Re: [PATCH] vhost: fix virtio_net cache sharing of broadcast_rarp

[Kevin Traynor <ktraynor@redhat.com>]

On 03/17/2017 10:01 AM, Maxime Coquelin wrote:
> 
> 
> On 03/17/2017 06:47 AM, Yuanhan Liu wrote:
>> On Thu, Mar 16, 2017 at 10:10:05AM +0000, Kevin Traynor wrote:
>>> On 03/16/2017 06:21 AM, Yuanhan Liu wrote:
>>>> On Wed, Mar 15, 2017 at 07:10:49PM +0000, Kevin Traynor wrote:
>>>>> The virtio_net structure is used in both enqueue and dequeue
>>>>> datapaths.
>>>>> broadcast_rarp is checked with cmpset in the dequeue datapath
>>>>> regardless
>>>>> of whether descriptors are available or not.
>>>>>
>>>>> It is observed in some cases where dequeue and enqueue are
>>>>> performed by
>>>>> different cores and no packets are available on the dequeue datapath
>>>>> (i.e. uni-directional traffic), the frequent checking of
>>>>> broadcast_rarp
>>>>> in dequeue causes performance degradation for the enqueue datapath.
>>>>>
>>>>> In OVS the issue can cause a uni-directional performance drop of up
>>>>> to 15%.
>>>>>
>>>>> Fix that by moving broadcast_rarp to a different cache line in
>>>>> virtio_net struct.
>>>>
>>>> Thanks, but I'm a bit confused. The drop looks like being caused by
>>>> cache false sharing, but I don't see anything would lead to a false
>>>> sharing. I mean, there is no write in the same cache line where the
>>>> broadcast_rarp belongs. Or, the "volatile" type is the culprit here?
>>>>
>>>
>>> Yes, the cmpset code uses cmpxchg and that performs a write regardless
>>> of the result - it either writes the new value or back the old value.
>>
>> Oh, right, I missed this part!
>>
>>>> Talking about that, I had actually considered to turn "broadcast_rarp"
>>>> to a simple "int" or "uint16_t" type, to make it more light weight.
>>>> The reason I used atomic type is to exactly send one broadcast RARP
>>>> packet once SEND_RARP request is recieved. Otherwise, we may send more
>>>> than one RARP packet when MQ is invovled. But I think we don't have
>>>> to be that accurate: it's tolerable when more RARP are sent. I saw 4
>>>> SEND_RARP requests (aka 4 RARP packets) in the last time I tried
>>>> vhost-user live migration after all. I don't quite remember why
>>>> it was 4 though.
>>>>
>>>> That said, I think it also would resolve the performance issue if you
>>>> change "rte_atomic16_t" to "uint16_t", without moving the place?
>>>>
>>>
>>> Yes, that should work fine, with the side effect you mentioned of
>>> possibly some more rarps - no big deal.
>>>
>>> I tested another solution also - as it is unlikely we would need to send
>>> the broadcast_rarp, you can first read and only do the cmpset if it is
>>> likely to succeed. This resolved the issue too.
>>>
>>> --- a/lib/librte_vhost/virtio_net.c
>>> +++ b/lib/librte_vhost/virtio_net.c
>>> @@ -1057,7 +1057,8 @@ static inline bool __attribute__((always_inline))
>>>          *
>>>          * Check user_send_rarp() for more information.
>>>          */
>>> -       if (unlikely(rte_atomic16_cmpset((volatile uint16_t *)
>>> +       if (unlikely(rte_atomic16_read(&dev->broadcast_rarp) &&
>>> +                       rte_atomic16_cmpset((volatile uint16_t *)
>>>                                          &dev->broadcast_rarp.cnt, 1,
>>> 0))) {
>>>                 rarp_mbuf = rte_pktmbuf_alloc(mbuf_pool);
>>>                 if (rarp_mbuf == NULL) {
>>
>> I'm okay with this one. It's simple and clean enough, that it could
>> be picked to a stable release. Later, I'd like to send another patch
>> to turn it to "uint16_t". Since it changes the behaviour a bit, it
>> is not a good candidate for stable release.
>>
>> BTW, would you please include the root cause (false sharing) into
>> your commit log?
> And maybe also adds the info to the comment just above?
> I will help people wondering why we read before cmpset.
> 

Sure, I will re-spin, do some testing and submit a v2.

> Maxime