RFR: 8239559: Cgroups: Incorrect detection logic on some systems

[Bob Vandette]

> On Feb 24, 2020, at 5:36 AM, Severin Gehwolf <sgehwolf at redhat.com> wrote:
> 
> Hi Bob,
> 
> Note: I've changed the bug title to be version-less. This is really an
> issue in the detection logic irrespective of cgroup version in use.
> 
> On Fri, 2020-02-21 at 10:12 -0500, Bob Vandette wrote:
>>> On Feb 21, 2020, at 9:30 AM, Severin Gehwolf <sgehwolf at redhat.com> wrote:
>>> 
>>> Hi Bob,
>>> 
>>> On Fri, 2020-02-21 at 09:11 -0500, Bob Vandette wrote:
>>>> Severin,
>>>> 
>>>> Don’t we need the contents of /proc/self/mountinfo in order to construct the path to the cgroup controllers?
>>> 
>>> There is only one for unified (cgroups v2), but yes it's beeing used.
>>> See CgroupV2Subsystem.initSubsystem() and
>>> CgroupV1Subsystem.initSubsystem(). For affected systems, no controllers
>>> are mounted, so the effect will be null Metrics, as before JDK-8231111. 
>>> Maybe I didn't understand the question, sorry.
>> 
>> If you don’t have access to the information required to get metrics, I just assumed that
>> you would return NULL in CgroupSubsystemFactory.create() rather than making the
>> assumption that it works only to fail later.
> 
> You are right. It makes little sense to continue in that case. Updated
> webrev:
> http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8239559/02/webrev/

Looks good.

> 
>> Alternatively, we could consider assuming the mount point is /sys/fs/cgroup for cgroupv1 in
>> the case you are trying to support.  This would involve using /proc/self/cgroup to get the list
>> of controllers and then use that list to call createSubSystemController and
>> setSubSystemControllerPath with the default path.
>> 
>> I think we need to understand the extent of the problem on these older systems before 
>> deciding a course of action.  Do we see the same empty mountinfo file in a docker container
>> running on these older systems or is this just a host issue?  If docker containers work fine, then
>> I wouldn’t bother trying to make this work.
> 
> That's the thing. I don't think any of those older systems support
> docker in the first place.

If that’s the case then you are doing the right thing.

Bob.


> 
> Thanks,
> Severin
> 
>> Bob.
>> 
>>>> On Thu, 2020-02-20 at 14:50 +0000, Baesken, Matthias wrote:
>>>>> Hi  Severin,
>>>>> 
>>>>> grep cgroup /proc/self/mountinfo 
>>>>> 
>>>>> returns  nothing.
>>>>> 
>>>>> Best Regards, Matthias
>>>>> 
>>>> 
>>>> Assuming your fix is correct, don’t we also need to apply the same change to the hotspot source cgroupSubsystem_linux.cpp?
>>> 
>>> Yes, tracked with JDK-8239785.
>>> 
>>> Thanks,
>>> Severin
>>> 
>>>> Bob;
>>>> 
>>>> 
>>>>> On Feb 21, 2020, at 8:32 AM, Severin Gehwolf <sgehwolf at redhat.com> wrote:
>>>>> 
>>>>> Hi,
>>>>> 
>>>>> Could I please get a review of this fix to the detection heuristic of
>>>>> cgroup v1 vs cgroup v2? Matthias (in CC) discovered that on some old
>>>>> systems the JDK Metrics code throws InternalError caused by wrong
>>>>> detection logic when Metrics are being created on Linux.
>>>>> 
>>>>> The reason for this is that hierarchy IDs of 0 in /proc/cgroups is
>>>>> being used as a heuristic to detect cgroups v2 systems. Apparently some
>>>>> old systems like RHEL 6 and SLES 11 have no cgroups controllers
>>>>> mounted, thus, triggering a false positive.
>>>>> 
>>>>> The fix is to also look at /proc/self/mountinfo and correct logic in
>>>>> this case.
>>>>> 
>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8239559
>>>>> webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8239559/01/webrev/
>>>>> 
>>>>> Testing: docker/cgroups tests on hybrid (cgroups v1) and unified
>>>>> hierarchy (cgroups v2). New regression test. Looks good here.
>>>>> 
>>>>> Unfortunately, I wasn't able to reproduce this on an actual affected
>>>>> system. I somewhat reproduced via the derived regression test based on
>>>>> data from reporters. I'd appreciate any testing on systems where this
>>>>> reproduces.
>>>>> 
>>>>> Thanks,
>>>>> Severin
>>>>> 
>