On Mon, 28 Sep 2020 12:14:11 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 incrementally with one additional > commit since the last revision: > > Review: Albert CR2 and defensive programming src/hotspot/share/compiler/oopMap.cpp line 319: > 317: #ifdef ASSERT > 318: if ((((uintptr_t)loc & (sizeof(oop)-1)) != 0) || > 319: > !Universe::heap()->is_in_or_null((oop)NativeAccess<AS_NO_KEEPALIVE>::oop_load(&val))) > { This NativeAccess call causes the troubles with Shenandoah GC. It triggers evacuation of the object referenced by the stack location during a safepoint, which we'd rather avoid. Why is it needed? ------------- PR: https://git.openjdk.java.net/jdk/pull/296
