Linux Virtualization - Jul 2017 - [PATCH net] Revert "vhost: cache used event for better performance"

On 2017?07?26? 20:57, Michael S. Tsirkin wrote:
> On Wed, Jul 26, 2017 at 04:03:17PM +0800, Jason Wang wrote:
>> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it
>> was reported to break vhost_net. We want to cache used event and use
>> it to check for notification. We try to valid cached used event by
>> checking whether or not it was ahead of new, but this is not correct
>> all the time, it could be stale and there's no way to know about
this.
>>
>> Signed-off-by: Jason Wang<jasowang at redhat.com>
> Could you supply a bit more data here please?  How does it get stale?
> What does guest need to do to make it stale?  This will be helpful if
> anyone wants to bring it back, or if we want to extend the protocol.
>
The problem we don't know whether or not guest has published a new used 
event. The check vring_need_event(vq->last_used_event, new + vq->num, 
new) is not sufficient to check for this.

Thanks

On 2017?07?26? 21:18, Jason Wang wrote:
>
>
> On 2017?07?26? 20:57, Michael S. Tsirkin wrote:
>> On Wed, Jul 26, 2017 at 04:03:17PM +0800, Jason Wang wrote:
>>> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since
it
>>> was reported to break vhost_net. We want to cache used event and
use
>>> it to check for notification. We try to valid cached used event by
>>> checking whether or not it was ahead of new, but this is not
correct
>>> all the time, it could be stale and there's no way to know
about this.
>>>
>>> Signed-off-by: Jason Wang<jasowang at redhat.com>
>> Could you supply a bit more data here please?  How does it get stale?
>> What does guest need to do to make it stale?  This will be helpful if
>> anyone wants to bring it back, or if we want to extend the protocol.
>>
>
> The problem we don't know whether or not guest has published a new 
> used event. The check vring_need_event(vq->last_used_event, new + 
> vq->num, new) is not sufficient to check for this.
>
> Thanks
More notes, the previous assumption is that we don't move used event 
back, but this could happen in fact if idx is wrapper around. Will 
repost and add this into commit log.

Thanks

On Wed, Jul 26, 2017 at 09:18:02PM +0800, Jason Wang
wrote:
> 
> 
> On 2017?07?26? 20:57, Michael S. Tsirkin wrote:
> > On Wed, Jul 26, 2017 at 04:03:17PM +0800, Jason Wang wrote:
> > > This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92.
Since it
> > > was reported to break vhost_net. We want to cache used event and
use
> > > it to check for notification. We try to valid cached used event
by
> > > checking whether or not it was ahead of new, but this is not
correct
> > > all the time, it could be stale and there's no way to know
about this.
> > > 
> > > Signed-off-by: Jason Wang<jasowang at redhat.com>
> > Could you supply a bit more data here please?  How does it get stale?
> > What does guest need to do to make it stale?  This will be helpful if
> > anyone wants to bring it back, or if we want to extend the protocol.
> > 
> 
> The problem we don't know whether or not guest has published a new used
> event. The check vring_need_event(vq->last_used_event, new + vq->num,
new)
> is not sufficient to check for this.
> 
> Thanks
Figured it out after an offline discussion.

On Wed, Jul 26, 2017 at 09:37:15PM +0800, Jason Wang
wrote:
> 
> 
> On 2017?07?26? 21:18, Jason Wang wrote:
> > 
> > 
> > On 2017?07?26? 20:57, Michael S. Tsirkin wrote:
> > > On Wed, Jul 26, 2017 at 04:03:17PM +0800, Jason Wang wrote:
> > > > This reverts commit
809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it
> > > > was reported to break vhost_net. We want to cache used event
and use
> > > > it to check for notification. We try to valid cached used
event by
> > > > checking whether or not it was ahead of new, but this is not
correct
> > > > all the time, it could be stale and there's no way to
know about this.
> > > > 
> > > > Signed-off-by: Jason Wang<jasowang at redhat.com>
> > > Could you supply a bit more data here please?  How does it get
stale?
> > > What does guest need to do to make it stale?  This will be
helpful if
> > > anyone wants to bring it back, or if we want to extend the
protocol.
> > > 
> > 
> > The problem we don't know whether or not guest has published a new
used
> > event. The check vring_need_event(vq->last_used_event, new +
vq->num,
> > new) is not sufficient to check for this.
> > 
> > Thanks
> 
> More notes, the previous assumption is that we don't move used event
back,
> but this could happen in fact if idx is wrapper around.
You mean if the 16 bit index wraps around after 64K entries.
Makes sense.
> Will repost and add
> this into commit log.
> 
> Thanks