RFR: 8360817: [ubsan] zDirector select_worker_threads - outside the range of representable values issue

[Axel Boldt-Christmas]

On Tue, 29 Jul 2025 09:36:30 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> When running the jtreg test
>> gc/z/TestMappedCacheHarvest.java
>> with ubsan-enabled binaries on macOS aarch64, the following ubsan error is reported :
>> 
>> 
>>  stderr: [/jdk/src/hotspot/share/gc/z/zDirector.cpp:715:33: runtime error: 6.1962e+10 is outside the range of representable values of type 'unsigned int'
>>     #0 0x109e368fc in select_worker_threads(ZDirectorStats const&, unsigned int, ZWorkerSelectionType) zDirector.cpp:715
>>     #1 0x109e3658c in initial_workers(ZDirectorStats const&, ZWorkerSelectionType) zDirector.cpp:804
>>     #2 0x109e35df0 in ZDirector::run_thread() zDirector.cpp:932
>>     #3 0x109ecd5f4 in ZThread::run_service() zThread.cpp:28
>>     #4 0x108cd1e50 in ConcurrentGCThread::run() concurrentGCThread.cpp:47
>>     #5 0x109ce770c in Thread::call_run() thread.cpp:243
>>     #6 0x10985be28 in thread_native_entry(Thread*) os_bsd.cpp:599
>>     #7 0x19fa8ef90 in _pthread_start+0x84 (libsystem_pthread.dylib:arm64e+0x6f90)
>>     #8 0x19fa89d30 in thread_start+0x4 (libsystem_pthread.dylib:arm64e+0x1d30)
>> 
>> 
>> After I added a bit of tracing to `select_worker_threads` it seems we get WAY too high young_to_old_ratio values; in my locally failing example
>> young_to_old_ratio:27561965412.478878
>> this leads to values out of range of uint and that makes ubsan complain about the uint cast 
>> `uint(young_workers * young_to_old_ratio)` .
>> We clamp the value anyway to the range 1u to ZOldGCThreads ; so it probably makes sense to avoid such high factors for young_to_old_ratio .
>
> src/hotspot/share/gc/z/zDirector.cpp line 714:
> 
>> 712:   }
>> 713: 
>> 714:   double young_to_old_ratio = calculate_young_to_old_worker_ratio(stats);
> 
> I think we might as well just clamp the return value in `calculate_young_to_old_worker_ratio`. Especially since it is a local helper function which is only used here. 
> ```c++
>   return MIN2(old_vs_young_efficiency_ratio, (double)ZOldGCThreads);
> 
> It is suppose to calculate the worker ratio. And the largest ratio we can have is ZOldGCThreads old workers to 1 young worker.

The ratio should probably also be called `old_to_young` as it is what it actually is, not `young_to_old`. But that is a preexisting issue which we can fix in a separate rfe.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26186#discussion_r2239169493