Re: [dpdk-dev] [PATCH 03/21] vhost: protect virtio_net device struct

[Tiwei Bie <tiwei.bie@intel.com>]

On Tue, Sep 05, 2017 at 11:24:14AM +0200, Maxime Coquelin wrote:
> On 09/05/2017 06:45 AM, Tiwei Bie wrote:
> > On Thu, Aug 31, 2017 at 11:50:05AM +0200, Maxime Coquelin wrote:
> > > virtio_net device might be accessed while being reallocated
> > > in case of NUMA awareness. This case might be theoretical,
> > > but it will be needed anyway to protect vrings pages against
> > > invalidation.
> > > 
> > > The virtio_net devs are now protected with a readers/writers
> > > lock, so that before reallocating the device, it is ensured
> > > that it is not being referenced by the processing threads.
> > > 
> > [...]
> > > +struct virtio_net *
> > > +get_device(int vid)
> > > +{
> > > +	struct virtio_net *dev;
> > > +
> > > +	rte_rwlock_read_lock(&vhost_devices[vid].lock);
> > > +
> > > +	dev = __get_device(vid);
> > > +	if (unlikely(!dev))
> > > +		rte_rwlock_read_unlock(&vhost_devices[vid].lock);
> > > +
> > > +	return dev;
> > > +}
> > > +
> > > +void
> > > +put_device(int vid)
> > > +{
> > > +	rte_rwlock_read_unlock(&vhost_devices[vid].lock);
> > > +}
> > > +
> > 
> > This patch introduced a per-device rwlock which needs to be acquired
> > unconditionally in the data path. So for each vhost device, the IO
> > threads of different queues will need to acquire/release this lock
> > during each enqueue and dequeue operation, which will cause cache
> > contention when multiple queues are enabled and handled by different
> > cores. With this patch alone, I saw ~7% performance drop when enabling
> > 6 queues to do 64bytes iofwd loopback test. Is there any way to avoid
> > introducing this lock to the data path?
> 
> First, I'd like to thank you for running the MQ test.
> I agree it may have a performance impact in this case.
> 
> This lock has currently two purposes:
> 1. Prevent referencing freed virtio_dev struct in case of numa_realloc.
> 2. Protect vring pages against invalidation.
> 
> For 2., it can be fixed by using the per-vq IOTLB lock (it was not the
> case in my early prototypes that had per device IOTLB cache).
> 
> For 1., this is an existing problem, so we might consider it is
> acceptable to keep current state. Maybe it could be improved by only
> reallocating in case VQ0 is not on the right NUMA node, the other VQs
> not being initialized at this point.
> 
> If we do this we might be able to get rid of this lock, I need some more
> time though to ensure I'm not missing something.
> 
> What do you think?
> 

Cool. So it's possible that the lock in the data path will be
acquired only when the IOMMU feature is enabled. It will be
great!

Besides, I just did a very simple MQ test to verify my thoughts.
Lei (CC'ed in this mail) may do a thorough performance test for
this patch set to evaluate the performance impacts.

Best regards,
Tiwei Bie