RFC: Backport cgroups v2 to 8u

[Severin Gehwolf]

Hi,

We, Red Hat, think that getting cgroups v2 support backported to JDK
8u will be needed long-term. Systems which use cgroups v2 by default
become more an more widely used. For example Red Hat Enterprise Linux
9[1] will have it turned on by default. Recent Fedora and Ubuntu
distributions have it on by default. Similarly, container technology
such as Docker and Kubernetes support cgroups v2 as well.

With more and more systems defaulting to cgroups v2, the likelihood of
users deploying legacy JDKs such as JDK 8u on a cgroups v2 system
increases. With container technology it's possible, to deploy an
application based on, say, UBI 8 onto a cgroups v2 host system, like
Red Hat Enterprise Linux 9. As cgroups is a kernel feature, the host
kernel (and cgroup configuration) is the one that counts as far as
application runtime is concerned.

The risk of NOT backporting those patches is that the JVM would wrongly
detect resource limits in such a scenario. Even though,
-XX:+UseContainerSupport is on by default in 8u, container detection
code would just give up early and would use the host systems' resource
limits instead. That gets us back to pre-JDK-8146115[2] behaviour of
the JDK when running on such systems. I.e. random OOM kills and other
strange behaviour. IMHO, that would be very bad.

Cgroups v2 support has been available in the JDK since version 15 and
up and has recently been backported to 11.0.16. I believe, most bugs
have been shaken out in due course. Support code has been pretty stable
over the last couple of major JDK releases. The code is Linux-only and
is pretty self-contained. It is our belief that the risks of
introducing those patches to 8u is manageable. At the very least, we
think NOT doing the backport is riskier.

While JDK 8u is in maintenance mode, this would be one of the few
exceptional enhancements needed to adapt to new computing environments.

Therefore, we propose to (roughly) backport the following patch-set
from JDK 15+:

Cgroups v2: Container awareness
https://bugs.openjdk.java.net/browse/JDK-8230305

Cgroups v2: Rework Metrics in java.base so as to recognize unified hierarchy
https://bugs.openjdk.java.net/browse/JDK-8231111

8230305 causes slowdebug build failure
https://bugs.openjdk.java.net/browse/JDK-8237479

[cgroups v2] Soft memory limit incorrectly using memory.high
https://bugs.openjdk.java.net/browse/JDK-8253714

Cgroups: Incorrect detection logic on old systems in hotspot
https://bugs.openjdk.java.net/browse/JDK-8239785

Cgroups: Incorrect detection logic on some systems
https://bugs.openjdk.java.net/browse/JDK-8239559

jtreg test error in test/hotspot/jtreg/containers/docker/TestMemoryAwareness.java
https://bugs.openjdk.java.net/browse/JDK-8244500

containers/cgroup/PlainRead.java fails on Ubuntu 21.10
https://bugs.openjdk.java.net/browse/JDK-8278951

[cgroups v2] Account for the fact that swap accounting is disabled on some systems
https://bugs.openjdk.java.net/browse/JDK-8253797

Cgroups: Incorrect detection logic on some systems (still reproducible)
https://bugs.openjdk.java.net/browse/JDK-8245543

[Metrics] Enhance parsing of cgroup interface files for version detection
https://bugs.openjdk.java.net/browse/JDK-8254001

HotSpot Not Identifying it is Running in a Container
https://bugs.openjdk.java.net/browse/JDK-8252359

Cgroup: 'stomping of _mount_path' crash if manually mounted cpusets exist
https://bugs.openjdk.java.net/browse/JDK-8253435

[TESTBUG] Increase coverage of the cgroups detection code
https://bugs.openjdk.java.net/browse/JDK-8253939

Thoughts? Objections?

Thanks,
Severin