Re: Flow Control and Port Mirroring Revisited

[Eric Dumazet <eric.dumazet@gmail.com>]

Le jeudi 06 janvier 2011 à 18:33 +0900, Simon Horman a écrit :
> Hi,
> 
> Back in October I reported that I noticed a problem whereby flow control
> breaks down when openvswitch is configured to mirror a port[1].
> 
> I have (finally) looked into this further and the problem appears to relate
> to cloning of skbs, as Jesse Gross originally suspected.
> 
> More specifically, in do_execute_actions[2] the first n-1 times that an skb
> needs to be transmitted it is cloned first and the final time the original
> skb is used.
> 
> In the case that there is only one action, which is the normal case, then
> the original skb will be used. But in the case of mirroring the cloning
> comes into effect. And in my case the cloned skb seems to go to the (slow)
> eth1 interface while the original skb goes to the (fast) dummy0 interface
> that I set up to be a mirror. The result is that dummy0 "paces" the flow,
> and its a cracking pace at that.
> 
> As an experiment I hacked do_execute_actions() to use the original skb
> for the first action instead of the last one.  In my case the result was
> that eth1 "paces" the flow, and things work reasonably nicely.
> 
> Well, sort of. Things work well for non-GSO skbs but extremely poorly for
> GSO skbs where only 3 (yes 3, not 3%) end up at the remote host running
> netserv. I'm unsure why, but I digress.
> 
> It seems to me that my hack illustrates the point that the flow ends up
> being "paced" by one interface. However I think that what would be
> desirable is that the flow is "paced" by the slowest link. Unfortunately
> I'm unsure how to achieve that.
> 

Hi Simon !

"pacing" is done because skb is attached to a socket, and a socket has a
limited (but configurable) sndbuf. sk->sk_wmem_alloc is the current sum
of all truesize skbs in flight.

When you enter something that :

1) Get a clone of the skb, queue the clone to device X
2) queue the original skb to device Y

Then :	Socket sndbuf is not affected at all by device X queue.
	This is speed on device Y that matters.

You want to get servo control on both X and Y

You could try to

1) Get a clone of skb
   Attach it to socket too (so that socket get a feedback of final
orphaning for the clone) with skb_set_owner_w()
   queue the clone to device X

Unfortunatly, stacked skb->destructor() makes this possible only for
known destructor (aka sock_wfree())

> One idea that I had was to skb_get() the original skb each time it is
> cloned - that is easy enough. But unfortunately it seems to me that
> approach would require some sort of callback mechanism in kfree_skb() so
> that the cloned skbs can kfree_skb() the original skb.
> 
> Ideas would be greatly appreciated.
> 
> [1] http://openvswitch.org/pipermail/dev_openvswitch.org/2010-October/003806.html
> [2] http://openvswitch.org/cgi-bin/gitweb.cgi?p=openvswitch;a=blob;f=datapath/actions.c;h=5e16143ca402f7da0ee8fc18ee5eb16c3b7598e6;hb=HEAD
> --