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

  On 09/06/2017 09:30 AM, Tiwei Bie wrote:
> On Wed, Sep 06, 2017 at 09:15:47AM +0200, Maxime Coquelin wrote:
>> Hi Tiwei,
>>
>> On 09/06/2017 03:15 AM, Tiwei Bie wrote:
>>> On Tue, Sep 05, 2017 at 01:00:42PM +0200, Maxime Coquelin wrote:
>>>> On 09/05/2017 12:07 PM, Tiwei Bie wrote:
>>>>> 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.
>>>>
>>>> I'll try to post v2 this week including the proposed change.
>>>> Maybe it'll be better Lei waits for the v2.
>>>>
>>>
>>> Cool. Sure. Thank you! :)
>>
>> I have done the changes, you can find the v2 on my gitlab repo:
>> https://gitlab.com/mcoquelin/dpdk-next-virtio/commits/vhost_iotlb_v2
>>
>> I'm testing it right now, but if you'd like to run some early benchmark
>> before I post the series, there it is!
>>
> 
> Got it. Thanks! :)

Just to let you know that I have updated my branch to remove another
regression with iommu=off by inlining the noiommu part of
vhost_iova_to_vva call (See below for the patch, that is squashed into
my branch).

Without this, when running microbenchmarks (txonly, rxonly, ...) I
noticed a 4% perf degradation.

I think I'll have to post the series without testing PVP, because I had
to change the machine I use as packet generator, and now I have X710
NICs that seems to be unsupported with Moongen :(.

I have been advised to us TRex instead, but I'll need some time to set
it up...

Regards,
Maxime

ps: Are you coming to Dublin?

> Best regards,
> Tiwei Bie
> 
Subject: [PATCH] vhost: inline IOMMU feature check

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
  lib/librte_vhost/vhost.c |  5 +----
  lib/librte_vhost/vhost.h | 12 +++++++++++-
  2 files changed, 12 insertions(+), 5 deletions(-)

  int vring_translate(struct virtio_net *dev, struct vhost_virtqueue *vq);
  void vring_invalidate(struct virtio_net *dev, struct vhost_virtqueue *vq);

Context	Check	Description
ci/checkpatch	warning	coding style issues
ci/Intel-compilation	success	Compilation OK

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

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