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

Chris Plummer chris.plummer at oracle.com
Mon Aug 8 20:22:41 UTC 2016

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()?



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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