RFR: JDK-8132849: Increased stop time in cleanup phase because of single-threaded walk of thread stacks in NMethodSweeper::mark_active_nmethods()

[Roman Kennke]

Hi Zhengyu,

>> Zhengyu noted off-list that the !ThreadLocalHandshakes version requires
>> to call Threads::change_thread_parity() before using
>> Threads::possibly_parallel_threads_do(), and that we can assert
>> is_Java_thread() instead of explicit filtering.
> 
> My bad suggestion on assertion for Java thread,
> Threads::possibly_parallel_threads_do also walks VMThread, sorry!

Yes, I noticed that, and updated/reverted the webrev accordingly:
http://cr.openjdk.java.net/~rkennke/JDK-8132849/webrev.05/

> Otherwise, looks good to me.

Thanks for reviewing!
Roman

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