netdev - Re: [net PATCH] i40e/i40evf: Limit TSO to 7 descriptors for payload instead of 8 per packet

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Date:	Wed, 30 Mar 2016 14:20:40 -0700
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Jesse Brandeburg <jesse.brandeburg@...el.com>
Cc:	Sowmini Varadhan <sowmini.varadhan@...cle.com>,
	Alexander Duyck <aduyck@...antis.com>,
	Netdev <netdev@...r.kernel.org>,
	intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Subject: Re: [net PATCH] i40e/i40evf: Limit TSO to 7 descriptors for payload
 instead of 8 per packet

On Wed, Mar 30, 2016 at 12:41 PM, Jesse Brandeburg
<jesse.brandeburg@...el.com> wrote:
> On Wed, 30 Mar 2016 10:35:55 -0700
> Alexander Duyck <alexander.duyck@...il.com> wrote:
>
>> On Wed, Mar 30, 2016 at 10:20 AM, Sowmini Varadhan
>> <sowmini.varadhan@...cle.com> wrote:
>> > On (03/30/16 10:12), Alexander Duyck wrote:
>> >> Yeah.  The patch was sort of a knee-jerk reaction to being told that
>> >> the patch referenced caused a regression.  From what I can tell that
>> >> is not the case as I am also seeing the Tx hangs when I run the test
>> >> with the frames being linearized.
>> >
>> > I'm not sure how important of a subtlety this is, but the actual
>> > console log after the patch is the following:
>> >
>> >  i40e 0000:82:00.0: TX driver issue detected, PF reset issued
>> >  i40e 0000:82:00.0 eth2: adding 68:05:ca:30:dd:18 vid=0
>> >  i40e 0000:82:00.0: TX driver issue detected, PF reset issued
>> >  i40e 0000:82:00.0 eth2: adding 68:05:ca:30:dd:18 vid=0
>> >  i40e 0000:82:00.0: TX driver issue detected, PF reset issued
>> >
>> > Comparing with what I'd pasted in the sourceforge thread earlier,
>> > I see that it does not say "Hung Tx queue etc."  any more, though
>> > it still resets.
>> >
>> > Not sure if that changed info is significant?
>>
>> It might be.  Right now I am chasing down the Tx driver issue as that
>> I what I am reproducing in my environment as well.
>
> This gets "Even Uglier", I've turned off all offloads at my receiver,
> enabled calling skb_linearize on *all* frames, which works fine for
> scp, but the receiver shows > MSS sized frames on the wire for
> rds-stress traffic.

Are you sure it isn't just GRO reassembling frames on the receive
side.  I know that one always trips me up when I am using the Rx path
to validate Tx checksums.

> This implies to me we have some issue with skb_linearize, possibly in
> how the stack linearizes the data, or how the driver interprets the
> linearized packets (which should always work)
>
> Wheee......

With the descriptor dump code you have you should be able to verify
what the layout is after the descriptor is linearized.  I would think
in most cases you would end up with at most something like 4 to maybe
5 descriptors for a 64K frame.

- Alex