RE: [PATCH V3 net-next 1/1] hv_netvsc: Use the xmit_more skb flag to optimize signaling the host

-----Original Message-----
From: Eric Dumazet [mailto:eric.dumazet@gmail.com]
Sent: Wednesday, May 6, 2015 11:09 AM
To: KY Srinivasan
Cc: davem@davemloft.net; netdev@vger.kernel.org; linux-
kernel@vger.kernel.org; devel@linuxdriverproject.org; olaf@aepfle.de;
apw@canonical.com; jasowang@redhat.com
Subject: Re: [PATCH V3 net-next 1/1] hv_netvsc: Use the xmit_more skb flag
to optimize signaling the host

On Wed, 2015-05-06 at 11:20 -0700, K. Y. Srinivasan wrote:

quoted

Based on the information given to this driver (via the xmit_more skb flag),
we can defer signaling the host if more packets are on the way. This will

help

quoted

make the host more efficient since it can potentially process a larger batch

of

quoted

packets. Implement this optimization.

quoted

diff --git a/drivers/net/hyperv/netvsc_drv.c

b/drivers/net/hyperv/netvsc_drv.c

quoted

index 5993c7e..4efaa6e 100644

--- a/drivers/net/hyperv/netvsc_drv.c
+++ b/drivers/net/hyperv/netvsc_drv.c

@@ -435,6 +435,8 @@ check_size:
 	packet->page_buf = page_buf;

 	packet->q_idx = skb_get_queue_mapping(skb);
+	if (netif_tx_queue_stopped(netdev_get_tx_queue(net, packet-

q_idx)))
+		packet->xmit_more = false;

Have you tested this condition ever triggers ?

It should not.

netvsc_start_xmit() should not be called if the queue is stopped.

The problem is the following :

netvsc_start_xmit() is called with the packet that is going to stop the
queue, filling last slot, but with skb->xmit_more = 1;

So you need to do something about xmit_more, after calling
netif_tx_stop_queue(), not before.

Ah! I too was wondering how we could get into this situation. The condition you mention
is already handled in the lower level - if the attempt to put the last packet on vmbus were to
fail because the ring is full, we will notify the host independent of kick_q parameter - look
at the function vmbus_sendpacket_ctl() in drivers/hv/channel.c

I will resend the patch by reverting this to version 2.

Regards,

K. Y