On Tue, 22 Sep 2020 13:17:05 GMT, Stefan Karlsson <stef...@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. > > src/hotspot/cpu/x86/templateInterpreterGenerator_x86_64.cpp line 194: > >> 192: >> 193: Label slow_path; >> 194: __ safepoint_poll(slow_path, r15_thread, true /* at_return */, >> false /* in_nmethod */); > > Why is this tagged with 'true /* at_return */? Same for line 240. The _x86_32 > version uses false. Good point. > src/hotspot/share/c1/c1_Runtime1.cpp line 515: > >> 513: if (thread->last_frame().is_runtime_frame()) { >> 514: // The Runtime1::handle_exception_from_callee_id handler is invoked >> after the >> 515: // frame has been unwinded. It instead builds its own stub frame, >> to call the > > Unwided -> unwound, maybe? Same below and probably other places as well. Good point. ------------- PR: https://git.openjdk.java.net/jdk/pull/296
