netdev - Re: [PATCH net-next] xen-netfront: convert to GRO API and advertise this feature

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Message-ID: <A3521C70-77D5-4219-8C8B-6137E7931DE5@juniper.net>
Date:	Mon, 23 Sep 2013 20:27:26 +0000
From:	Anirban Chakraborty <abchak@...iper.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Wei Liu <wei.liu2@...rix.com>,
	"<netdev@...r.kernel.org>" <netdev@...r.kernel.org>,
	"<xen-devel@...ts.xen.org>" <xen-devel@...ts.xen.org>,
	Ian Campbell <ian.campbell@...rix.com>
Subject: Re: [PATCH net-next] xen-netfront: convert to GRO API and advertise
 this feature


On Sep 22, 2013, at 10:58 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:

> On Sun, 2013-09-22 at 23:09 +0000, Anirban Chakraborty wrote:
>> On Sep 22, 2013, at 7:55 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> 
>>> On Sat, 2013-09-21 at 17:05 +0100, Wei Liu wrote:
>>>> Anirban was seeing netfront received MTU size packets, which downgraded
>>>> throughput. The following patch makes netfront use GRO API which
>>>> improves throughput for that case.
>>> 
>>>> -	netdev->hw_features	= NETIF_F_IP_CSUM | NETIF_F_SG | NETIF_F_TSO;
>>>> +	netdev->hw_features	= NETIF_F_IP_CSUM | NETIF_F_SG | NETIF_F_TSO |
>>>> +				  NETIF_F_GRO;
>>> 
>>> 
>>> This part is not needed.
>> 
>> Shouldn't the flag be set? In dev_gro_receive() we do check if this flag is set or not:
>> 
>>        if (!(skb->dev->features & NETIF_F_GRO) || netpoll_rx_on(skb))
>>               goto normal;
> 
> Drivers do not set NETIF_F_GRO themselves, they do not need to.
> 
> Look at other drivers which are GRO ready : NETIF_F_GRO is enabled by
> default by core networking stack, in register_netdevice()
> 
> 
> dev->hw_features |= NETIF_F_SOFT_FEATURES;
> dev->features |= NETIF_F_SOFT_FEATURES;

I didn't realize that the drivers no longer need to set the GRO flag explicitly. It looks like it has been changed since 3.2. I was looking at the kernel version 2.6.32.43 (which corresponds to the dom0 kernel) where the problem is happening. 

-Anirban
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