On Mon, Jan 24, 2011 at 8:05 PM, Kevin Wolf <kw...@redhat.com> wrote: > Am 24.01.2011 20:47, schrieb Michael S. Tsirkin: >> 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 <stefa...@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? > > I was building with the default options, i.e. there is no IO thread. > > Now I'm just running the test with IO threads enabled, and so far > everything looks good. So I can only reproduce the problem with IO > threads disabled.
Hrm...aio uses SIGUSR2 to force the vcpu to process aio completions (relevant when --enable-io-thread is not used). I will take a look at that again and see why we're spinning without checking for ioeventfd completion. Stefan
