Re: [Qemu-devel] [PATCH v5 2/4] virtio-pci: Use ioeventfd for virtqueue notify

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jan 24, 2011 at 08:48:05PM +0100, Kevin Wolf wrote:
> Am 24.01.2011 20:36, schrieb Michael S. Tsirkin:
> > On Mon, Jan 24, 2011 at 07:54:20PM +0100, Kevin Wolf wrote:
> >> Am 12.12.2010 16:02, schrieb Stefan Hajnoczi:
> >>> Virtqueue notify is currently handled synchronously in userspace virtio.  This
> >>> prevents the vcpu from executing guest code while hardware emulation code
> >>> handles the notify.
> >>>
> >>> On systems that support KVM, the ioeventfd mechanism can be used to make
> >>> virtqueue notify a lightweight exit by deferring hardware emulation to the
> >>> iothread and allowing the VM to continue execution.  This model is similar to
> >>> how vhost receives virtqueue notifies.
> >>>
> >>> The result of this change is improved performance for userspace virtio devices.
> >>> Virtio-blk throughput increases especially for multithreaded scenarios and
> >>> virtio-net transmit throughput increases substantially.
> >>>
> >>> Some virtio devices are known to have guest drivers which expect a notify to be
> >>> processed synchronously and spin waiting for completion.  Only enable ioeventfd
> >>> for virtio-blk and virtio-net for now.
> >>>
> >>> Care must be taken not to interfere with vhost-net, which uses host
> >>> notifiers.  If the set_host_notifier() API is used by a device
> >>> virtio-pci will disable virtio-ioeventfd and let the device deal with
> >>> host notifiers as it wishes.
> >>>
> >>> After migration and on VM change state (running/paused) virtio-ioeventfd
> >>> will enable/disable itself.
> >>>
> >>>  * VIRTIO_CONFIG_S_DRIVER_OK -> enable virtio-ioeventfd
> >>>  * !VIRTIO_CONFIG_S_DRIVER_OK -> disable virtio-ioeventfd
> >>>  * virtio_pci_set_host_notifier() -> disable virtio-ioeventfd
> >>>  * vm_change_state(running=0) -> disable virtio-ioeventfd
> >>>  * vm_change_state(running=1) -> enable virtio-ioeventfd
> >>>
> >>> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
> >>
> >> On current git master I'm getting hangs when running iozone on a
> >> virtio-blk disk. "Hang" means that it's not responsive any more and has
> >> 100% CPU consumption.
> >>
> >> I bisected the problem to this patch. Any ideas?
> >>
> >> Kevin
> > 
> > Does it help if you set ioeventfd=off on command line?
> 
> Yes, with ioeventfd=off it seems to work fine.
> 
> Kevin

Then it's the ioeventfd that is to blame.
Is it the io thread that consumes 100% CPU?
Or the vcpu thread?

-- 
MST