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

From: jasowang <[email-masked]>
Date: Thu, 24 Nov 2011 18:13:41 +0800

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.
> 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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