8271513: support JavaThreadIteratorWithHandle replacement by new ThreadsList::Iterator #4948

dcubed-ojdk · 2021-07-30T19:19:49Z

A trivial fix to support JavaThreadIteratorWithHandle replacement by new ThreadsList::Iterator.

This fix was tested with Mach5 Tier[1-3].

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Issue

JDK-8271513: support JavaThreadIteratorWithHandle replacement by new ThreadsList::Iterator

Reviewers

Kim Barrett (@kimbarrett - Reviewer) ⚠️ Review applies to b3edb5c
David Holmes (@dholmes-ora - Reviewer)
Coleen Phillimore (@coleenp - Reviewer)

Contributors

Kim Barrett <kbarrett@openjdk.org>

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.java.net/jdk pull/4948/head:pull/4948
$ git checkout pull/4948

Update a local copy of the PR:
$ git checkout pull/4948
$ git pull https://git.openjdk.java.net/jdk pull/4948/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 4948

View PR using the GUI difftool:
$ git pr show -t 4948

Using diff file

Download this PR as a diff file:
https://git.openjdk.java.net/jdk/pull/4948.diff

…res.

…adsList::Iterator

bridgekeeper · 2021-07-30T19:20:02Z

👋 Welcome back dcubed! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2021-07-30T19:21:38Z

@dcubed-ojdk The following label will be automatically applied to this pull request:

hotspot-runtime

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

dcubed-ojdk · 2021-07-30T21:33:41Z

/contributor add @kimbarrett

openjdk · 2021-07-30T21:35:15Z

@dcubed-ojdk
Contributor Kim Barrett <kbarrett@openjdk.org> successfully added.

mlbridge · 2021-07-30T21:39:47Z

Webrevs

kimbarrett

Looks good, and trivial.

dcubed-ojdk · 2021-07-31T14:22:21Z

@kimbarrett - Thanks for the review!

dcubed-ojdk · 2021-08-01T13:37:18Z

Since @kimbarrett is listed as the Contributor, I need another (R)eviewer.
I can't be listed as a (R)eviewer because I created the PR. Any takers?

dholmes-ora

Sorry Dan, while I like the use of the for-each loops very much I don't understand the details of the iterator declaration. I definitely do not classify this as a trivial change.

Thanks,
David

dholmes-ora · 2021-08-02T04:11:35Z

src/hotspot/share/runtime/threadSMR.hpp

    return _list_ptr.list();
  }

+  using Iterator = ThreadsList::Iterator;


Is this like a typedef? I don't think I want to see plain "Iterator" and not know what the actual type is here.

It's a type alias, which in this simple form is just like a typedef. Type aliases can also have template parameters. The iterator type used for iterating over a ThreadsListHandle today happens to be the same type as used for iterating over the underlying ThreadsList. There's no particular reason to wire that information in to clients.

So ThreadsListHandle::Iterator is a global(?) alias for ThreadsList::Iterator?

If find this name/type aliasing somewhat obscure and confusing. I mean ideally the thing returned would just be an Iterator and the details of whether it is a Foo::Iterator or Bar::Iterator would not be evident in the API.

dholmes-ora · 2021-08-02T04:13:25Z

src/hotspot/share/runtime/threadSMR.inline.hpp

  }
 }

+ThreadsListHandle::Iterator ThreadsListHandle::begin() { return list()->begin(); }


I don't understand this - why is the return type not ThreadsList::Iterator as per the declaration in the hpp file? That using directive is apparently doing something more bizarre than I thought.

See above. I think I understand this now. Don't like it; but understand it.

I'm not sure what declaration in the hpp file you are referring to. The function declaration specifies a return type of (unqualified) Iterator which is declared (via the type alias) in that class. Outside the scope of that class it needs to be qualified, i.e. ThreadsListHandle::Iterator.

There's nothing bizarre about it. Maybe this looks unfamiliar because nested types are relatively uncommon in HotSpot, and even more so using such in a return type for a definition outside the body of the class.

The definition can't use unqualified Iterator because that wouldn't compile. The leading return type isn't in the scope of the class of the member, so must itself be qualified too. It's qualified by the class of the member, so that it refers to the name in that class. The scoping rules for the return type of a member function are an annoying bit of C++ syntax that can be dealt with differently using C++11 trailing return types , but that's a not yet discussed feature in the style guide. That is, this could have been written as
auto ThreadsListHandle::begin() -> Iterator { return list()->begin(); }
(note the arrow pointing to the return type, which doesn't need qualification because the trailing return type is in the scope of ThreadsListHandle). Or it can be even shorter using type deduction
auto ThreadsListHandle::begin() { return list()->begin(); }
though I'm not sure type deduction helps readability here.

@kimbarrett not knowing how the aliasing works I was looking at it more as a textual substitution so ThreadsListHandle::Iterator looked to me like it would "expand" to ThreadsListHandler::ThreadsList::Iterator (which makes no sense) and I was not reading plain "Iterator" as ThreadsListHandle::Iterator.
I don't see why the ThreadsListHandle methods couldn't just return ThreadsList::Iterator as we know a ThreadsListHandle is just a wrapper for a ThreadsList. I don't think that is something we have to worry about "wiring" into clients. As I said the right style here would have an abstract Iterator type and we wouldn't care about the concrete implementation type. YMMV.

openjdk-notifier · 2021-08-02T16:04:01Z

The dependent pull request has now been integrated, and the target branch of this pull request has been updated. This means that changes from the dependent pull request can start to show up as belonging to this pull request, which may be confusing for reviewers. To remedy this situation, simply merge the latest changes from the new target branch into this pull request by running commands similar to these in the local repository for your personal fork:

git checkout JDK-8193559
git fetch https://git.openjdk.java.net/jdk master
git merge FETCH_HEAD
# if there are conflicts, follow the instructions given by git merge
git commit -m "Merge master"
git push

openjdk · 2021-08-02T16:05:32Z

@dcubed-ojdk This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8271513: support JavaThreadIteratorWithHandle replacement by new ThreadsList::Iterator

Co-authored-by: Kim Barrett <kbarrett@openjdk.org>
Reviewed-by: kbarrett, dholmes, coleenp

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 69 new commits pushed to the master branch:

adb0ae5: 8261441: JFR: Filename expansion
e38e365: 8271208: Typo in ModuleDescriptor.read javadoc
14692d5: Merge
dfacda4: 8270872: Final nroff manpage update for JDK 17
90f85ff: 8271588: JFR Recorder Thread crashed with SIGSEGV in write_klass
f312f28: 8271863: ProblemList serviceability/sa/TestJmapCore.java on linux-x64 with ZGC
62e72ad: 8271293: Monitor class should use ThreadBlockInVMPreprocess
cb36880: 8270116: Expand ButtonGroupLayoutTraversalTest.java to run in all LaFs, including Aqua on macOS
3ab95d1: 8271905: mark hotspot runtime/Metaspace tests which ignore external VM flags
e2c5bfe: 8271308: (fc) FileChannel.transferTo() transfers no more than Integer.MAX_VALUE bytes in one call
... and 59 more: https://git.openjdk.java.net/jdk/compare/0a85236396c667c8d2c890e4384c623b39455075...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

openjdk · 2021-08-02T16:05:40Z

⚠️ @dcubed-ojdk This pull request contains merges that bring in commits not present in the target repository. Since this is not a "merge style" pull request, these changes will be squashed when this pull request in integrated. If this is your intention, then please ignore this message. If you want to preserve the commit structure, you must change the title of this pull request to Merge <project>:<branch> where <project> is the name of another project in the OpenJDK organization (for example Merge jdk:master).

coleenp · 2021-08-03T18:50:10Z

src/hotspot/share/runtime/thread.cpp

    array[i] = _jvmci_old_thread_counters[i];
  }
-  for (JavaThreadIteratorWithHandle jtiwh; JavaThread *tp = jtiwh.next(); ) {
+  for (JavaThread* tp : ThreadsListHandle()) {


This is a nicer interface with underlying C++ magic. There are lots of these in the source code. Is it the intention to replace these as we go?

There's definitely lots of these:

$ grep -r JavaThreadIteratorWithHandle src/hotspot | sed 's/:.*//' | sort | uniq -c 1 src/hotspot/os/linux/os_linux.cpp 1 src/hotspot/share/compiler/compileBroker.cpp 1 src/hotspot/share/gc/parallel/mutableNUMASpace.cpp 2 src/hotspot/share/gc/shared/collectedHeap.cpp 1 src/hotspot/share/gc/shared/gcLocker.cpp 1 src/hotspot/share/gc/shenandoah/shenandoahCodeRoots.cpp 5 src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp 2 src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp 1 src/hotspot/share/jfr/utilities/jfrThreadIterator.cpp 1 src/hotspot/share/jfr/utilities/jfrThreadIterator.hpp 1 src/hotspot/share/opto/idealGraphPrinter.cpp 1 src/hotspot/share/prims/jvmtiEnvBase.cpp 1 src/hotspot/share/prims/jvmtiEventController.cpp 1 src/hotspot/share/prims/jvmtiImpl.cpp 1 src/hotspot/share/prims/jvmtiTagMap.cpp 1 src/hotspot/share/prims/whitebox.cpp 5 src/hotspot/share/runtime/escapeBarrier.cpp 2 src/hotspot/share/runtime/handshake.cpp 1 src/hotspot/share/runtime/os.cpp 8 src/hotspot/share/runtime/safepoint.cpp 1 src/hotspot/share/runtime/thread.cpp 3 src/hotspot/share/runtime/threadSMR.hpp 3 src/hotspot/share/runtime/vmOperations.cpp 1 src/hotspot/share/services/heapDumper.cpp 1 src/hotspot/share/services/management.cpp 1 src/hotspot/share/services/threadService.cpp 1 src/hotspot/share/utilities/globalCounter.cpp

I believe the plan is to migrate to this newer, cleaner interface.
Obviously not with this bug, but down the road...

Awesome, thanks!

dcubed-ojdk · 2021-08-05T16:44:11Z

@kimbarrett, @dholmes-ora and @coleenp - I'm not seeing any commentary in this PR
indicating that there are any agreed up changes to make here. I'm going to double
check my email folder for this bug ID since I know that not everything sent via email
has made it into the PR...

So far, only @kimbarrett has marked this PR as reviewed. However, since this code is
actually his, that doesn't count. My own review doesn't count since I created the PR
so if we don't get another (R)eviewer approval, then this PR won't be moving forward.

coleenp

Looks good! Sorry for the delay, I didn't see the answer to my question at first.

dholmes-ora

Changes are fine now I have a better understanding of the aliasing. Not sure it was really needed but ...

Thanks,
David

dcubed-ojdk · 2021-08-06T14:17:33Z

@coleenp and @dholmes-ora - Thanks for closing the loop on this review thread.

/integrate

openjdk · 2021-08-06T14:19:03Z

Going to push as commit fa36e33.
Since your change was applied there have been 76 commits pushed to the master branch:

cc61520: 8270842: G1: Only young regions need to redirty outside references in remset.
f4cf2f7: 8272095: ProblemList java/nio/channels/FileChannel/Transfer2GPlus.java on linux-aarch64
0aca4f7: 8271868: Warn user when using mac-sign option with unsigned app-image.
b6a19f1: 8271128: InlineIntrinsics support for 32-bit ARM
c2b7fac: 8271896: Remove unnecessary top address checks in BOT
e7b6f48: 8265602: -XX:DumpLoadedClassList should support custom loaders
ea02dad: 8272067: Initial nroff manpage generation for JDK 18
adb0ae5: 8261441: JFR: Filename expansion
e38e365: 8271208: Typo in ModuleDescriptor.read javadoc
14692d5: Merge
... and 66 more: https://git.openjdk.java.net/jdk/compare/0a85236396c667c8d2c890e4384c623b39455075...master

Your commit was automatically rebased without conflicts.

openjdk · 2021-08-06T14:19:15Z

@dcubed-ojdk Pushed as commit fa36e33.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

dcubed-ojdk added 5 commits July 1, 2021 19:59

8193559.cr0

3f653c5

kbarrett CR - delete unused _list member.

5ee4991

Merge branch 'master' into JDK-8193559

a133ca4

8193559.kbarrett.part1 - Kim's proposed rewrite using newer C++ featu…

3be5dc7

…res.

8271513: support JavaThreadIteratorWithHandle replacement by new Thre…

8c255c4

…adsList::Iterator

openjdk bot added the hotspot-runtime hotspot-runtime-dev@openjdk.org label Jul 30, 2021

dcubed-ojdk changed the base branch from master to pr/4671 July 30, 2021 19:52

dcubed-ojdk added 2 commits July 30, 2021 16:36

kbarrett CR - simplify 'ThreadsList::Iterator::operator!=(Iterator i)'

a8865b8

Merge branch 'pull/4671' into JDK-8271513

b3edb5c

dcubed-ojdk marked this pull request as ready for review July 30, 2021 21:33

openjdk bot added the rfr Pull request is ready for review label Jul 30, 2021

openjdk bot mentioned this pull request Jul 30, 2021

8271514: support JFR use of new ThreadsList::Iterator #4949

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kimbarrett approved these changes Jul 31, 2021

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openjdk-notifier bot changed the base branch from pr/4671 to master August 2, 2021 16:03

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Merge branch 'master' into JDK-8271513

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openjdk bot closed this Aug 6, 2021

openjdk bot added integrated Pull request has been integrated and removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Aug 6, 2021

dcubed-ojdk deleted the JDK-8271513 branch August 6, 2021 15:42

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8271513: support JavaThreadIteratorWithHandle replacement by new ThreadsList::Iterator #4948

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