RFR: 8298853: JvmtiVTMSTransitionDisabler should support disabling one virtual thread transitions [v5]

[Patricio Chilano Mateo]

On Sat, 24 Dec 2022 04:14:07 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>> Now the `JvmtiVTMSTransitionDisabler` mechanism supports disabling VTMS transitions for all virtual threads only. It should also support disabling transitions for any specific virtual thread as well. This will improve scalability of the JVMTI functions operating on target virtual threads as the functions can be executed concurrently without blocking each other execution when target virtual threads are different.
>> New constructor `JvmtiVTMSTransitionDisabler(jthread vthread)` is added which has jthread parameter of the target virtual thread.
>> 
>> Testing:
>>   mach5 jobs are TBD (preliminary testing was completed):
>>    - all JVMTI, JDWP, JDI and JDB tests have to be run
>>    - Kitchensink
>>    - tier5
>
> Serguei Spitsyn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix race between VTMS_transition_disable_for_one and start_VTMS_transition

Hi Serguei,

I looked at the latest changes and it all looks good to me. Thanks!

Patricio

src/hotspot/share/prims/jvmtiThreadState.cpp line 376:

> 374:   Atomic::dec(&_VTMS_transition_disable_for_one_count);
> 375:   if (_VTMS_transition_disable_for_one_count == 0 || _is_SR) {
> 376:     ml.notify_all();

I was going to say that we don't really need the global _VTMS_transition_disable_for_one_count since we can notify here if java_lang_Thread::VTMS_transition_disable_count(vth()) is zero, which is what the target checks. But I realized everybody uses the same lock so we would be notifying everybody. Using _VTMS_transition_disable_for_one_count as it is at least we notify everybody at once when there are no more single disablers. Not sure if that was the purpose, otherwise I think we could remove it.

src/hotspot/share/prims/jvmtiThreadState.cpp line 479:

> 477: 
> 478:   // Unblock waiting VTMS transition disablers.
> 479:   if (_VTMS_transition_disable_for_one_count > 0 ||

In here it would actually be the other way. If we would check java_lang_Thread::VTMS_transition_disable_count(vth()) > 0 instead of the global _VTMS_transition_disable_for_one_count we would avoid the notify in case there are singler disablers waiting but not for this thread.

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Marked as reviewed by pchilanomate (Reviewer).

PR: https://git.openjdk.org/jdk/pull/11690