netdev - Re: [PATCH] loopback: better handling of packet drops

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Date:	Fri, 17 Apr 2009 17:05:42 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Stephen Hemminger <shemminger@...tta.com>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] loopback: better handling of packet drops

Stephen Hemminger a écrit :
> On Fri, 17 Apr 2009 12:33:33 +0200
> Eric Dumazet <dada1@...mosbay.com> wrote:
>>  static int loopback_xmit(struct sk_buff *skb, struct net_device *dev)
>>  {
>>  	struct pcpu_lstats *pcpu_lstats, *lb_stats;
>> +	int len;
>>  
>>  	skb_orphan(skb);
>>  
>> -	skb->protocol = eth_type_trans(skb,dev);
>> +	skb->protocol = eth_type_trans(skb, dev);
>>  
>>  	/* it's OK to use per_cpu_ptr() because BHs are off */
>>  	pcpu_lstats = dev->ml_priv;
>>  	lb_stats = per_cpu_ptr(pcpu_lstats, smp_processor_id());
>> -	lb_stats->bytes += skb->len;
>> -	lb_stats->packets++;
>>  
>> -	netif_rx(skb);
>> +	len = skb->len;
>> +	if (likely(__netif_rx(skb) == NET_RX_SUCCESS)) {
>> +		lb_stats->bytes += len;
>> +		lb_stats->packets++;
>> +		return NETDEV_TX_OK;
>> +	}
>> +	lb_stats->drops++;
>>  
>> -	return 0;
>> +	return NETDEV_TX_BUSY;
>>  }
> 
> If you return NETDEV_TX_BUSY, then the xmit logic will retry
> so it is not really a drop but a stall. I think it is confusing
> to call this a packet loss.

Good point, thanks.
So we should not account this stall in dev stats ? Maybe in 'collisions' ?

I also discovered we had to do
skb_push(skb, ETH_HLEN); /* undo the skb_pull() done in eth_type_trans() */
before returning NETDEV_TX_BUSY;


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