netdev - Re: Tun congestion/BQL

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Date:   Wed, 10 Apr 2019 16:01:46 +0300
From:   David Woodhouse <dwmw2@...radead.org>
To:     netdev@...r.kernel.org
Subject: Re: Tun congestion/BQL

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.


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. 

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