Subject: Re: [PATCH] macvtap: Fix macvtap_get_queue to use rxhash first

From: jasowang <[email-masked]>
Date: Thu, 24 Nov 2011 20:56:45 +0800

On 11/24/2011 06:34 PM, Michael S. Tsirkin wrote:
> On Thu, Nov 24, 2011 at 06:13:41PM +0800, jasowang wrote:
>> On 11/24/2011 05:59 PM, Michael S. Tsirkin wrote:
>>> On Thu, Nov 24, 2011 at 01:47:14PM +0530, Krishna Kumar wrote:
>>>> It was reported that the macvtap device selects a
>>>> different vhost (when used with multiqueue feature)
>>>> for incoming packets of a single connection. Use
>>>> packet hash first. Patch tested on MQ virtio_net.
>>> So this is sure to address the problem, why exactly does this happen?
>> Ixgbe has flow director and bind queue to host cpu, so it can make
>> sure the packet of a flow to be handled by the same queue/cpu. So
>> when vhost thread moves from one host cpu to another, ixgbe would
>> therefore send the packet to the new cpu/queue.
> Confused. How does ixgbe know about vhost thread moving?

As far as I can see, ixgbe binds queues to physical cpu, so let consider:

vhost thread transmits packets of flow A on processor M
during packet transmission, ixgbe driver programs the card to deliver 
the packet of flow A to queue/cpu M through flow director (see ixgbe_atr())
vhost thread then receives packet of flow A with from M
...
vhost thread transmits packets of flow A on processor N
ixgbe driver programs the flow director to change the delivery of flow A 
to queue N ( cpu N )
vhost thread then receives packet of flow A with from N
...

So, for a single flow A, we may get different queue mappings. Using 
rxhash instead may solve this issue.

>
>>> Does your device spread a single flow across multiple RX queues? Would
>>> not that cause trouble in the TCP layer?
>>> It would seem that using the recorded queue should be faster with
>>> less cache misses. Before we give up on that, I'd
>>> like to understand why it's wrong. Do you know?
>>>
>>>> Signed-off-by: Krishna Kumar<[email-masked]>
>>>> ---
>>>>   drivers/net/macvtap.c |   16 ++++++++--------
>>>>   1 file changed, 8 insertions(+), 8 deletions(-)
>>>>
>>>> diff -ruNp org/drivers/net/macvtap.c new/drivers/net/macvtap.c
>>>> --- org/drivers/net/macvtap.c	2011-10-22 08:38:01.000000000 +0530
>>>> +++ new/drivers/net/macvtap.c	2011-11-16 18:34:51.000000000 +0530
>>>> @@ -175,6 +175,14 @@ static struct macvtap_queue *macvtap_get
>>>>   	if (!numvtaps)
>>>>   		goto out;
>>>>
>>>> +	/* Check if we can use flow to select a queue */
>>>> +	rxq = skb_get_rxhash(skb);
>>>> +	if (rxq) {
>>>> +		tap = rcu_dereference(vlan->taps[rxq % numvtaps]);
>>>> +		if (tap)
>>>> +			goto out;
>>>> +	}
>>>> +
>>>>   	if (likely(skb_rx_queue_recorded(skb))) {
>>>>   		rxq = skb_get_rx_queue(skb);
>>>>
>>>> @@ -186,14 +194,6 @@ static struct macvtap_queue *macvtap_get
>>>>   			goto out;
>>>>   	}
>>>>
>>>> -	/* Check if we can use flow to select a queue */
>>>> -	rxq = skb_get_rxhash(skb);
>>>> -	if (rxq) {
>>>> -		tap = rcu_dereference(vlan->taps[rxq % numvtaps]);
>>>> -		if (tap)
>>>> -			goto out;
>>>> -	}
>>>> -
>>>>   	/* Everything failed - find first available queue */
>>>>   	for (rxq = 0; rxq<   MAX_MACVTAP_QUEUES; rxq++) {
>>>>   		tap = rcu_dereference(vlan->taps[rxq]);
>>> --
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