Message-ID: <AANLkTinJK-nbkP5_ee2cuS8RA7jTB4-bcWmAf4bjSouP@mail.gmail.com>
Date:	Thu, 6 Jan 2011 17:38:01 -0500
From:	Jesse Gross <jesse@...ira.com>
To:	Simon Horman <horms@...ge.net.au>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org, dev@...nvswitch.org,
	virtualization@...ts.osdl.org, netdev@...r.kernel.org,
	kvm@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: Flow Control and Port Mirroring Revisited

On Thu, Jan 6, 2011 at 7:44 AM, Simon Horman <horms@...ge.net.au> wrote:
> On Thu, Jan 06, 2011 at 11:22:42AM +0100, Eric Dumazet wrote:
>> 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())
>
> Hi Eric !
>
> Thanks for the advice. I had thought about the socket buffer but at some
> point it slipped my mind.
>
> In any case the following patch seems to implement the change that I had in
> mind. However my discussions Michael Tsirkin elsewhere in this thread are
> beginning to make me think that think that perhaps this change isn't the
> best solution.

I know that everyone likes a nice netperf result but I agree with
Michael that this probably isn't the right question to be asking.  I
don't think that socket buffers are a real solution to the flow
control problem: they happen to provide that functionality but it's
more of a side effect than anything.  It's just that the amount of
memory consumed by packets in the queue(s) doesn't really have any
implicit meaning for flow control (think multiple physical adapters,
all with the same speed instead of a virtual device and a physical
device with wildly different speeds).  The analog in the physical
world that you're looking for would be Ethernet flow control.
Obviously, if the question is limiting CPU or memory consumption then
that's a different story.

This patch also double counts memory, since the full size of the
packet will be accounted for by each clone, even though they share the
actual packet data.  Probably not too significant here but it might be
when flooding/mirroring to many interfaces.  This is at least fixable
(the Xen-style accounting through page tracking deals with it, though
it has its own problems).
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