Intel Ethernet driver igb causes huge latencies with cyclictest (rt-tests)

["Koehrer Mathias (ETAS/ESW5)" <mathias.koehrer@etas.com>]

Hi all,

I noticed that with fairly new versions of the Linux kernel, the igb driver
causes huge latencies with the cyclictest in a RT_PREEMPT environment.
The root cause seems to be the number of interrupts that are used for the igb
NIC devices as multiple of these irqs may occur at the same time (see below).

With the kernel 4.6.7-rt14 the igb uses 9 (!) irqs per NIC on an Intel Core i7 PC (x86-64):
E.g. eth2, and eth2-TxRx-0, eth2-TxRx-1, ... , eth2-TxRx-7.

Running the very same machine with kernel 3.18.27-rt27 there are only 2 irqs:
eth2 and eth2-TxRx0

The issue with the many irqs is now that they are all fired roughly the same time
even if the link is down as nothing is connected to the NIC.
I analyzed the execution of the cyclictest tool using the kernel tracer on kernel 4.6.7-rt14:

kworker/-5       0dN.h2.. 1504647372us : sched_wakeup: comm=cyclictest pid=5887 prio=19 target_cpu=000
kworker/-5       0dN.h3.. 1504647374us : sched_wakeup: comm=irq/54-eth2-TxR pid=5883 prio=49 target_cpu=000
kworker/-5       0dN.h3.. 1504647375us : sched_wakeup: comm=irq/53-eth2-TxR pid=5882 prio=49 target_cpu=000
kworker/-5       0dN.h3.. 1504647377us : sched_wakeup: comm=irq/52-eth2-TxR pid=5881 prio=49 target_cpu=000
kworker/-5       0dN.h3.. 1504647378us : sched_wakeup: comm=irq/51-eth2-TxR pid=5880 prio=49 target_cpu=000
kworker/-5       0dN.h3.. 1504647380us : sched_wakeup: comm=irq/50-eth2-TxR pid=5879 prio=49 target_cpu=000
kworker/-5       0dN.h3.. 1504647381us : sched_wakeup: comm=irq/49-eth2-TxR pid=5878 prio=49 target_cpu=000
kworker/-5       0dN.h3.. 1504647382us : sched_wakeup: comm=irq/48-eth2-TxR pid=5877 prio=49 target_cpu=000
kworker/-5       0dN.h3.. 1504647383us : sched_wakeup: comm=irq/47-eth2-TxR pid=5876 prio=49 target_cpu=000
kworker/-5       0d...2.. 1504647384us : sched_switch: prev_comm=kworker/0:0 prev_pid=5 prev_prio=120 prev_state=R+ ==> next_comm=cyclictest next_pid=5887 next_prio=19

Here it can be clearly seen that eight irqs from the igb are coming in at the same time.
This leads to a fairly long phase of running in irq mode which hurts the real time latency.

In my setup I have no cable connected to the eth2,
I do a 
# modprobe igb
# ifconfig eth2 up 192.168.100.111

I did multiple tests with analyzing and modifying the igb driver.
The function "igb_watchdog_task" seems to be the root cause of the issue.
Whenever I disable this function the cyclictest shows great results.

There has been lengthy discussion on that topic on the rt-users mailing list:
http://marc.info/?t=147454836600003&r=1&w=2 

My question is now:
How can I either use only 1 irq per NIC using the igb driver or how can 
the driver be reorganized to let the watchdog task trigger the irqs alternately.

Thanks for any feedback

Regards

Mathias