RFR: 8369283: Improve trace logs in safepoint machinery [v9]

[Aleksey Shipilev]

On Thu, 9 Oct 2025 13:40:29 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
>>  - [x] Linux x86_64 server fastdebug, `tier1`
>>  - [x] Linux x86_64 server fastdebug, `all`
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Missing include header

Thanks for reviews, all! The crude parsing tool I now have tells me the logging is fairly complete for the safepoint investigation purposes. The re-run of `make test TEST=all` finished without issues. We can do more stuff as followups, if needed. I am integrating this meanwhile.

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PR Comment: https://git.openjdk.org/jdk/pull/27673#issuecomment-3387113025