Subject: Re: [PATCH] tuntap: add flow control to support back pressure

From: Jason Wang <[email-masked]>
Date: Fri, 11 Apr 2014 10:57:55 +0800

On Thu, 2014-04-10 at 13:29 +0300, Michael S. Tsirkin wrote:
> On Wed, Apr 09, 2014 at 10:19:40PM -0400, Steven Galgano wrote:
> > Add tuntap flow control support for use by back pressure routing protocols. Setting the new TUNSETIFF flag IFF_FLOW_CONTROL, will signal resources as unavailable when the tx queue limit is reached by issuing a netif_tx_stop_all_queues() rather than discarding frames. A netif_tx_wake_all_queues() is issued after reading a frame from the queue to signal resource availability.
> > 
> > Back pressure capability was previously supported by the legacy tun default mode. This change restores that functionality, which was last present in v3.7.
> > 
> > Reported-by: Brian Adamson <[email-masked]>
> > Tested-by: Joseph Giovatto <[email-masked]>
> > Signed-off-by: Steven Galgano <[email-masked]>
> 
> I don't think it's a good idea.
> 
> This trivial flow control really created more problems than it was worth.
> 
> In particular this blocks all flows so it's trivially easy for one flow
> to block and starve all others: just send a bunch of packets to loopback
> destinations that get queued all over the place.
> 
> Luckily it was never documented so we changed the default and nothing
> seems to break, but we won't be so lucky if we add an explicit API.
> 
> 
> One way to implement this would be with ubuf_info callback this is
> already invoked in most places where a packet might get stuck for a long
> time.  It's still incomplete though: this will prevent head of queue
> blocking literally forever, but a single bad flow can still degrade
> performance significantly.

This is send queue for tuntap. Like all other real nics, we can solve
this through fairness qdiscs?
> 
> Another alternative is to try and isolate the flows that we
> can handle and throttle them.
> 
> It's all fixable but we really need to fix the issues *before*
> exposing the interface to userspace.
> 
> 
> 
> > ---
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index ee328ba..268130c 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -783,8 +783,19 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
> >  	 * number of queues.
> >  	 */
> >  	if (skb_queue_len(&tfile->socket.sk->sk_receive_queue) * numqueues
> > -			  >= dev->tx_queue_len)
> > -		goto drop;
> > +			>= dev->tx_queue_len) {
> > +		if (tun->flags & TUN_FLOW_CONTROL) {
> > +			/* Resources unavailable stop transmissions */
> > +			netif_tx_stop_all_queues(dev);
> > +
> > +			/* We won't see all dropped packets individually, so
> > +			 * over run error is more appropriate.
> > +			 */
> > +			dev->stats.tx_fifo_errors++;
> > +		} else {
> > +			goto drop;
> > +		}
> > +	}
> >  
> >  	if (unlikely(skb_orphan_frags(skb, GFP_ATOMIC)))
> >  		goto drop;
> > @@ -1362,6 +1373,9 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
> >  			continue;
> >  		}
> >  
> > +		/* Wake in case resources previously signaled unavailable */
> > +		netif_tx_wake_all_queues(tun->dev);
> > +
> >  		ret = tun_put_user(tun, tfile, skb, iv, len);
> >  		kfree_skb(skb);
> >  		break;
> > @@ -1550,6 +1564,9 @@ static int tun_flags(struct tun_struct *tun)
> >  	if (tun->flags & TUN_PERSIST)
> >  		flags |= IFF_PERSIST;
> >  
> > +	if (tun->flags & TUN_FLOW_CONTROL)
> > +		flags |= IFF_FLOW_CONTROL;
> > +
> >  	return flags;
> >  }
> >  
> > @@ -1732,6 +1749,11 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
> >  	else
> >  		tun->flags &= ~TUN_TAP_MQ;
> >  
> > +	if (ifr->ifr_flags & IFF_FLOW_CONTROL)
> > +		tun->flags |= TUN_FLOW_CONTROL;
> > +	else
> > +		tun->flags &= ~TUN_FLOW_CONTROL;
> > +
> >  	/* Make sure persistent devices do not get stuck in
> >  	 * xoff state.
> >  	 */
> > @@ -1900,7 +1922,8 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
> >  		 * This is needed because we never checked for invalid flags on
> >  		 * TUNSETIFF. */
> >  		return put_user(IFF_TUN | IFF_TAP | IFF_NO_PI | IFF_ONE_QUEUE |
> > -				IFF_VNET_HDR | IFF_MULTI_QUEUE,
> > +				IFF_VNET_HDR | IFF_MULTI_QUEUE |
> > +				IFF_FLOW_CONTROL,
> >  				(unsigned int __user*)argp);
> >  	} else if (cmd == TUNSETQUEUE)
> >  		return tun_set_queue(file, &ifr);
> > diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h
> > index e9502dd..bcf2790 100644
> > --- a/include/uapi/linux/if_tun.h
> > +++ b/include/uapi/linux/if_tun.h
> > @@ -36,6 +36,7 @@
> >  #define TUN_PERSIST 	0x0100	
> >  #define TUN_VNET_HDR 	0x0200
> >  #define TUN_TAP_MQ      0x0400
> > +#define TUN_FLOW_CONTROL 0x0800
> >  
> >  /* Ioctl defines */
> >  #define TUNSETNOCSUM  _IOW('T', 200, int) 
> > @@ -70,6 +71,7 @@
> >  #define IFF_MULTI_QUEUE 0x0100
> >  #define IFF_ATTACH_QUEUE 0x0200
> >  #define IFF_DETACH_QUEUE 0x0400
> > +#define IFF_FLOW_CONTROL 0x0010
> >  /* read-only flag */
> >  #define IFF_PERSIST	0x0800
> >  #define IFF_NOFILTER	0x1000