[Intel-wired-lan] i40e card Tx resets

[Sowmini Varadhan <sowmini.varadhan@oracle.com>]

Hi,

I am trying out some DB stress tests on both i40e and ixgbe. The 
stress test that I use is rds-stress (http://linux.die.net/man/1/rds-stress),
and I can list out the entire set of steps and parameters that I
am using to run this if that info is  interesting.

My test bed is a pair of X5-2 (Haswell) servers, each with a
Niantic (X540-AT2) card and a Fortville card. The Niantic/fortville
cards are connected back-to-back, so I essentially have a 10G
connection and a 40G connection.

Everything else (kernel, RDS modules, stress test and parameters)
remaining the same, I get  the expected throughput on the 10G
connection, but the i40e connection goes through a lot of TX
errors that result in console messages like this:

  i40e 0000:81:00.0: TX driver issue detected, PF reset issued
  i40e 0000:81:00.0 eth2: adding 68:05:ca:30:db:30 vid=0
  i40e 0000:81:00.0: TX driver issue detected, PF reset issued
  i40e 0000:81:00.0 eth2: VSI_seid 390, Hung TX queue 32, tx_pending: 82, NTC:0xeb, HWB: 0xeb, NTU: 0x13d, TAIL: 0x13d
  i40e 0000:81:00.0 eth2: VSI_seid 390, Issuing force_wb for TX queue 32, Interrupt Reg

I understand these are "mdd errors", but how can I find out what 
triggered these errors, any hints?

The other data-point here is that if I disable tso, and fall back
to gso, there are no tx errors, and throughput matches the 10G 
connection (for the same set of test parameters).

Please let me know if there is any other info that would help.
The kernel is a 4.5.0-rc2 kernel. Info for the i40e card is

    # ethtool -i eth3
    driver: i40e
    version: 1.4.8-k
    firmware-version: 5.02 0x80002285 0.0.0
    bus-info: 0000:81:00.1 
       :

Thanks in advance,
--Sowmini