RFR: 8229517: Support for optional asynchronous/buffered logging [v12]

[Xin Liu]

On Fri, 7 May 2021 08:24:03 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Xin Liu has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Change option AsyncLogBufferEntries to AsyncLogBufferSize.
>>   
>>   AsyncLogBufferSize is the memory budget in bytes for Asynchronous Logging.
>>   User can specify it in shorthand form. eg. -XX:AsyncLogBufferSize=10M.
>
> src/hotspot/share/logging/logAsyncFlusher.cpp line 206:
> 
>> 204: 
>> 205: LogAsyncFlusher* LogAsyncFlusher::instance() {
>> 206:   if (Thread::current_or_null() != nullptr) {
> 
> I don't understand why this depends on the existence of a current thread? Ideally we should be able to log just fine in corner cases, e.g. when thread is detached or if we have an error (during signal handling e..g.)

`LogAsyncFlusher::enqueue` needs Thread::current() to use _lock. 

void Mutex::lock_without_safepoint_check() {
  lock_without_safepoint_check(Thread::current());
}


This check is for a very rare corner case.  This can be trigger using the following cmdline.
`
java -Xlog:'thread+smr=debug:file=/tmp/t.log' -Xlog:async -version
`
the root cause is the following code in thread


void ThreadsSMRSupport::smr_delete(JavaThread *thread) {
  elapsedTimer timer;
  if (EnableThreadSMRStatistics) {
    timer.start();
  }

  wait_until_not_protected(thread);

  delete thread; // Thread::current() will trigger assert(current != __null) failed: Thread::current() called on detached thread after ~Thread()
  if (EnableThreadSMRStatistics) {
    timer.stop();
    uint millis = (uint)timer.milliseconds();
    ThreadsSMRSupport::inc_deleted_thread_cnt();
    ThreadsSMRSupport::add_deleted_thread_times(millis);
    ThreadsSMRSupport::update_deleted_thread_time_max(millis);
  }
  // logsite will crash  in async mode because we can't use Thread::current() here. 
  log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_delete: thread=" INTPTR_FORMAT " is deleted.", os::current_thread_id(), p2i(thread));
}

-------------

PR: https://git.openjdk.java.net/jdk/pull/3135