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

Hi Julia!

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Have you tested on a vanilla (non-RT) kernel?  I doubt there is
anything RT specific about what you are seeing, but it might be nice
to get confirmation.  Also, bisection would probably be easier if you confirm on a

vanilla kernel.

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I find it unlikely that it's a kernel config option that changed
which regressed you, but instead was a code change to a driver.
Which driver is now the question, and the surface area is still big
(processor mapping attributes for this region, PCI root complex configuration,

PCI brige configuration, igb driver itself, etc.).

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Big enough that I'd recommend a bisection.  It looks like a
bisection between 3.18 and 4.8 would take you about 18 tries to narrow down,

assuming all goes well.

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I have now repeated my tests using the vanilla kernel.
There I got the very same issue.
Using kernel 4.0 is fine, however starting with kernel 4.1, the issue appears.

Great, thanks for confirming!  That helps narrow things down quite a bit.

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Here is my exact (reproducible) test description:
I applied the following patch to the kernel to get the igb trace.
This patch instruments the igb_rd32() function to measure the call to
readl() which is used to access registers of the igb NIC.

I took your test setup and ran it between 4.0 and 4.1 on the hardware on my desk,
which is an Atom-based board with dual I210s, however I didn't see much
difference.

However, it's a fairly simple board, with a much simpler PCI topology than your
workstation.  I'll see if I can find some other hardware to test on.

[..]

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This means, that I think that some other stuff in kernel 4.1 has
changed, which has impact on the igb accesses.

Any idea what component could cause this kind of issue?

Can you continue your bisection using 'git bisect'?  You've already narrowed it down
between 4.0 and 4.1, so you're well on your way.

OK - done.
And finally I was successful!
The following git commit is the one that is causing the trouble!
(The full commit is in the attachment).
+++++++++++++++++++++ BEGIN +++++++++++++++++++++++++++
commit 387d37577fdd05e9472c20885464c2a53b3c945f
Author: Matthew Garrett [off-list ref]
Date:   Tue Apr 7 11:07:00 2015 -0700

    PCI: Don't clear ASPM bits when the FADT declares it's unsupported

    Communications with a hardware vendor confirm that the expected behaviour
    on systems that set the FADT ASPM disable bit but which still grant full
    PCIe control is for the OS to leave any BIOS configuration intact and
    refuse to touch the ASPM bits.  This mimics the behaviour of Windows.

    Signed-off-by: Matthew Garrett [off-list ref]
    Signed-off-by: Bjorn Helgaas [off-list ref]
+++++++++++++++++++++ HEADER +++++++++++++++++++++++++++

The only files that are modified by this commit are 
drivers/acpi/pci_root.c
drivers/pci/pcie/aspm.c
include/linux/pci-aspm.h

This is all generic PCIe stuff - however I do not really understand what
the changes of the commit are...

In my setup I am using a dual port igb Ethernet adapter.
This has an onboard PCIe switch and it might be that the configuration of this
PCIe switch on the Intel board is causing the trouble.

Please see also the output of "lspci -v" in the attachment.
The relevant PCI address of the NIC is 04:00.0 / 04:00.1

Any feedback on this is welcome!

Thanks

Mathias