RFR(XS): 8215727: Restore JFR thread sampler loop to old / previous behavior (was: [PATCH] JFR thread sampling issue)

[Erik Gahlin]

Looks good.

Erik

> Greetings,
>
> I am sending this RFR on behalf of Milan Mimica to formalize the patch he submitted to address this issue.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8215727
> Webrev: http://cr.openjdk.java.net/~mgronlun/8215727/webrev/
>
> Tests: jdk_jfr
>
> I am a Reviewer for this change.
>
> Thanks a lot Milan for your contribution and patience.
>
> Thanks
> Markus
>
> -----Original Message-----
> From: Milan Mimica <milan.mimica at gmail.com>
> Sent: den 19 november 2018 13:42
> To: hotspot-jfr-dev at openjdk.java.net
> Subject: [PATCH] JFR thread sampling issue
>
> Hello
>
> I was regularly using JFR to profile my service and thread sampling results have been very useful. Unfortunately, when I have switched to Java 9 and afterwards, it stopped working. The recording would catch too few samples for the results to be useful. My service has about 500 threads, most of them being on some blocked state.
>
> With the release of Java 11, I have started digging into the JFR source code and I think I have found the problem. I think JfrThreadSampler::task_stacktrace is supposed to find at most 5 threads that are in state _thread_in_Java, or at most 1 thread that is in _thread_in_native, but instead it just picks next 5 (or 1) threads and then ignores them of they are not in right state. Without the insight into code change prior to JDK 11 I can just guess, but there are some clues that lead me to think that's how it was supposed to work. One of the clues is that sample_task.do_sample_thread returns a result that is otherwise unused.
>
> I'm attaching a patch.
> Tested test-jdk_jfr_sanity on fastdebug, and in my production.
>
> I'm waiting for my Author status approval so I'll be able to create a proper changeset. I believe I also need a ticket for this.
>
>
> --
> Milan Mimica