Kernel 4.6.7-rt13: Intel Ethernet driver igb causes huge latencies in cyclictest

Hi all,

I have detected the strange effect, that the igb driver for Intel's Ethernet adapters is causing huge latencies in cyclictest on
a Core i7 PC (64bit) using kernel 4.6.7-rt13.

I run the cyclictest with the following options:
# cyclictest -a -i 100 -d 10 -m -n -t -p 80

The max latencies are really great, all below 5 microseconds (on an idle system, after 1 minute run time of cyclictest).
Of course the 2 minutes run-time of cyclictest is only a rough first estimate.
With this first test there is no igb base NIC configured yet.


In the PC I have also 2 port Ethernet NIC i350 from Intel that uses the igb driver.
Once I configure one of the i350 ports
# ifconfig eth2 up 192.168.100.100
the cyclictest shows directly and reproducibly significant larger max latency values (40 microseconds, using the same conditions).


I did the very same test with kernel version 3.18.27-rt27.
With that version I did not see anything like that.

Also, only the igb driver seems to cause the trouble. I have also an e1000e based NIC in this PC and 
the usage of this driver does not add any significant latency.

Any idea on this?

Thanks for any feedback

Best regards

Mathias