4.15 kernel's ip_vs module gets refcount errors with --ops usage

[Marc Hasson]

On our 16.04LTS system we use the ipvsadm --ops UDP support (one-packet scheduling) to get a better distribution amongst our real servers behind the load-balancer for some small subset of applications.

This has worked fine through the 4.4.0-xxx kernels. But when we started a program to upgrade systems to use the 4.15 series of kernels to take advantage of new facilities, the subset of systems which used the --ops option ran into problems. Everything else with the 4.15 kernels appeared to work well.

The issue appears to have been the change in the ip_vs module from using "atomic_*()" increment/decrement functions in the 4.4 kernel to instead use "refcount_*()" functions in a later kernel, including the 4.15 one we switched to. Unfortunately, the simple refcount_dec() function was inadequate, in putting out a time-consuming message and handling when the refcount dropped to zero, which is expected in the case of --ops support that retains no state post packet delivery. I will upload an attachment with the sample messages that get put out at packet arrival rate, which destroys performance of course. On this VM, with far more limited # of CPUs than production servers, the system actually hangs (crashes?) for quite some time.

This issue was apparently already recognized as an error and has appeared as a fix in upstream kernels. This is a reference to the 4.17 version of the fix that we'd like to see incorporated into the next possible kernel maintenance release:

https://github.com/torvalds/linux/commit/a050d345cef0dc6249263540da1e902bba617e43#diff-75923493f6e3f314b196a8223b0d6342

We have successfully used the livepatch facility to build a livepatch .ko with the above diffs on our 4.15.0-36 system and successfully demonstrated the contrast in good/bad behavior with/without the livepatch module loaded. But we'd rather not have to build a version of livepatch.ko for each kernel maintenance release.

The problem is easy to generate, with only a couple of packets and a simple configuration. Here's a very basic test (addresses rewritten/obscured) version of an example configuration for 2 servers that worked on my test VM:

ipvsadm -A -f 100 -s rr --ops
ipvsadm -a -f 100 -r 10.129.131.227:0 -g -w 9999
ipvsadm -a -f 100 -r 10.129.131.228:0 -g -w 9999
iptables -t mangle -A PREROUTING -d 172.16.5.1/32 -j MARK --set-xmark 0x64/0xffffffff
ifconfig lo:0 172.16.5.1/32 up

Routing and addressing to achieve the above, or adaptation for one's own test environment, is left to the tester.

I set up routing and addresses on my 2 NIC test such that packets arrived on my test machine's eth1 NIC and were directed by ip_vs out the eth2. To test, all I did was throw a few UDP packets via traceroute at the address on the iptables/firewall mark rule so that the eth1 interface of the test system was the traceroute system's default gateway:

  traceroute -m 2 172.16.5.1

Without the fix my test ip_vs system either hangs or puts out messages as per the attached. With our livepatch module using the above commit's contents, all is well. Both real servers configured above, get packets
and no errors are reported in the logs.

Just read that, despite using apport-bug, that I should include the lsb_release info requested. Here it is inline, with the uname -a to emphasize the kernel we are running which is the issue:

$ lsb_release -rd
Description: Ubuntu 16.04.5 LTS
Release: 16.04
$ uname -a
Linux director-16-04 4.15.0-36-generic #39~16.04.1-Ubuntu SMP Tue Sep 25 08:59:23 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Let me know of anything I can do to help accelerate addressing of this issue or understanding. It seems that the fix incorporation is fairly straightforward, and is a performance disaster without it for anyone using the --ops facility to any significant degree.

Thanks!

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: linux-image-4.15.0-36-generic 4.15.0-36.39~16.04.1
ProcVersionSignature: Ubuntu 4.15.0-36.39~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-36-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Feb 21 20:13:18 2019
InstallationDate: Installed on 2017-06-21 (611 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
SourcePackage: linux-signed-hwe
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Marc Hasson (mhassonsuspect)]

This report does not appear to have received any human's attention yet at Ubuntu, even though it was written weeks ago and is a significant performance/functionality concern that the --ops facility can not be used as is. It must be patched. Unfortunately for us in the interim, it was discovered that the -36 kernel we did the livepatch remedy for now has to be quickly replaced by a later kernel due to a TCP window performance problem fixed in a later maintenance release.

So, in case the combination of the 16.04LTS with HWE 4.15 kernel is not sufficient to garner attention I have also tested this exact issue on a stock 18.04 system with the distributed kernel. As one would expect from the similar 4.15 kernel, the issue exists there as well and is reported at the following URL with a title that better conveys the seriousness of the issue:

  https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1819786

[Marc Hasson (mhassonsuspect)]

The test kernel supplied for the 4.15 kernel in the 18.04LTS release fixed the identical issue there:

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1819786

With our wider 16.04 deployment with the HWE 4.15 kernel we'd especially wish to see the same version of the fix applied here too.

Thanks!

[Amr Ibrahim (amribrahim1987)]

The correct source is linux-hwe, linux-signed-hwe is the signed binary of the former.

[Marc Hasson (mhassonsuspect)]

Thanks, that source change makes sense. Hopefully now that the similar bug:

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1819786

Has been marked with a status of "Fix Released" we can get a similar result here?

[Amr Ibrahim (amribrahim1987)]

This is fixed now based on the updated version.