netdev - Re: [PATCH] macvtap: Fix macvtap_get

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Date:	Thu, 24 Nov 2011 11:59:03 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Krishna Kumar <krkumar2@...ibm.com>
Cc:	arnd@...db.de, netdev@...r.kernel.org, jasowang@...hat.com,
	virtualization@...ts.linux-foundation.org, levinsasha928@...il.com,
	davem@...emloft.net
Subject: Re: [PATCH] macvtap: Fix macvtap_get_queue to use rxhash first

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?
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 <krkumar2@...ibm.com>
> ---
>  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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