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Message-ID: <1416242717.5102.16.camel@edumazet-glaptop2.roam.corp.google.com>
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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