netdev - Re: [PATCH net-next V3 3/3] tun: rx batching

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Date:   Tue, 3 Jan 2017 11:12:22 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     David Miller <davem@...emloft.net>
Cc:     mst@...hat.com, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        wexu@...hat.com
Subject: Re: [PATCH net-next V3 3/3] tun: rx batching



On 2017年01月01日 01:31, David Miller wrote:
> From: Jason Wang <jasowang@...hat.com>
> Date: Fri, 30 Dec 2016 13:20:51 +0800
>
>> @@ -1283,10 +1314,15 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>>   	skb_probe_transport_header(skb, 0);
>>   
>>   	rxhash = skb_get_hash(skb);
>> +
>>   #ifndef CONFIG_4KSTACKS
>> -	local_bh_disable();
>> -	netif_receive_skb(skb);
>> -	local_bh_enable();
>> +	if (!rx_batched) {
>> +		local_bh_disable();
>> +		netif_receive_skb(skb);
>> +		local_bh_enable();
>> +	} else {
>> +		tun_rx_batched(tfile, skb, more);
>> +	}
>>   #else
>>   	netif_rx_ni(skb);
>>   #endif
> If rx_batched has been set, and we are talking to clients not using
> this new MSG_MORE facility (or such clients don't have multiple TX
> packets to send to you, thus MSG_MORE is often clear), you are doing a
> lot more work per-packet than the existing code.
>
> You take the queue lock, you test state, you splice into a local queue
> on the stack, then you walk that local stack queue to submit just one
> SKB to netif_receive_skb().
>
> I think you want to streamline this sequence in such cases so that the
> cost before and after is similar if not equivalent.

Yes, so I will do a skb_queue_empty() check if !MSG_MORE and call 
netif_receive_skb() immediately in this case. This can save the wasted 
efforts.

Thanks