RFR(M): 8133749, 8133747, 8133740: NMT detail stack trace cleanup

Chris Plummer chris.plummer at oracle.com
Wed Aug 10 03:56:18 UTC 2016

On 8/8/16 5:52 PM, David Holmes wrote:
> On 9/08/2016 6:22 AM, Chris Plummer wrote:
>> Hi David,
>> Did you want me to implement any of the additional cleanup work I
>> mentioned: manually inline _get_previous_fp, change os::current_frame()
>> to walk back one less frame, possibly rename os::current_frame()?
> Up to you. I'm not insisting on anything, but the less reliance we 
> have on uncheckable (at build time) compiler behaviour, the better.
Ok. Given the relatively short amount of time left to resolve p4s, I 
think I will just leave it as-is. I've started some more robust testing 
the NMT detailed enabled. Once that completes I'll do the push. I'll 
also file a couple of RFEs for further cleanup/improvements.

Can I consider the changes officially reviewed by you now?


> Thanks,
> David
>> thanks,
>> Chris
>> On 8/7/16 4:26 PM, David Holmes wrote:
>>> Hi Chris,
>>> I don't have any good suggestions for this. So go with (2) and lets
>>> work on (3).
>>> Thanks,
>>> David
>>> On 5/08/2016 5:05 PM, Chris Plummer wrote:
>>>> Hi David,
>>>> If fixing os::current_frame() to have a better name and also make 
>>>> it go
>>>> up one less frame makes these changes more palatable, I'm willing to
>>>> make that change. I would prefer to do it with a follow up CR (it 
>>>> would
>>>> probably have to be an RFE), but will do it with these changes if
>>>> necessary. I still pull hairs over the proper name for this method, 
>>>> even
>>>> if it is modified to return the frame of whoever called it. Usually 
>>>> the
>>>> meaning conveyed by a method's name does not change based on 
>>>> whether you
>>>> choose the caller's or callee's point of view, but in this case it 
>>>> does,
>>>> and I'm not sure which point of view makes more sense. If we choose 
>>>> the
>>>> caller's point of view, then the proper name remains
>>>> os::current_frame(). If we choose the callee's point of view, then it
>>>> should be os::callers_frame(). Maybe there's a name that is 
>>>> agnostic and
>>>> means the same thing from both view points. I just haven't thought of
>>>> one yet.
>>>> With respect to ALWAYSINLINE, it does not work for solaris and windows
>>>> slowdebug builds. Note the special case in the test I wrote to 
>>>> allow for
>>>> AllocateHeap() in the stack trace in this case, even though it 
>>>> shouldn't
>>>> be there because it uses ALWAYSINLINE. I could have made changes in 
>>>> the
>>>> source to get rid of it from the stack trace, but I didn't feel the
>>>> source code disruption was worth it for a slowdebug build, especially
>>>> since there are only a allocation call sites where it is a problem. I
>>>> could use ALWAYSINLINE for the cases where it will work to inline
>>>> _get_previous_fp, but I don't really see that as being any more 
>>>> reliable
>>>> than what is there now.
>>>> As for making _get_previous_fp() a macro, that's made more complicated
>>>> because it has #ifdefs already. I could move its implementation 
>>>> directly
>>>> into os::current_frame(). That would fix the inlining problem. I think
>>>> it could also use some cleanup with the #ifdefs. For example, for
>>>> linux-x86 do we have to worry about the SPARC_WORKS and __clang__ 
>>>> cases?
>>>> And yes, even with my changes the code is no less fragile, and no less
>>>> misdirected in its approach to getting a consistent allocation back
>>>> trace.  As I see it, there are 3 options:
>>>> (1) Do nothing, and leave it both broken and fragile.
>>>> (2) Do the cleanup I've done to at least correct the known stack trace
>>>> issues.
>>>> (3) Find another solution that doesn't suffer from these fragility
>>>> issues.
>>>> Note that (3) does not preclude doing (2) first, and (2) seems a 
>>>> better
>>>> alternative than leaving it in its broken state (1). That's why I have
>>>> pursued these changes even though I know things will still be fragile.
>>>> thanks,
>>>> Chris
>>>> On 8/4/16 9:47 PM, David Holmes wrote:
>>>>> Hi Chris,
>>>>> On 5/08/2016 7:53 AM, Chris Plummer wrote:
>>>>>> Ping!
>>>>> I took another look at this and my earlier comments from JDK-8133749.
>>>>> I hate to see the functionality "fixed" yet still have a completely
>>>>> confusing and mis-named API. I'm still far from convinced that
>>>>> returning the callers caller wasn't an "error" that was done due to
>>>>> the lack of inlining and the appearance of an unexpected stackframe.
>>>>> You've now made things consistent - but os::current_frame() is
>>>>> completely mis-leading in name. And I'm still concerned that
>>>>> correctness here depends on C compiler inlining choices, with no way
>>>>> to verify at build time that they were indeed inlined or not! 
>>>>> Don't we
>>>>> have ALWAYSINLINE to mark things like _get_previous_fp ? For that
>>>>> matter shouldn't _get_previous_fp be a macro so inlining plays no
>>>>> role ?
>>>>> Sorry but this code seems to simply limp from one broken state to
>>>>> another due to its fragility.
>>>>> Thanks,
>>>>> David
>>>>> -----
>>>>>> On 8/2/16 1:31 PM, Chris Plummer wrote:
>>>>>>> Hello,
>>>>>>> Please review the following:
>>>>>>> webrev:
>>>>>>> http://cr.openjdk.java.net/~cjplummer/8133749-8133747-8133740/webrev-01/webrev.hotspot/ 
>>>>>>> Bugs fixed:
>>>>>>> JDK-8133749: os::current_frame() is not returning the proper 
>>>>>>> frame on
>>>>>>> ARM and solaris-x64
>>>>>>> https://bugs.openjdk.java.net/browse/JDK-8133749
>>>>>>> JDK-8133747: NMT includes an extra stack frame due to assumption 
>>>>>>> NMT
>>>>>>> is making on tail calls being used
>>>>>>> https://bugs.openjdk.java.net/browse/JDK-8133747
>>>>>>> JDK-8133740: NMT for Linux/x86/x64 and bsd/x64 slowdebug builds
>>>>>>> includes NativeCallStack::NativeCallStack() frame in backtrace
>>>>>>> https://bugs.openjdk.java.net/browse/JDK-8133740
>>>>>>> The above bugs all result in the NMT detail stack traces including
>>>>>>> extra frames in the stack traces. Certain frames are suppose to be
>>>>>>> skipped, but sometimes are not. The frames that show up are:
>>>>>>>   NativeCallStack::NativeCallStack
>>>>>>>   os::get_native_stack
>>>>>>> These are both methods used to generate the stack trace, and
>>>>>>> therefore
>>>>>>> should not be included it.  However, under some (most) 
>>>>>>> circumstances,
>>>>>>> they were.
>>>>>>> Also, there was no test to make sure that any NMT detail output is
>>>>>>> generated, or that it is correct. I've added one with this 
>>>>>>> webrev. Of
>>>>>>> the 27 possible builds (9 platforms * 3 build flavors), only 9 
>>>>>>> of the
>>>>>>> 27 initially passed this new test. They were the product and
>>>>>>> fastdebug
>>>>>>> builds for solaris-sparc, bsd-x64, and linux-x64; and the slowdebug
>>>>>>> builds for solaris-x64, windows-x86, and windows-x64. All the rest
>>>>>>> failed. They now all pass with my fixes in place.
>>>>>>> Here's a summary of the changes:
>>>>>>> src/os/posix/vm/os_posix.cpp
>>>>>>> src/os/windows/vm/os_windows.cpp
>>>>>>> JDK-8133747 fixes: There was some frame skipping logic here 
>>>>>>> which was
>>>>>>> sort of correct, but was misplace. There are no extra frames being
>>>>>>> added in os::get_native_stack() due to lack of inlining or lack 
>>>>>>> of a
>>>>>>> tail call, so no need for toSkip++ here. The logic has been 
>>>>>>> moved to
>>>>>>> NativeCallStack::NativeCallStack, which is where the tail call is
>>>>>>> (sometimes) made, and also corrected (see nativeCallStack.cpp 
>>>>>>> below).
>>>>>>> src/share/vm/utilities/nativeCallStack.cpp
>>>>>>> JDK-8133747 fixes: The frame skipping logic that was moved here
>>>>>>> assumed that NativeCallStack::NativeCallStack would not appear 
>>>>>>> in the
>>>>>>> call stack (due to a tail call be using to call 
>>>>>>> os::get_native_stack)
>>>>>>> except in slow debug builds. However, some platforms also don't 
>>>>>>> use a
>>>>>>> tail call even when optimized. From what I can tell that is the 
>>>>>>> case
>>>>>>> for 32-bit platforms and for windows.
>>>>>>> src/os_cpu/bsd_x86/vm/os_bsd_x86.cpp
>>>>>>> src/os_cpu/windows_x86/vm/os_windows_x86.cpp
>>>>>>> src/os_cpu/linux_x86/vm/os_linux_x86.cpp
>>>>>>> JDK-8133740 fixes: When _get_previous_fp is not inlined, we need to
>>>>>>> skip one extra frame
>>>>>>> src/os_cpu/solaris_x86/vm/os_solaris_x86.cpp
>>>>>>> JDK-8133749 fixes: os:current_frame() was not consistent with other
>>>>>>> platforms and needs to skip one more frame. This means it 
>>>>>>> returns the
>>>>>>> frame for the caller's caller. So when called by
>>>>>>> os:get_native_stack(), it returns the frame for whoever called
>>>>>>> os::get_native_stack(). Although not intuitive, this is what
>>>>>>> os:get_native_stack() expects. Probably a method rename and/or a
>>>>>>> behavior change is justified here, but I would prefer to do that 
>>>>>>> with
>>>>>>> a followup CR if anyone has a good suggestion on what to do.
>>>>>>> test/runtime/NMT/CheckForProperDetailStackTrace.java
>>>>>>> This is the new NTM detail test. It checks for frames that 
>>>>>>> shouldn't
>>>>>>> be present and validates at least one stack trace is what is
>>>>>>> expected.
>>>>>>> I verified that the above test now passes on all supported 
>>>>>>> platforms,
>>>>>>> and also did a full jprt "-testset hotpot" run. I plan on doing 
>>>>>>> some
>>>>>>> RBT testing with NMT detail enabled before committing.
>>>>>>> Regarding the community contributed ports that Oracle does not
>>>>>>> support, I didn't make any changes there, but it looks like some of
>>>>>>> these bugs do exist. Notably:
>>>>>>> -linux-aarch64: Looks like it suffers from JDK-8133740. The changes
>>>>>>> done to the
>>>>>>>  os_linux_x86.cp should also be applied here.
>>>>>>> -linux-ppc: Hard to say for sure since the implementation of
>>>>>>> os::current_frame is
>>>>>>>  different than others, but it looks to me like it suffers from 
>>>>>>> both
>>>>>>> JDK-8133749
>>>>>>>  and JDK-8133740.
>>>>>>> -aix-ppc: Looks to be the same implementation as linux-ppc, so 
>>>>>>> would
>>>>>>> need the
>>>>>>>  same changes.
>>>>>>> These ports may also be suffering from JDK-8133747, but that fix
>>>>>>> is in
>>>>>>> shared code (nativeCallStack.cpp). My changes there will need some
>>>>>>> tweaking for these ports they don't use a tail call to call
>>>>>>> os::get_native_stack().
>>>>>>> If the maintainers of these ports could send me some NMT detail
>>>>>>> output, I can advise better on what changes are needed. Then you 
>>>>>>> can
>>>>>>> implement and test them, and then send them back to me and I'll
>>>>>>> include them with my changes. What I need is the following command
>>>>>>> run
>>>>>>> on product and slowdebug builds. Initially run without any of my
>>>>>>> changes applied. If needed I may followup with a request that 
>>>>>>> they be
>>>>>>> run with the changes applied:
>>>>>>> bin/java -XX:+UnlockDiagnosticVMOptions
>>>>>>> -XX:NativeMemoryTracking=detail -XX:+PrintNMTStatistics -version
>>>>>>> thanks,
>>>>>>> Chris

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