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

[Kevin Wolf <kwolf@redhat.com>]

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 <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?

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.

Kevin