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

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Date:	Wed, 06 May 2015 11:08:53 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"K. Y. Srinivasan" <kys@...rosoft.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
	olaf@...fle.de, apw@...onical.com, jasowang@...hat.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:
> 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
> make the host more efficient since it can potentially process a larger batch of
> packets. Implement this optimization.


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



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