RFR: 8253180: ZGC: Implementation of JEP 376: ZGC: Concurrent Thread-Stack Processing

[Erik Österlund]

On Tue, 22 Sep 2020 13:17:05 GMT, Stefan Karlsson <stefank at 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.

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PR: https://git.openjdk.java.net/jdk/pull/296