netdev - Re: [net-next 03/12] i40e: Handle a single mss packet with more than 8 frags

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Date:	Mon, 17 Nov 2014 08:45:17 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Nelson, Shannon" <shannon.nelson@...el.com>
Cc:	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"Kong, Serey" <serey.kong@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"nhorman@...hat.com" <nhorman@...hat.com>,
	"sassmann@...hat.com" <sassmann@...hat.com>,
	"jogreene@...hat.com" <jogreene@...hat.com>
Subject: Re: [net-next 03/12] i40e: Handle a single mss packet with more
 than 8 frags

On Mon, 2014-11-17 at 16:04 +0000, Nelson, Shannon wrote:

> This typically hits when there are many little TSO packets in a single
> MTU size frame.  The hardware doesn't like dealing with more than 8
> separate buffers.
> 
> If skb_linearize() fails and we try to send the skb as is, the
> hardware will throw an MDD event and disable that particular queue,
> possibly causing a Tx hang, which will lead to a PF reset and then we
> continue on.  It's a bit of a hit to throughput, and most folks get a
> little annoyed when seeing the Tx hang stack trace in their log, but
> it generally isn't fatal.
> 
> If we disable the MDD detection, the hardware ends up putting garbage
> on the wire.
> 
> It looks to me that the way for skb_linearize() to fail is with
> -ENOMEM, in which case we probably have other issues.
> 
This kind of events happens in real workloads, trust me.

> If there are some alternative suggestions for dealing with this, we'd
> be happy to hear about them.

Nelson, all you have to do is to check skb_linearize(skb) return code.

If its a failure, then drop this packet. How hard is this ?

This should hardly trigger in normal conditions.

If some customer complains, then you might implement something more
complex in the future.

Lets fix the bug first in a 100%  safe way.


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