netdev - Re: [PATCH] rfc: ethtool: early-orphan control

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Date:	Sat, 11 Dec 2010 09:03:38 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Simon Horman <horms@...ge.net.au>
Cc:	netdev@...r.kernel.org, Ben Hutchings <bhutchings@...arflare.com>
Subject: Re: [PATCH] rfc: ethtool: early-orphan control

Le samedi 11 décembre 2010 à 13:24 +0900, Simon Horman a écrit :
> On Sat, Dec 11, 2010 at 01:13:35PM +0900, Simon Horman wrote:
> > Early orphaning is an optimisation which avoids unnecessary cache misses by
> > orphaning an skb just before it is handed to a device for transmit thus
> > avoiding the case where the orphaning occurs on a different CPU.
> > 
> > In the case of bonded devices this has the unfortunate side-effect of
> > breaking down flow control allowing a socket to send UDP packets as fast as
> > the CPU will allow. This is particularly undesirable in virtualised
> > network environments.
> > 
> > This patch introduces ethtool control of early orphaning.
> > It remains on by default by it now may be disabled on a per-interface basis.
> > 
> > I have implemented this as a generic flag.
> > As it seems to be the first generic flag that requires
> > no driver awareness I also supplied a default flag handler.
> > I am unsure if any aspect of this approach is acceptable.
> > 
> > I believe Eric has it in mind that some of the calls
> > to skb_orphan() in drivers can be removed with the addition
> > of this feature. I need to discuss that with him further.
> > 
> > A patch for the ethtool user-space utility accompanies this patch.
> 
> The following results were measured using kvm using virto without vhost net.
> The virtio device is bridged to a bond device which has one gigabit slave.
> 

As you know, vhost net does the orphaning, as well as some NIC drivers,
so one UDP flood would have same problem.

I wonder if this problem could not be solved in other ways.


We might do early orphaning only for sockets with SOCK_USE_WRITE_QUEUE
flag asserted. (tcp sets it)

Then, we could also say : Why tcp use sock_wfree() at all...


Hmm...


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