RFR(S): 8239785: Cgroups: Incorrect detection logic on old systems in hotspot

[Severin Gehwolf]

Hi,

Could I please get reviews of this cgroup-related patch? After JDK-
8230305 old systems with no mounted cgroup controllers would get
detected as cgroups v2 (wrong). Then, when initializing the v2
subsystem, it would fail. The trace about cgroupv2 mount point not
found is misleading in this case. While the outcome is similar pre/post
patch (NULL cgroup subsystem), I'd like to be explicit about this case.

The suggested fix is to look at /proc/self/mountinfo in order to
correctly detect whether this is cgroup v2 or cgroup v1 with no mounted
controllers. In the latter case we just stop initialization as we'd
fail later in cgroupv1 code anyway. This aligns hotspot code with core-
libs after JDK-8239559.

Bug: https://bugs.openjdk.java.net/browse/JDK-8239785
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8239785/01/webrev/

Testing: jdk-submit, hotspot docker tests on cgroup v1 and cgroup v2. All pass.

Thoughts?

Thanks,
Severin