RFR: 8369283: Improve trace logs in safepoint machinery

[Aleksey Shipilev]

On Tue, 7 Oct 2025 12:42:20 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> During the performance investigations in safepoint machinery (notably [JDK-8350324](https://bugs.openjdk.org/browse/JDK-8350324)), I found the trace logging lacking. It would be good to improve it in order to finely profile various microscopic things like thread list walks, the blocking/resuming of Java threads, etc. For [JDK-8350324](https://bugs.openjdk.org/browse/JDK-8350324), for example, I want to be able to measure the Java thread delays if they assist in avalanche wakeups.
> 
> This leans heavily on unified logging and invariant clocks to do the right thing, but I think it is a fair compromise for simplicity. The configuration I found most useful for testing is:
> 
> 
> -Xlog:async -Xlog:safepoint=trace:file=safepoint.log:uptimenanos
> 
> 
> My tentative plan is to visualize this more comprehensively with a little tool that digests that log.
> 
> I am open for bikeshedding on logging wording. The log example is in the comment below.
> 
> Additional testing:
>  - [x] Ad-hoc log peeking
>  - [ ] Linux x86_64 server fastdebug, `tier1`

Oops, cannot touch `JavaThread::name()` on this safepoint path, because it can allocate. Back to thread pointers only then.

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PR Comment: https://git.openjdk.org/jdk/pull/27673#issuecomment-3377463872