RFR: 8020962: dump loaded java classes when vm crash

[Yumin Qi]

Chris, David

   Yes. This can be after crash with core file, but some time no core 
file generated (also if the error could not repeat, we will never got 
information again), so dumping  target process is also a choice. To 
avoid more confusion, this step was launched at the very last moment, 
just before raise abort to exit. At this moment, if client process could 
not attach to the target process it will exit right away.

Answers to David:

Note that the SA is only present in a full JDK, not a JRE (full or 
compact profile).
   Yes, in code, if no sa-jdi.jar found, skip fork.

- What mechanism will the SA try to use to query the VM?

   After successfully attached to target process, SA will read via proc 
APIs and vmStructs (named TYPEDB) to iterate  memory of system 
dictionary read (only)  from target process to dump class information.

- What if the state of the crashed VM stops the SA from being able to 
attach properly (ie both processes hang)?

   That should be an attaching API problem. We haven't watched such case 
happened so far.

- What if it the SA also crashes, will it launch a third VM then a 
fourth etc?

   Definitely don't want to see this happened in a chain. The solution 
may use a property such as 
sun.jvm.hotspot.DumpLoadedClasses.dumpingInProcess=true to pass into SA 
process, at launching call, check if the property set, if set, do not 
fork. When SA process died, it will generate core file first, note the 
target process still waiting for its exit, so when target exit, the core 
file (if both use default core as name) will be override by target. The 
SA process will only leave a hs_err_pid*.log file. (? read such property 
in handler is possible?)

  Also what is the nature of this dump? How big is it? Where will it go?
  The jars includes *app.jar and *boot.jar, the later usually can be 
ignored (rt.jar etc system jars) unless it's rewritten by JVMTI agent. 
The app.jar will contain all classes by customer, so we can do whatever 
we can to the jar.


Thanks
Yumin


On 8/12/2013 5:51 AM, Christian Tornqvist wrote:
> Hi Yumin,
>
> The idea is to do as little as possible in the VM error handler, since we've crashed for some reason we don't know what state the process is in and we have to be extremely careful in when we're gathering the information. This seems like a step that is risky for all of the reasons David mentioned below.
>
> It's also information that can easily be extracted post-mortem from the core/mdmp using the method you described for OSX, so gathering this at the time of a crash seems like an unnecessary risk.
>
> Thanks,
> Christian
>
> -----Original Message-----
> From: hotspot-compiler-dev-bounces at openjdk.java.net [mailto:hotspot-compiler-dev-bounces at openjdk.java.net] On Behalf Of David Holmes
> Sent: Monday, August 12, 2013 1:56 AM
> To: Yumin Qi
> Cc: hotspot compiler; hotspot-runtime-dev at openjdk.java.net
> Subject: Re: RFR: 8020962: dump loaded java classes when vm crash
>
> Hi Yumin,
>
> Note that the SA is only present in a full JDK, not a JRE (full or compact profile).
>
> I have quite a few concerns with this:
>
> We already do a lot of things that are not valid to be done from a signal handling context but this really takes that to an extreme. Doing fork-exec from a signal handler seems like a recipe for disaster (Note the existing onError facility is typically used for synchronous failures.)
>
> The idea of launching a second VM to try and query a VM that has crashed also seems somewhat problematic:
> - What mechanism will the SA try to use to query the VM?
> - What if the state of the crashed VM stops the SA from being able to attach properly (ie both processes hang)?
> - What if it the SA also crashes, will it launch a third VM then a fourth etc?
>
> Also what is the nature of this dump? How big is it? Where will it go?
>
> Thanks,
> David
>
> On 12/08/2013 9:36 AM, Yumin Qi wrote:
>> Hi, all
>>
>>     I would like to have your review for
>>
>> http://cr.openjdk.java.net/~minqi/8020962/webrev0/
>> <http://cr.openjdk.java.net/%7Eminqi/8020962/webrev0/>
>>
>> Description: When JVM crashed, we also want to check the application
>> class files especially we got core file from customers.  The aftermath
>> analysis will benefit from all loaded java classes available. In this
>> change, spawn another process running SA to do the job when JVM
>> crashes, this way also avoid further messing up with the error report
>> which already in signal handler.
>>
>> Note: The test has done with following two bugs worked around:
>>     8022655: ClassDump ignored jarStream setting (This will be fixed
>> and integrated by Kevin Walls soon)
>>     8011888: sa.js: TypeError: [object JSAdapter] has no such function
>> "__has__" (Not know when it will be integrated)
>>     That is, without those two fixed, the jars of loaded classes will
>> not be successfully dumped.
>>     Also, on MacOS it requires security access permission to attach to
>> another process, so omit doing so. To get loaded jar file s, with core
>> file available (on all platforms), one can (only after this
>> change) do
>>
>>     $JAVA_HOME/bin/java [-d64] -cp $JAVA_HOME/lib/sa-jdi.jar
>> sun.jvm.hotspot.DumpLoadedClasses $JAVA_HOME/bin/java corefile
>>
>>
>> Thanks
>> Yumin