netdev - Re: [PATCH] macvtap: Fix macvtap_get

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Date:	Thu, 24 Nov 2011 12:34:49 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	jasowang <jasowang@...hat.com>
Cc:	Krishna Kumar <krkumar2@...ibm.com>, arnd@...db.de,
	netdev@...r.kernel.org, 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 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?

> >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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