netdev - Re: net: tso: add UDP segmentation support: adds regression for ax200 upload

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Message-ID: <adbee2ec-c6ba-7a17-eb98-1c53365fa911@candelatech.com>
Date:   Thu, 17 Dec 2020 09:56:33 -0800
From:   Ben Greear <greearb@...delatech.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: net: tso: add UDP segmentation support: adds regression for ax200
 upload

On 12/17/20 2:11 AM, Eric Dumazet wrote:
> On Thu, Dec 17, 2020 at 12:59 AM Ben Greear <greearb@...delatech.com> wrote:
>>
>> On 12/16/20 3:09 PM, Ben Greear wrote:
>>> Hello Eric,
>>>
>>> The patch below evidently causes TCP throughput to be about 50Mbps instead of 700Mbps
>>> when using ax200 to upload tcp traffic.
>>>
>>> When I disable TSO, performance goes back up to around 700Mbps.
>>
>> As a followup, when I revert the patch, upload speed goes to ~900Mbps,
>> so even better than just disabling TSO (I left TSO enabled after reverting the patch).
>>
>> Thanks,
>> Ben
>>
> 
> Thanks for the report !
> 
> It seems drivers/net/wireless/intel/iwlwifi/pcie/tx.c:iwl_fill_data_tbs_amsdu()
> calls tso_build_hdr() with extra bytes (SNAP header),
> it is not yet clear to me what is broken :/

Your patch is guessing tcp vs udp by looking at header length
from what I could tell.  So if something uses a different size,
it probably gets confused?

> 
> Can you confirm which driver is used for ax200 ?
> 
> I see tso_build_hdr() also being used from
> drivers/net/wireless/intel/iwlwifi/queue/tx.c

I tested against the un-modified ax200 5.10.0 kernel driver, and it has the issue.

The ax200 backports release/core56 driver acts a bit different (poorer performance over all than
in-kernel driver), but has similar upstream issues that are mitigated by
disabling TSO.

Thanks,
Ben

-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com