RFR: 8369283: Improve trace logs in safepoint machinery [v6]

[Aleksey Shipilev]

On Thu, 9 Oct 2025 04:40:07 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> I reasoned it is way too noisy to be useful even for debugging. 
>> 
>> Note this is `thread_not_running()` checker, which is called from two places in `SafepointSynchronize::synchronize_threads`: initial filtering and on-going checks for state changes. In well-behaved applications most of the threads are actually not running, blocked somewhere in idle thread pool. So prnting this from the initial filtering dumps info on potentially thousands of threads we do not care about 99.9% of the time.
>> 
>> I argued to myself that what we really care about are the threads that are _transiting_ from runnable to blocked. So the new `log_trace(safepoint)` statements are in caller code that uses this `thread_not_running()` checker, and is able to print exactly what they see. "thread is now blocked", for example.
>> 
>> I don't think there are useful bits in `TSS`, really. We can amend trace logs to pull some of that, if you want. Look what it prints in current mainline:
>> 
>> 
>> [792737314ns] Thread: 0x0000785db85bf4c0  [0x214944] State: _running _at_poll_safepoint 0
>> [792737594ns]    JavaThread state: _thread_in_Java
>> [792738025ns] Thread: 0x0000785db85c08e0  [0x214945] State: _running _at_poll_safepoint 1
>> [792738246ns]    JavaThread state: _thread_in_vm
>
> I thought it was intended to report which threads still running - i.e. are not reaching the safepoint.

Yes, I think so. This PR moves the logging in the code that _calls_ `thread_not_running()`, so that it give us a more useful messages on what we expected to see: still running, now blocked, etc. So it is only half  removed: it gets pulled to callers that we care about.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27673#discussion_r2416000769