RFR/RFA (M): 8185003: JMX: Add a version of ThreadMXBean.dumpAllThreads with a maxDepth argument

[Volker Simonis]

On Thu, Aug 20, 2020 at 10:06 PM serguei.spitsyn at oracle.com
<serguei.spitsyn at oracle.com> wrote:
>
> Hi Paul,
>
> I was also wondering if there is a compatibility risk involved with the JMM_VERSION change.
> So, thanks to Volker for asking these questions.
>
> One more question.
> I do not see a backport of the src/jdk.management/share/native/libmanagement_ext/management_ext.c change.
> Is it intentional, and if so, what is the reason to skip this file?
>

"libmanagement_ext/management_ext.c" doesn't exist in jdk8. It was
introduced with "8042901: Allow com.sun.management to be in a
different module to java.lang.management" in jdk9. In jdk8 all the
functionality is in "management/management.h" so there's no need to
backport the changes from "management_ext.c" .

[1] https://bugs.openjdk.java.net/browse/JDK-8042901

> Thanks,
> Serguei
>
>
> On 8/20/20 11:30, Volker Simonis wrote:
>
> On Wed, Aug 19, 2020 at 6:17 PM Hohensee, Paul <hohensee at amazon.com> wrote:
>
> Please review this backport to jdk8u. I especially need a CSR review, since the CSR approval process can be a bottleneck. The patch significantly reduces fleet profiling overhead, and a version of it has been in production at Amazon for over 3 years.
>
>
>
> Original JBS issue: https://bugs.openjdk.java.net/browse/JDK-8185003
>
> Original CSR: https://bugs.openjdk.java.net/browse/JDK-8185705
>
> Original patch: http://hg.openjdk.java.net/jdk10/master/rev/68d46cb9be45
>
>
>
> Backport JBS issue: https://bugs.openjdk.java.net/browse/JDK-8251494
>
> Backport CSR: https://bugs.openjdk.java.net/browse/JDK-8251498
>
> Backport JDK webrev: http://cr.openjdk.java.net/~phh/8185003/webrev.8u.jdk.05/
>
> JDK part looks good to me.
>
> Backport Hotspot webrev: http://cr.openjdk.java.net/~phh/8185003/webrev.8u.hotspot.05/
>
> HotSpot part looks good to me but see discussion below.
>
>
> Details of the interface changes needed for the backport are in the Description of the Backport CSR 8251498. The actual functional changes are minimal and low risk.
>
> I've also reviewed the CSR yesterday which I think is fine. But now,
> when looking at the implementation, I'm a little concerned about
> changing JMM_VERSION from " 0x20010203" to "0x20020000" in "jmm.h".
>
> This might be especially problematic in combination with the changes
> in "Management::get_jmm_interface()" which is called by
> JVM_GetManagement():
>
>  void* Management::get_jmm_interface(int version) {
>  #if INCLUDE_MANAGEMENT
> -  if (version == JMM_VERSION_1_0) {
> +  if (version == JMM_VERSION) {
>      return (void*) &jmm_interface;
>    }
>  #endif // INCLUDE_MANAGEMENT
>    return NULL;
>  }
>
> You've correctly fixed the single caller of "JVM_GetManagement()" in
> the JDK (in "JNI_OnLoad()" in "management.c"):
>
> -    jmm_interface = (JmmInterface*) JVM_GetManagement(JMM_VERSION_1_0);
> +    jmm_interface = (JmmInterface*) JVM_GetManagement(JMM_VERSION);
>
> but I wonder if there are other monitoring/serviceability tools out
> there which use this interface and which will break after this change.
> A quick search revealed at least two StackOverflow entries which
> recommend using "JVM_GetManagement(JMM_VERSION_1_0)" [1,2] and there's
> a talk and a blog entry doing the same [3, 4].
>
> I'm not sure how relevant this is but I think a much safer and
> backwards-compatible way of doing this downport would be the
> following:
>
> - don't change "Management::get_jmm_interface()" (i.e. still check for
> "JMM_VERSION_1_0") but return the "new" JMM_VERSION in
> "jmm_GetVersion()". This won't break anything but will make it
> possible for clients to detect the new version if they want.
>
> - don't change the signature of "DumpThreads()". Instead add a new
> version (e.g. "DumpThreadsMaxDepth()/jmm_DumpThreadsMaxDepth()") to
> the "JMMInterface" struct and to "jmm_interface" in "management.cpp".
> You can do this in one of  the two first, reserved fields of
> "JMMInterface" so you won't break binary compatibility.
> "jmm_DumpThreads()" will then be a simple wrapper which calls
> "jmm_DumpThreadsMaxDepth()" with Integer.MAX_VALUE as depth.
>
> - in the jdk you then simply call "DumpThreadsMaxDepth()" in
> "Java_sun_management_ThreadImpl_dumpThreads0()"
>
> I think this way we can maintain full binary compatibility while still
> using the new feature. What do you think?
>
> Best regards,
> Volker
>
> [1] https://stackoverflow.com/questions/23632653/generate-java-heap-dump-on-uncaught-exception
> [2] https://stackoverflow.com/questions/60887816/jvm-options-printnmtstatistics-save-info-to-file
> [3] https://sudonull.com/post/25841-JVM-TI-how-to-make-a-plugin-for-a-virtual-machine-Odnoklassniki-company-blog
> [4] https://2019.jpoint.ru/talks/2o8scc5jbaurnqqlsydzxv/
>
> Passes the included (suitably modified) test, as well as the tests in
>
>
>
> jdk/test/java/lang/management/ThreadMXBean
>
> jdk/test/com/sun/management/ThreadMXBean
>
>
>
> Thanks,
>
> Paul
>
>