netdev - Re: tuntap: Overload handling

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Date:	Thu, 14 Feb 2013 08:32:27 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Sebastian Pöhn <sebastian.poehn@...glemail.com>
Cc:	netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: tuntap: Overload handling

On Thu, 2013-02-14 at 12:50 +0100, Sebastian Pöhn wrote:
> I am having a look on the tun driver to realize an userspace network
> driver ( TAP + UIO ). Maybe that's not the use-case tun is intended
> for.
> 
> What I've noticed is that in tun.c Line 741
> 
> static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
> 
> /* Limit the number of packets queued by dividing txq length with the
>  * number of queues.
>  */
> if (skb_queue_len(&tfile->socket.sk->sk_receive_queue)
>  >= dev->tx_queue_len / tun->numqueues)
> goto drop;
> 
> If a frame can not be tx it is dropped by the driver.
> Wouldn't it be more correct to netif_tx_stop_queue() so that packet
> drops are performed by the overlying traffic control code?
> 
> Of course this is not very likely in virtual environments but as soon
> as any real network hop is involved it could be important.
> 
> (I also had a look on some two year old version of tun.c. There
> queue/tx stopping was done correctly.)

You should ask Michael S. Tsirkin, as he removed the flow control
in commit 5d097109257c03a71845729f8db6b5770c4bbedc
(tun: only queue packets on device)



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