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

Maxime Coquelin maxime.coquelin at redhat.com
Tue Sep 5 11:24:14 CEST 2017

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?


> Best regards,
> Tiwei Bie

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