netdev - Re: Tun congestion/BQL

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Date:   Wed, 10 Apr 2019 21:25:49 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>, netdev@...r.kernel.org
Subject: Re: Tun congestion/BQL


On 2019/4/10 下午9:01, David Woodhouse wrote:
> On Wed, 2019-04-10 at 15:01 +0300, David Woodhouse wrote:
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1125,7 +1128,9 @@ static netdev_tx_t tun_net_xmit(struct sk_buff
>> *skb, struct net_device *dev)
>>          if (tfile->flags & TUN_FASYNC)
>>                  kill_fasync(&tfile->fasync, SIGIO, POLL_IN);
>>          tfile->socket.sk->sk_data_ready(tfile->socket.sk);
>>   
>> +       if (!ptr_ring_empty(&tfile->tx_ring))
>> +               netif_stop_queue(tun->dev);
>>          rcu_read_unlock();
>>          return NETDEV_TX_OK;
>>   
>>
> Hm, that should be using ptr_ring_full() shouldn't it? So...
>
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1121,6 +1121,9 @@ static netdev_tx_t tun_net_xmit(struct s
>   	if (ptr_ring_produce(&tfile->tx_ring, skb))
>   		goto drop;
>   
> +	if (ptr_ring_full(&tfile->tx_ring))
> +		netif_stop_queue(tun->dev);
> +
>   	/* Notify and wake up reader process */
>   	if (tfile->flags & TUN_FASYNC)
>   		kill_fasync(&tfile->fasync, SIGIO, POLL_IN);
> @@ -2229,6 +2232,7 @@ static ssize_t tun_do_read(struct tun_st
>   			consume_skb(skb);
>   	}
>   
> +	netif_wake_queue(tun->dev);
>   	return ret;
>   }
>   
>
> That doesn't seem to make much difference at all; it's still dropping a
> lot of packets because ptr_ring_produce() is returning non-zero.


I think you need try to stop the queue just in this case? Ideally we may 
want to stop the queue when the queue is about to full, but we don't 
have such helper currently.

Thanks


>
>
> Socket  Message  Elapsed      Messages
> Size    Size     Time         Okay Errors   Throughput
> bytes   bytes    secs            #      #   10^6bits/sec
>
> 212992    1400   10.00     7747169      0    8676.81
> 212992           10.00     1471769           1648.38
>
> Making it call netif_stop_queue() when ptr_ring_produce() fails causes
> it to perform even worse...
>
> Socket  Message  Elapsed      Messages
> Size    Size     Time         Okay Errors   Throughput
> bytes   bytes    secs            #      #   10^6bits/sec
>
> 212992    1400   10.00     1906235      0    2134.98
> 212992           10.00      985428           1103.68
>
>
> At this point I'm mostly just confused.