Dead code in jdk.internal.platform.Metrics?

[Severin Gehwolf]

On Wed, 2019-09-18 at 11:07 -0700, Bob Vandette wrote:
> Do you handle the case where there are v1 and v2 subsystems enabled at the same time?

Do you mean hybrid hierarchy? I believe so, yes.

Right now, if cpu/cpuset/memory/cpuacct controllers are not bound to a
cgroupv1 hierarchy - 0 of the hierarchy field in /proc/cgroups (see man
7 cgroups) - cgroupv2 is being assumed. For hybrid hiearchy, which is
pretty common with systemd, this would detect to cgroupv1. It's all or
nothing for relevant controllers, though. Either all of them are
cgroups v1 or all of them are cgroups v2.

Examples:

hybrid hierarchy (note cgroup2 mounted at /sys/fs/cgroup/unified):
----------------------------------------------------
$ grep cgroup /proc/self/mountinfo
28 21 0:25 / /sys/fs/cgroup ro,nosuid,nodev,noexec shared:4 - tmpfs tmpfs ro,seclabel,mode=755
29 28 0:26 / /sys/fs/cgroup/unified rw,nosuid,nodev,noexec,relatime shared:5 - cgroup2 cgroup2 rw,seclabel,nsdelegate
30 28 0:27 / /sys/fs/cgroup/systemd rw,nosuid,nodev,noexec,relatime shared:6 - cgroup cgroup rw,seclabel,xattr,name=systemd
33 28 0:30 / /sys/fs/cgroup/blkio rw,nosuid,nodev,noexec,relatime shared:7 - cgroup cgroup rw,seclabel,blkio
34 28 0:31 / /sys/fs/cgroup/cpuset rw,nosuid,nodev,noexec,relatime shared:8 - cgroup cgroup rw,seclabel,cpuset
35 28 0:32 / /sys/fs/cgroup/net_cls,net_prio rw,nosuid,nodev,noexec,relatime shared:9 - cgroup cgroup rw,seclabel,net_cls,net_prio
36 28 0:33 / /sys/fs/cgroup/cpu,cpuacct rw,nosuid,nodev,noexec,relatime shared:10 - cgroup cgroup rw,seclabel,cpu,cpuacct
37 28 0:34 / /sys/fs/cgroup/devices rw,nosuid,nodev,noexec,relatime shared:11 - cgroup cgroup rw,seclabel,devices
38 28 0:35 / /sys/fs/cgroup/hugetlb rw,nosuid,nodev,noexec,relatime shared:12 - cgroup cgroup rw,seclabel,hugetlb
39 28 0:36 / /sys/fs/cgroup/pids rw,nosuid,nodev,noexec,relatime shared:13 - cgroup cgroup rw,seclabel,pids
40 28 0:37 / /sys/fs/cgroup/memory rw,nosuid,nodev,noexec,relatime shared:14 - cgroup cgroup rw,seclabel,memory
41 28 0:38 / /sys/fs/cgroup/freezer rw,nosuid,nodev,noexec,relatime shared:15 - cgroup cgroup rw,seclabel,freezer
42 28 0:39 / /sys/fs/cgroup/perf_event rw,nosuid,nodev,noexec,relatime shared:16 - cgroup cgroup rw,seclabel,perf_event

$ bin/java -Xlog:os+container=trace -version | grep Detected
[0.003s][debug][os,container] Detected cgroups hybrid or legacy hierarchy, using cgroups v1 controllers
openjdk version "14-internal" 2020-03-17
OpenJDK Runtime Environment (fastdebug build 14-internal+0-adhoc.sgehwolf.openjdk-head)
OpenJDK 64-Bit Server VM (fastdebug build 14-internal+0-adhoc.sgehwolf.openjdk-head, mixed mode, sharing)

unified hierarchy:
----------------------------------------------------
$ grep cgroup /proc/self/mountinfo 
28 21 0:25 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime shared:4 - cgroup2 cgroup2 rw,seclabel
$ bin/java -Xlog:os+container=trace -version | grep Detected
[0.001s][debug][os,container] Detected cgroups v2 unified hierarchy
openjdk version "14-internal" 2020-03-17
OpenJDK Runtime Environment (fastdebug build 14-internal+0-adhoc.sgehwolf.openjdk-head)
OpenJDK 64-Bit Server VM (fastdebug build 14-internal+0-adhoc.sgehwolf.openjdk-head, mixed mode, sharing)

Thanks,
Severin