On Thu, 24 Sep 2020 12:27:42 GMT, Erik Österlund <eosterl...@openjdk.org> wrote:
>> This PR the implementation of "JEP 376: ZGC: Concurrent Thread-Stack >> Processing" (cf. >> https://openjdk.java.net/jeps/376). >> Basically, this patch modifies the epilog safepoint when returning from a >> frame (supporting interpreter frames, c1, c2, >> and native wrapper frames), to compare the stack pointer against a >> thread-local value. This turns return polls into >> more of a swiss army knife that can be used to poll for safepoints, >> handshakes, but also returns into not yet safe to >> expose frames, denoted by a "stack watermark". ZGC will leave frames (and >> other thread oops) in a state of a mess in >> the GC checkpoint safepoints, rather than processing all threads and their >> stacks. Processing is initialized >> automagically when threads wake up for a safepoint, or get poked by a >> handshake or safepoint. Said initialization >> processes a few (3) frames and other thread oops. The rest - the bulk of the >> frame processing, is deferred until it is >> actually needed. It is needed when a frame is exposed to either 1) execution >> (returns or unwinding due to exception >> handling), or 2) stack walker APIs. A hook is then run to go and finish the >> lazy processing of frames. Mutator and GC >> threads can compete for processing. The processing is therefore performed >> under a per-thread lock. Note that disarming >> of the poll word (that the returns are comparing against) is only performed >> by the thread itself. So sliding the >> watermark up will require one runtime call for a thread to note that nothing >> needs to be done, and then update the poll >> word accordingly. Downgrading the poll word concurrently by other threads >> was simply not worth the complexity it >> brought (and is only possible on TSO machines). So left that one out. > > Erik Österlund has updated the pull request with a new target base due to a > merge or a rebase. The pull request now > contains seven commits: > - Review: Per CR 1 > - Merge branch 'master' into 8253180_conc_stack_scanning > - Review: Albert CR 1 > - Review: SteafanK CR 2 > - Merge branch 'master' into 8253180_conc_stack_scanning > - Review: SteafanK CR 1 > - 8253180: ZGC: Implementation of JEP 376: ZGC: Concurrent Thread-Stack > Processing src/hotspot/share/runtime/mutexLocker.cpp line 249: > 247: def(Patching_lock , PaddedMutex , special, true, > _safepoint_check_never); // used for > safepointing and code patching. 248: def(CompiledMethod_lock , > PaddedMutex , special-1, true, > _safepoint_check_never); 249: def(Service_lock , > PaddedMonitor, tty-2, true, > _safepoint_check_never); // used for service thread operations @pchilano Look. tty-2 ------------- PR: https://git.openjdk.java.net/jdk/pull/296