attach_host_notifier_no_poll(VirtQueue *vq, AioContext *ctx) +{ + aio_set_event_notifier(ctx, &vq->host_notifier, true, + virtio_queue_host_notifier_read, + NULL, NULL); +} + void virtio_queue_aio_detach_host_notifier(VirtQueue *vq, AioContext *ctx) { aio_set_event_notifier(ctx, &vq->host_notifier, true, NULL, NULL, NULL); -- cgit v1.2.3