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

David Holmes david.holmes at oracle.com
Tue Aug 9 00:52:28 UTC 2016

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.


> 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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