Subject: Re: [PATCH -next] tun: stop tx queue when limit is hit

From: Florian Westphal <[email-masked]>
Date: Mon, 21 Jul 2014 10:31:06 +0200

Florian Westphal <[email-masked]> wrote:
> David Miller <[email-masked]> wrote:
> > From: Florian Westphal <[email-masked]>
> > Date: Sun, 20 Jul 2014 20:51:25 +0200
> > 
> > > Currently tun just frees the skb and returns NETDEV_TX_OK
> > > when queue length exceeds txqlen.
> > > 
> > > This causes severe packetloss and unneeded resource
> > > consumption on host when sending to vm connected via tun.
> > > 
> > > Instead, lets stop the transmit queue and start it once
> > > packets are consumed from the queue.  This allows the network
> > > stack to control applications that send data via tun device.
> > 
> > I strongly suspect the current behavior is intentional, see
> > commit:
> > 
> > commit 5d097109257c03a71845729f8db6b5770c4bbedc
> > Author: Michael S. Tsirkin <[email-masked]>
> > Date:   Mon Dec 3 10:07:14 2012 +0000
> > 
> >     tun: only queue packets on device
> 
> Looks like you're right :-/
> 
> Alright, please ignore my patch.
> 
> That being said, the current behaviour isn't ideal either.
> 
> It took me quite some time to realize that packetloss
> was on the sender side inside tun driver and not on the receiver
> vm.  Not stopping the queue was a bit ... unexpected.

Michael, do you think we could restore the 'stop queue' default
behaviour?

Looking at your changelog, the only concern seems to be the
'packets never consumed'/'receiver is stuck forever' case.

What about reverting 5d097109257c03a7184, and then adding some
type of tun watchdog that will zap the rcv queue + tx queue wakeup?

That should be quite noisy if we combine it with the WARN_ON
you usually get when (physical) NIC driver detect stalled tx unit.

What do you think?