jdk8-mips64-public/hotspot: src/share/vm/runtime/safepoint.cpp@bfe29ec02863 (annotated)

src/share/vm/runtime/safepoint.cpp@bfe29ec02863 (annotated)

src/share/vm/runtime/safepoint.cpp

Mon, 17 May 2010 16:50:07 -0700

author: never
date: Mon, 17 May 2010 16:50:07 -0700
changeset 1893: bfe29ec02863
parent 1845: f03d0a26bf83
child 1907: c18cbe5936b8
permissions: -rw-r--r--

6950075: nmethod sweeper should operate concurrently
Reviewed-by: never, kvn
Contributed-by: eric.caspole@amd.com

 /*
  * Copyright 1997-2009 Sun Microsystems, Inc.  All Rights Reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 only, as
  * published by the Free Software Foundation.
  *
  * This code is distributed in the hope that it will be useful, but WITHOUT
  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  * version 2 for more details (a copy is included in the LICENSE file that
  * accompanied this code).
  *
  * You should have received a copy of the GNU General Public License version
  * 2 along with this work; if not, write to the Free Software Foundation,
  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  *
  * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
  * CA 95054 USA or visit www.sun.com if you need additional information or
  * have any questions.
  *
  */
 # include "incls/_precompiled.incl"
 # include "incls/_safepoint.cpp.incl"
 // --------------------------------------------------------------------------------------------------
 // Implementation of Safepoint begin/end
 SafepointSynchronize::SynchronizeState volatile SafepointSynchronize::_state = SafepointSynchronize::_not_synchronized;
 volatile int  SafepointSynchronize::_waiting_to_block = 0;
 volatile int SafepointSynchronize::_safepoint_counter = 0;
 long  SafepointSynchronize::_end_of_last_safepoint = 0;
 static volatile int PageArmed = 0 ;        // safepoint polling page is RO|RW vs PROT_NONE
 static volatile int TryingToBlock = 0 ;    // proximate value -- for advisory use only
 static bool timeout_error_printed = false;
 // Roll all threads forward to a safepoint and suspend them all
 void SafepointSynchronize::begin() {
   Thread* myThread = Thread::current();
   assert(myThread->is_VM_thread(), "Only VM thread may execute a safepoint");
   if (PrintSafepointStatistics || PrintSafepointStatisticsTimeout > 0) {
     _safepoint_begin_time = os::javaTimeNanos();
     _ts_of_current_safepoint = tty->time_stamp().seconds();
   }
 #ifndef SERIALGC
   if (UseConcMarkSweepGC) {
     // In the future we should investigate whether CMS can use the
     // more-general mechanism below.  DLD (01/05).
     ConcurrentMarkSweepThread::synchronize(false);
   } else if (UseG1GC) {
     ConcurrentGCThread::safepoint_synchronize();
   }
 #endif // SERIALGC
   // By getting the Threads_lock, we assure that no threads are about to start or
   // exit. It is released again in SafepointSynchronize::end().
   Threads_lock->lock();
   assert( _state == _not_synchronized, "trying to safepoint synchronize with wrong state");
   int nof_threads = Threads::number_of_threads();
   if (TraceSafepoint) {
     tty->print_cr("Safepoint synchronization initiated. (%d)", nof_threads);
   }
   RuntimeService::record_safepoint_begin();
   {
   MutexLocker mu(Safepoint_lock);
   // Set number of threads to wait for, before we initiate the callbacks
   _waiting_to_block = nof_threads;
   TryingToBlock     = 0 ;
   int still_running = nof_threads;
   // Save the starting time, so that it can be compared to see if this has taken
   // too long to complete.
   jlong safepoint_limit_time;
   timeout_error_printed = false;
   // PrintSafepointStatisticsTimeout can be specified separately. When
   // specified, PrintSafepointStatistics will be set to true in
   // deferred_initialize_stat method. The initialization has to be done
   // early enough to avoid any races. See bug 6880029 for details.
   if (PrintSafepointStatistics || PrintSafepointStatisticsTimeout > 0) {
     deferred_initialize_stat();
   }
   // Begin the process of bringing the system to a safepoint.
   // Java threads can be in several different states and are
   // stopped by different mechanisms:
   //
   //  1. Running interpreted
   //     The interpeter dispatch table is changed to force it to
   //     check for a safepoint condition between bytecodes.
   //  2. Running in native code
   //     When returning from the native code, a Java thread must check
   //     the safepoint _state to see if we must block.  If the
   //     VM thread sees a Java thread in native, it does
   //     not wait for this thread to block.  The order of the memory
   //     writes and reads of both the safepoint state and the Java
   //     threads state is critical.  In order to guarantee that the
   //     memory writes are serialized with respect to each other,
   //     the VM thread issues a memory barrier instruction
   //     (on MP systems).  In order to avoid the overhead of issuing
   //     a memory barrier for each Java thread making native calls, each Java
   //     thread performs a write to a single memory page after changing
   //     the thread state.  The VM thread performs a sequence of
   //     mprotect OS calls which forces all previous writes from all
   //     Java threads to be serialized.  This is done in the
   //     os::serialize_thread_states() call.  This has proven to be
   //     much more efficient than executing a membar instruction
   //     on every call to native code.
   //  3. Running compiled Code
   //     Compiled code reads a global (Safepoint Polling) page that
   //     is set to fault if we are trying to get to a safepoint.
   //  4. Blocked
   //     A thread which is blocked will not be allowed to return from the
   //     block condition until the safepoint operation is complete.
   //  5. In VM or Transitioning between states
   //     If a Java thread is currently running in the VM or transitioning
   //     between states, the safepointing code will wait for the thread to
   //     block itself when it attempts transitions to a new state.
   //
   _state            = _synchronizing;
   OrderAccess::fence();
   // Flush all thread states to memory
   if (!UseMembar) {
     os::serialize_thread_states();
   }
   // Make interpreter safepoint aware
   Interpreter::notice_safepoints();
   if (UseCompilerSafepoints && DeferPollingPageLoopCount < 0) {
     // Make polling safepoint aware
     guarantee (PageArmed == 0, "invariant") ;
     PageArmed = 1 ;
     os::make_polling_page_unreadable();
   }
   // Consider using active_processor_count() ... but that call is expensive.
   int ncpus = os::processor_count() ;
 #ifdef ASSERT
   for (JavaThread *cur = Threads::first(); cur != NULL; cur = cur->next()) {
     assert(cur->safepoint_state()->is_running(), "Illegal initial state");
   }
 #endif // ASSERT
   if (SafepointTimeout)
     safepoint_limit_time = os::javaTimeNanos() + (jlong)SafepointTimeoutDelay * MICROUNITS;
   // Iterate through all threads until it have been determined how to stop them all at a safepoint
   unsigned int iterations = 0;
   int steps = 0 ;
   while(still_running > 0) {
     for (JavaThread *cur = Threads::first(); cur != NULL; cur = cur->next()) {
       assert(!cur->is_ConcurrentGC_thread(), "A concurrent GC thread is unexpectly being suspended");
       ThreadSafepointState *cur_state = cur->safepoint_state();
       if (cur_state->is_running()) {
         cur_state->examine_state_of_thread();
         if (!cur_state->is_running()) {
            still_running--;
            // consider adjusting steps downward:
            //   steps = 0
            //   steps -= NNN
            //   steps >>= 1
            //   steps = MIN(steps, 2000-100)
            //   if (iterations != 0) steps -= NNN
         }
         if (TraceSafepoint && Verbose) cur_state->print();
       }
     }
     if (PrintSafepointStatistics && iterations == 0) {
       begin_statistics(nof_threads, still_running);
     }
     if (still_running > 0) {
       // Check for if it takes to long
       if (SafepointTimeout && safepoint_limit_time < os::javaTimeNanos()) {
         print_safepoint_timeout(_spinning_timeout);
       }
       // Spin to avoid context switching.
       // There's a tension between allowing the mutators to run (and rendezvous)
       // vs spinning.  As the VM thread spins, wasting cycles, it consumes CPU that
       // a mutator might otherwise use profitably to reach a safepoint.  Excessive
       // spinning by the VM thread on a saturated system can increase rendezvous latency.
       // Blocking or yielding incur their own penalties in the form of context switching
       // and the resultant loss of $ residency.
       //
       // Further complicating matters is that yield() does not work as naively expected
       // on many platforms -- yield() does not guarantee that any other ready threads
       // will run.   As such we revert yield_all() after some number of iterations.
       // Yield_all() is implemented as a short unconditional sleep on some platforms.
       // Typical operating systems round a "short" sleep period up to 10 msecs, so sleeping
       // can actually increase the time it takes the VM thread to detect that a system-wide
       // stop-the-world safepoint has been reached.  In a pathological scenario such as that
       // described in CR6415670 the VMthread may sleep just before the mutator(s) become safe.
       // In that case the mutators will be stalled waiting for the safepoint to complete and the
       // the VMthread will be sleeping, waiting for the mutators to rendezvous.  The VMthread
       // will eventually wake up and detect that all mutators are safe, at which point
       // we'll again make progress.
       //
       // Beware too that that the VMThread typically runs at elevated priority.
       // Its default priority is higher than the default mutator priority.
       // Obviously, this complicates spinning.
       //
       // Note too that on Windows XP SwitchThreadTo() has quite different behavior than Sleep(0).
       // Sleep(0) will _not yield to lower priority threads, while SwitchThreadTo() will.
       //
       // See the comments in synchronizer.cpp for additional remarks on spinning.
       //
       // In the future we might:
       // 1. Modify the safepoint scheme to avoid potentally unbounded spinning.
       //    This is tricky as the path used by a thread exiting the JVM (say on
       //    on JNI call-out) simply stores into its state field.  The burden
       //    is placed on the VM thread, which must poll (spin).
       // 2. Find something useful to do while spinning.  If the safepoint is GC-related
       //    we might aggressively scan the stacks of threads that are already safe.
       // 3. Use Solaris schedctl to examine the state of the still-running mutators.
       //    If all the mutators are ONPROC there's no reason to sleep or yield.
       // 4. YieldTo() any still-running mutators that are ready but OFFPROC.
       // 5. Check system saturation.  If the system is not fully saturated then
       //    simply spin and avoid sleep/yield.
       // 6. As still-running mutators rendezvous they could unpark the sleeping
       //    VMthread.  This works well for still-running mutators that become
       //    safe.  The VMthread must still poll for mutators that call-out.
       // 7. Drive the policy on time-since-begin instead of iterations.
       // 8. Consider making the spin duration a function of the # of CPUs:
       //    Spin = (((ncpus-1) * M) + K) + F(still_running)
       //    Alternately, instead of counting iterations of the outer loop
       //    we could count the # of threads visited in the inner loop, above.
       // 9. On windows consider using the return value from SwitchThreadTo()
       //    to drive subsequent spin/SwitchThreadTo()/Sleep(N) decisions.
       if (UseCompilerSafepoints && int(iterations) == DeferPollingPageLoopCount) {
          guarantee (PageArmed == 0, "invariant") ;
          PageArmed = 1 ;
          os::make_polling_page_unreadable();
       }
       // Instead of (ncpus > 1) consider either (still_running < (ncpus + EPSILON)) or
       // ((still_running + _waiting_to_block - TryingToBlock)) < ncpus)
       ++steps ;
       if (ncpus > 1 && steps < SafepointSpinBeforeYield) {
         SpinPause() ;     // MP-Polite spin
       } else
       if (steps < DeferThrSuspendLoopCount) {
         os::NakedYield() ;
       } else {
         os::yield_all(steps) ;
         // Alternately, the VM thread could transiently depress its scheduling priority or
         // transiently increase the priority of the tardy mutator(s).
       }
       iterations ++ ;
     }
     assert(iterations < (uint)max_jint, "We have been iterating in the safepoint loop too long");
   }
   assert(still_running == 0, "sanity check");
   if (PrintSafepointStatistics) {
     update_statistics_on_spin_end();
   }
   // wait until all threads are stopped
   while (_waiting_to_block > 0) {
     if (TraceSafepoint) tty->print_cr("Waiting for %d thread(s) to block", _waiting_to_block);
     if (!SafepointTimeout || timeout_error_printed) {
       Safepoint_lock->wait(true);  // true, means with no safepoint checks
     } else {
       // Compute remaining time
       jlong remaining_time = safepoint_limit_time - os::javaTimeNanos();
       // If there is no remaining time, then there is an error
       if (remaining_time < 0 || Safepoint_lock->wait(true, remaining_time / MICROUNITS)) {
         print_safepoint_timeout(_blocking_timeout);
       }
     }
   }
   assert(_waiting_to_block == 0, "sanity check");
 #ifndef PRODUCT
   if (SafepointTimeout) {
     jlong current_time = os::javaTimeNanos();
     if (safepoint_limit_time < current_time) {
       tty->print_cr("# SafepointSynchronize: Finished after "
                     INT64_FORMAT_W(6) " ms",
                     ((current_time - safepoint_limit_time) / MICROUNITS +
                      SafepointTimeoutDelay));
     }
   }
 #endif
   assert((_safepoint_counter & 0x1) == 0, "must be even");
   assert(Threads_lock->owned_by_self(), "must hold Threads_lock");
   _safepoint_counter ++;
   // Record state
   _state = _synchronized;
   OrderAccess::fence();
   if (TraceSafepoint) {
     VM_Operation *op = VMThread::vm_operation();
     tty->print_cr("Entering safepoint region: %s", (op != NULL) ? op->name() : "no vm operation");
   }
   RuntimeService::record_safepoint_synchronized();
   if (PrintSafepointStatistics) {
     update_statistics_on_sync_end(os::javaTimeNanos());
   }
   // Call stuff that needs to be run when a safepoint is just about to be completed
   do_cleanup_tasks();
   if (PrintSafepointStatistics) {
     // Record how much time spend on the above cleanup tasks
     update_statistics_on_cleanup_end(os::javaTimeNanos());
   }
   }
 }
 // Wake up all threads, so they are ready to resume execution after the safepoint
 // operation has been carried out
 void SafepointSynchronize::end() {
   assert(Threads_lock->owned_by_self(), "must hold Threads_lock");
   assert((_safepoint_counter & 0x1) == 1, "must be odd");
   _safepoint_counter ++;
   // memory fence isn't required here since an odd _safepoint_counter
   // value can do no harm and a fence is issued below anyway.
   DEBUG_ONLY(Thread* myThread = Thread::current();)
   assert(myThread->is_VM_thread(), "Only VM thread can execute a safepoint");
   if (PrintSafepointStatistics) {
     end_statistics(os::javaTimeNanos());
   }
 #ifdef ASSERT
   // A pending_exception cannot be installed during a safepoint.  The threads
   // may install an async exception after they come back from a safepoint into
   // pending_exception after they unblock.  But that should happen later.
   for(JavaThread *cur = Threads::first(); cur; cur = cur->next()) {
     assert (!(cur->has_pending_exception() &&
               cur->safepoint_state()->is_at_poll_safepoint()),
             "safepoint installed a pending exception");
   }
 #endif // ASSERT
   if (PageArmed) {
     // Make polling safepoint aware
     os::make_polling_page_readable();
     PageArmed = 0 ;
   }
   // Remove safepoint check from interpreter
   Interpreter::ignore_safepoints();
   {
     MutexLocker mu(Safepoint_lock);
     assert(_state == _synchronized, "must be synchronized before ending safepoint synchronization");
     // Set to not synchronized, so the threads will not go into the signal_thread_blocked method
     // when they get restarted.
     _state = _not_synchronized;
     OrderAccess::fence();
     if (TraceSafepoint) {
        tty->print_cr("Leaving safepoint region");
     }
     // Start suspended threads
     for(JavaThread *current = Threads::first(); current; current = current->next()) {
       // A problem occurring on Solaris is when attempting to restart threads
       // the first #cpus - 1 go well, but then the VMThread is preempted when we get
       // to the next one (since it has been running the longest).  We then have
       // to wait for a cpu to become available before we can continue restarting
       // threads.
       // FIXME: This causes the performance of the VM to degrade when active and with
       // large numbers of threads.  Apparently this is due to the synchronous nature
       // of suspending threads.
       //
       // TODO-FIXME: the comments above are vestigial and no longer apply.
       // Furthermore, using solaris' schedctl in this particular context confers no benefit
       if (VMThreadHintNoPreempt) {
         os::hint_no_preempt();
       }
       ThreadSafepointState* cur_state = current->safepoint_state();
       assert(cur_state->type() != ThreadSafepointState::_running, "Thread not suspended at safepoint");
       cur_state->restart();
       assert(cur_state->is_running(), "safepoint state has not been reset");
     }
     RuntimeService::record_safepoint_end();
     // Release threads lock, so threads can be created/destroyed again. It will also starts all threads
     // blocked in signal_thread_blocked
     Threads_lock->unlock();
   }
 #ifndef SERIALGC
   // If there are any concurrent GC threads resume them.
   if (UseConcMarkSweepGC) {
     ConcurrentMarkSweepThread::desynchronize(false);
   } else if (UseG1GC) {
     ConcurrentGCThread::safepoint_desynchronize();
   }
 #endif // SERIALGC
   // record this time so VMThread can keep track how much time has elasped
   // since last safepoint.
   _end_of_last_safepoint = os::javaTimeMillis();
 }
 bool SafepointSynchronize::is_cleanup_needed() {
   // Need a safepoint if some inline cache buffers is non-empty
   if (!InlineCacheBuffer::is_empty()) return true;
   return false;
 }
 jlong CounterDecay::_last_timestamp = 0;
 static void do_method(methodOop m) {
   m->invocation_counter()->decay();
 }
 void CounterDecay::decay() {
   _last_timestamp = os::javaTimeMillis();
   // This operation is going to be performed only at the end of a safepoint
   // and hence GC's will not be going on, all Java mutators are suspended
   // at this point and hence SystemDictionary_lock is also not needed.
   assert(SafepointSynchronize::is_at_safepoint(), "can only be executed at a safepoint");
   int nclasses = SystemDictionary::number_of_classes();
   double classes_per_tick = nclasses * (CounterDecayMinIntervalLength * 1e-3 /
                                         CounterHalfLifeTime);
   for (int i = 0; i < classes_per_tick; i++) {
     klassOop k = SystemDictionary::try_get_next_class();
     if (k != NULL && k->klass_part()->oop_is_instance()) {
       instanceKlass::cast(k)->methods_do(do_method);
     }
   }
 }
 // Various cleaning tasks that should be done periodically at safepoints
 void SafepointSynchronize::do_cleanup_tasks() {
   {
     TraceTime t1("deflating idle monitors", TraceSafepointCleanupTime);
     ObjectSynchronizer::deflate_idle_monitors();
   }
   {
     TraceTime t2("updating inline caches", TraceSafepointCleanupTime);
     InlineCacheBuffer::update_inline_caches();
   }
   if(UseCounterDecay && CounterDecay::is_decay_needed()) {
     TraceTime t3("decaying counter", TraceSafepointCleanupTime);
     CounterDecay::decay();
   }
   TraceTime t4("sweeping nmethods", TraceSafepointCleanupTime);
   NMethodSweeper::scan_stacks();
 }
 bool SafepointSynchronize::safepoint_safe(JavaThread *thread, JavaThreadState state) {
   switch(state) {
   case _thread_in_native:
     // native threads are safe if they have no java stack or have walkable stack
     return !thread->has_last_Java_frame() || thread->frame_anchor()->walkable();
    // blocked threads should have already have walkable stack
   case _thread_blocked:
     assert(!thread->has_last_Java_frame() || thread->frame_anchor()->walkable(), "blocked and not walkable");
     return true;
   default:
     return false;
   }
 }
 // -------------------------------------------------------------------------------------------------------
 // Implementation of Safepoint callback point
 void SafepointSynchronize::block(JavaThread *thread) {
   assert(thread != NULL, "thread must be set");
   assert(thread->is_Java_thread(), "not a Java thread");
   // Threads shouldn't block if they are in the middle of printing, but...
   ttyLocker::break_tty_lock_for_safepoint(os::current_thread_id());
   // Only bail from the block() call if the thread is gone from the
   // thread list; starting to exit should still block.
   if (thread->is_terminated()) {
      // block current thread if we come here from native code when VM is gone
      thread->block_if_vm_exited();
      // otherwise do nothing
      return;
   }
   JavaThreadState state = thread->thread_state();
   thread->frame_anchor()->make_walkable(thread);
   // Check that we have a valid thread_state at this point
   switch(state) {
     case _thread_in_vm_trans:
     case _thread_in_Java:        // From compiled code
       // We are highly likely to block on the Safepoint_lock. In order to avoid blocking in this case,
       // we pretend we are still in the VM.
       thread->set_thread_state(_thread_in_vm);
       if (is_synchronizing()) {
          Atomic::inc (&TryingToBlock) ;
       }
       // We will always be holding the Safepoint_lock when we are examine the state
       // of a thread. Hence, the instructions between the Safepoint_lock->lock() and
       // Safepoint_lock->unlock() are happening atomic with regards to the safepoint code
       Safepoint_lock->lock_without_safepoint_check();
       if (is_synchronizing()) {
         // Decrement the number of threads to wait for and signal vm thread
         assert(_waiting_to_block > 0, "sanity check");
         _waiting_to_block--;
         thread->safepoint_state()->set_has_called_back(true);
         // Consider (_waiting_to_block < 2) to pipeline the wakeup of the VM thread
         if (_waiting_to_block == 0) {
           Safepoint_lock->notify_all();
         }
       }
       // We transition the thread to state _thread_blocked here, but
       // we can't do our usual check for external suspension and then
       // self-suspend after the lock_without_safepoint_check() call
       // below because we are often called during transitions while
       // we hold different locks. That would leave us suspended while
       // holding a resource which results in deadlocks.
       thread->set_thread_state(_thread_blocked);
       Safepoint_lock->unlock();
       // We now try to acquire the threads lock. Since this lock is hold by the VM thread during
       // the entire safepoint, the threads will all line up here during the safepoint.
       Threads_lock->lock_without_safepoint_check();
       // restore original state. This is important if the thread comes from compiled code, so it
       // will continue to execute with the _thread_in_Java state.
       thread->set_thread_state(state);
       Threads_lock->unlock();
       break;
     case _thread_in_native_trans:
     case _thread_blocked_trans:
     case _thread_new_trans:
       if (thread->safepoint_state()->type() == ThreadSafepointState::_call_back) {
         thread->print_thread_state();
         fatal("Deadlock in safepoint code.  "
               "Should have called back to the VM before blocking.");
       }
       // We transition the thread to state _thread_blocked here, but
       // we can't do our usual check for external suspension and then
       // self-suspend after the lock_without_safepoint_check() call
       // below because we are often called during transitions while
       // we hold different locks. That would leave us suspended while
       // holding a resource which results in deadlocks.
       thread->set_thread_state(_thread_blocked);
       // It is not safe to suspend a thread if we discover it is in _thread_in_native_trans. Hence,
       // the safepoint code might still be waiting for it to block. We need to change the state here,
       // so it can see that it is at a safepoint.
       // Block until the safepoint operation is completed.
       Threads_lock->lock_without_safepoint_check();
       // Restore state
       thread->set_thread_state(state);
       Threads_lock->unlock();
       break;
     default:
      fatal(err_msg("Illegal threadstate encountered: %d", state));
   }
   // Check for pending. async. exceptions or suspends - except if the
   // thread was blocked inside the VM. has_special_runtime_exit_condition()
   // is called last since it grabs a lock and we only want to do that when
   // we must.
   //
   // Note: we never deliver an async exception at a polling point as the
   // compiler may not have an exception handler for it. The polling
   // code will notice the async and deoptimize and the exception will
   // be delivered. (Polling at a return point is ok though). Sure is
   // a lot of bother for a deprecated feature...
   //
   // We don't deliver an async exception if the thread state is
   // _thread_in_native_trans so JNI functions won't be called with
   // a surprising pending exception. If the thread state is going back to java,
   // async exception is checked in check_special_condition_for_native_trans().
   if (state != _thread_blocked_trans &&
       state != _thread_in_vm_trans &&
       thread->has_special_runtime_exit_condition()) {
     thread->handle_special_runtime_exit_condition(
       !thread->is_at_poll_safepoint() && (state != _thread_in_native_trans));
   }
 }
 // ------------------------------------------------------------------------------------------------------
 // Exception handlers
 #ifndef PRODUCT
 #ifdef _LP64
 #define PTR_PAD ""
 #else
 #define PTR_PAD "        "
 #endif
 static void print_ptrs(intptr_t oldptr, intptr_t newptr, bool wasoop) {
   bool is_oop = newptr ? ((oop)newptr)->is_oop() : false;
   tty->print_cr(PTR_FORMAT PTR_PAD " %s %c " PTR_FORMAT PTR_PAD " %s %s",
                 oldptr, wasoop?"oop":"   ", oldptr == newptr ? ' ' : '!',
                 newptr, is_oop?"oop":"   ", (wasoop && !is_oop) ? "STALE" : ((wasoop==false&&is_oop==false&&oldptr !=newptr)?"STOMP":"     "));
 }
 static void print_longs(jlong oldptr, jlong newptr, bool wasoop) {
   bool is_oop = newptr ? ((oop)(intptr_t)newptr)->is_oop() : false;
   tty->print_cr(PTR64_FORMAT " %s %c " PTR64_FORMAT " %s %s",
                 oldptr, wasoop?"oop":"   ", oldptr == newptr ? ' ' : '!',
                 newptr, is_oop?"oop":"   ", (wasoop && !is_oop) ? "STALE" : ((wasoop==false&&is_oop==false&&oldptr !=newptr)?"STOMP":"     "));
 }
 #ifdef SPARC
 static void print_me(intptr_t *new_sp, intptr_t *old_sp, bool *was_oops) {
 #ifdef _LP64
   tty->print_cr("--------+------address-----+------before-----------+-------after----------+");
   const int incr = 1;           // Increment to skip a long, in units of intptr_t
 #else
   tty->print_cr("--------+--address-+------before-----------+-------after----------+");
   const int incr = 2;           // Increment to skip a long, in units of intptr_t
 #endif
   tty->print_cr("---SP---|");
   for( int i=0; i<16; i++ ) {
     tty->print("blob %c%d |"PTR_FORMAT" ","LO"[i>>3],i&7,new_sp); print_ptrs(*old_sp++,*new_sp++,*was_oops++); }
   tty->print_cr("--------|");
   for( int i1=0; i1<frame::memory_parameter_word_sp_offset-16; i1++ ) {
     tty->print("argv pad|"PTR_FORMAT" ",new_sp); print_ptrs(*old_sp++,*new_sp++,*was_oops++); }
   tty->print("     pad|"PTR_FORMAT" ",new_sp); print_ptrs(*old_sp++,*new_sp++,*was_oops++);
   tty->print_cr("--------|");
   tty->print(" G1     |"PTR_FORMAT" ",new_sp); print_longs(*(jlong*)old_sp,*(jlong*)new_sp,was_oops[incr-1]); old_sp += incr; new_sp += incr; was_oops += incr;
   tty->print(" G3     |"PTR_FORMAT" ",new_sp); print_longs(*(jlong*)old_sp,*(jlong*)new_sp,was_oops[incr-1]); old_sp += incr; new_sp += incr; was_oops += incr;
   tty->print(" G4     |"PTR_FORMAT" ",new_sp); print_longs(*(jlong*)old_sp,*(jlong*)new_sp,was_oops[incr-1]); old_sp += incr; new_sp += incr; was_oops += incr;
   tty->print(" G5     |"PTR_FORMAT" ",new_sp); print_longs(*(jlong*)old_sp,*(jlong*)new_sp,was_oops[incr-1]); old_sp += incr; new_sp += incr; was_oops += incr;
   tty->print_cr(" FSR    |"PTR_FORMAT" "PTR64_FORMAT"       "PTR64_FORMAT,new_sp,*(jlong*)old_sp,*(jlong*)new_sp);
   old_sp += incr; new_sp += incr; was_oops += incr;
   // Skip the floats
   tty->print_cr("--Float-|"PTR_FORMAT,new_sp);
   tty->print_cr("---FP---|");
   old_sp += incr*32;  new_sp += incr*32;  was_oops += incr*32;
   for( int i2=0; i2<16; i2++ ) {
     tty->print("call %c%d |"PTR_FORMAT" ","LI"[i2>>3],i2&7,new_sp); print_ptrs(*old_sp++,*new_sp++,*was_oops++); }
   tty->print_cr("");
 }
 #endif  // SPARC
 #endif  // PRODUCT
 void SafepointSynchronize::handle_polling_page_exception(JavaThread *thread) {
   assert(thread->is_Java_thread(), "polling reference encountered by VM thread");
   assert(thread->thread_state() == _thread_in_Java, "should come from Java code");
   assert(SafepointSynchronize::is_synchronizing(), "polling encountered outside safepoint synchronization");
   // Uncomment this to get some serious before/after printing of the
   // Sparc safepoint-blob frame structure.
   /*
   intptr_t* sp = thread->last_Java_sp();
   intptr_t stack_copy[150];
   for( int i=0; i<150; i++ ) stack_copy[i] = sp[i];
   bool was_oops[150];
   for( int i=0; i<150; i++ )
     was_oops[i] = stack_copy[i] ? ((oop)stack_copy[i])->is_oop() : false;
   */
   if (ShowSafepointMsgs) {
     tty->print("handle_polling_page_exception: ");
   }
   if (PrintSafepointStatistics) {
     inc_page_trap_count();
   }
   ThreadSafepointState* state = thread->safepoint_state();
   state->handle_polling_page_exception();
   // print_me(sp,stack_copy,was_oops);
 }
 void SafepointSynchronize::print_safepoint_timeout(SafepointTimeoutReason reason) {
   if (!timeout_error_printed) {
     timeout_error_printed = true;
     // Print out the thread infor which didn't reach the safepoint for debugging
     // purposes (useful when there are lots of threads in the debugger).
     tty->print_cr("");
     tty->print_cr("# SafepointSynchronize::begin: Timeout detected:");
     if (reason ==  _spinning_timeout) {
       tty->print_cr("# SafepointSynchronize::begin: Timed out while spinning to reach a safepoint.");
     } else if (reason == _blocking_timeout) {
       tty->print_cr("# SafepointSynchronize::begin: Timed out while waiting for threads to stop.");
     }
     tty->print_cr("# SafepointSynchronize::begin: Threads which did not reach the safepoint:");
     ThreadSafepointState *cur_state;
     ResourceMark rm;
     for(JavaThread *cur_thread = Threads::first(); cur_thread;
         cur_thread = cur_thread->next()) {
       cur_state = cur_thread->safepoint_state();
       if (cur_thread->thread_state() != _thread_blocked &&
           ((reason == _spinning_timeout && cur_state->is_running()) ||
            (reason == _blocking_timeout && !cur_state->has_called_back()))) {
         tty->print("# ");
         cur_thread->print();
         tty->print_cr("");
       }
     }
     tty->print_cr("# SafepointSynchronize::begin: (End of list)");
   }
   // To debug the long safepoint, specify both DieOnSafepointTimeout &
   // ShowMessageBoxOnError.
   if (DieOnSafepointTimeout) {
     char msg[1024];
     VM_Operation *op = VMThread::vm_operation();
     sprintf(msg, "Safepoint sync time longer than " INTX_FORMAT "ms detected when executing %s.",
             SafepointTimeoutDelay,
             op != NULL ? op->name() : "no vm operation");
     fatal(msg);
   }
 }
 // -------------------------------------------------------------------------------------------------------
 // Implementation of ThreadSafepointState
 ThreadSafepointState::ThreadSafepointState(JavaThread *thread) {
   _thread = thread;
   _type   = _running;
   _has_called_back = false;
   _at_poll_safepoint = false;
 }
 void ThreadSafepointState::create(JavaThread *thread) {
   ThreadSafepointState *state = new ThreadSafepointState(thread);
   thread->set_safepoint_state(state);
 }
 void ThreadSafepointState::destroy(JavaThread *thread) {
   if (thread->safepoint_state()) {
     delete(thread->safepoint_state());
     thread->set_safepoint_state(NULL);
   }
 }
 void ThreadSafepointState::examine_state_of_thread() {
   assert(is_running(), "better be running or just have hit safepoint poll");
   JavaThreadState state = _thread->thread_state();
   // Check for a thread that is suspended. Note that thread resume tries
   // to grab the Threads_lock which we own here, so a thread cannot be
   // resumed during safepoint synchronization.
   // We check to see if this thread is suspended without locking to
   // avoid deadlocking with a third thread that is waiting for this
   // thread to be suspended. The third thread can notice the safepoint
   // that we're trying to start at the beginning of its SR_lock->wait()
   // call. If that happens, then the third thread will block on the
   // safepoint while still holding the underlying SR_lock. We won't be
   // able to get the SR_lock and we'll deadlock.
   //
   // We don't need to grab the SR_lock here for two reasons:
   // 1) The suspend flags are both volatile and are set with an
   //    Atomic::cmpxchg() call so we should see the suspended
   //    state right away.
   // 2) We're being called from the safepoint polling loop; if
   //    we don't see the suspended state on this iteration, then
   //    we'll come around again.
   //
   bool is_suspended = _thread->is_ext_suspended();
   if (is_suspended) {
     roll_forward(_at_safepoint);
     return;
   }
   // Some JavaThread states have an initial safepoint state of
   // running, but are actually at a safepoint. We will happily
   // agree and update the safepoint state here.
   if (SafepointSynchronize::safepoint_safe(_thread, state)) {
       roll_forward(_at_safepoint);
       return;
   }
   if (state == _thread_in_vm) {
     roll_forward(_call_back);
     return;
   }
   // All other thread states will continue to run until they
   // transition and self-block in state _blocked
   // Safepoint polling in compiled code causes the Java threads to do the same.
   // Note: new threads may require a malloc so they must be allowed to finish
   assert(is_running(), "examine_state_of_thread on non-running thread");
   return;
 }
 // Returns true is thread could not be rolled forward at present position.
 void ThreadSafepointState::roll_forward(suspend_type type) {
   _type = type;
   switch(_type) {
     case _at_safepoint:
       SafepointSynchronize::signal_thread_at_safepoint();
       break;
     case _call_back:
       set_has_called_back(false);
       break;
     case _running:
     default:
       ShouldNotReachHere();
   }
 }
 void ThreadSafepointState::restart() {
   switch(type()) {
     case _at_safepoint:
     case _call_back:
       break;
     case _running:
     default:
        tty->print_cr("restart thread "INTPTR_FORMAT" with state %d",
                       _thread, _type);
        _thread->print();
       ShouldNotReachHere();
   }
   _type = _running;
   set_has_called_back(false);
 }
 void ThreadSafepointState::print_on(outputStream *st) const {
   const char *s;
   switch(_type) {
     case _running                : s = "_running";              break;
     case _at_safepoint           : s = "_at_safepoint";         break;
     case _call_back              : s = "_call_back";            break;
     default:
       ShouldNotReachHere();
   }
   st->print_cr("Thread: " INTPTR_FORMAT
               "  [0x%2x] State: %s _has_called_back %d _at_poll_safepoint %d",
                _thread, _thread->osthread()->thread_id(), s, _has_called_back,
                _at_poll_safepoint);
   _thread->print_thread_state_on(st);
 }
 // ---------------------------------------------------------------------------------------------------------------------
 // Block the thread at the safepoint poll or poll return.
 void ThreadSafepointState::handle_polling_page_exception() {
   // Check state.  block() will set thread state to thread_in_vm which will
   // cause the safepoint state _type to become _call_back.
   assert(type() == ThreadSafepointState::_running,
          "polling page exception on thread not running state");
   // Step 1: Find the nmethod from the return address
   if (ShowSafepointMsgs && Verbose) {
     tty->print_cr("Polling page exception at " INTPTR_FORMAT, thread()->saved_exception_pc());
   }
   address real_return_addr = thread()->saved_exception_pc();
   CodeBlob *cb = CodeCache::find_blob(real_return_addr);
   assert(cb != NULL && cb->is_nmethod(), "return address should be in nmethod");
   nmethod* nm = (nmethod*)cb;
   // Find frame of caller
   frame stub_fr = thread()->last_frame();
   CodeBlob* stub_cb = stub_fr.cb();
   assert(stub_cb->is_safepoint_stub(), "must be a safepoint stub");
   RegisterMap map(thread(), true);
   frame caller_fr = stub_fr.sender(&map);
   // Should only be poll_return or poll
   assert( nm->is_at_poll_or_poll_return(real_return_addr), "should not be at call" );
   // This is a poll immediately before a return. The exception handling code
   // has already had the effect of causing the return to occur, so the execution
   // will continue immediately after the call. In addition, the oopmap at the
   // return point does not mark the return value as an oop (if it is), so
   // it needs a handle here to be updated.
   if( nm->is_at_poll_return(real_return_addr) ) {
     // See if return type is an oop.
     bool return_oop = nm->method()->is_returning_oop();
     Handle return_value;
     if (return_oop) {
       // The oop result has been saved on the stack together with all
       // the other registers. In order to preserve it over GCs we need
       // to keep it in a handle.
       oop result = caller_fr.saved_oop_result(&map);
       assert(result == NULL || result->is_oop(), "must be oop");
       return_value = Handle(thread(), result);
       assert(Universe::heap()->is_in_or_null(result), "must be heap pointer");
     }
     // Block the thread
     SafepointSynchronize::block(thread());
     // restore oop result, if any
     if (return_oop) {
       caller_fr.set_saved_oop_result(&map, return_value());
     }
   }
   // This is a safepoint poll. Verify the return address and block.
   else {
     set_at_poll_safepoint(true);
     // verify the blob built the "return address" correctly
     assert(real_return_addr == caller_fr.pc(), "must match");
     // Block the thread
     SafepointSynchronize::block(thread());
     set_at_poll_safepoint(false);
     // If we have a pending async exception deoptimize the frame
     // as otherwise we may never deliver it.
     if (thread()->has_async_condition()) {
       ThreadInVMfromJavaNoAsyncException __tiv(thread());
       VM_DeoptimizeFrame deopt(thread(), caller_fr.id());
       VMThread::execute(&deopt);
     }
     // If an exception has been installed we must check for a pending deoptimization
     // Deoptimize frame if exception has been thrown.
     if (thread()->has_pending_exception() ) {
       RegisterMap map(thread(), true);
       frame caller_fr = stub_fr.sender(&map);
       if (caller_fr.is_deoptimized_frame()) {
         // The exception patch will destroy registers that are still
         // live and will be needed during deoptimization. Defer the
         // Async exception should have defered the exception until the
         // next safepoint which will be detected when we get into
         // the interpreter so if we have an exception now things
         // are messed up.
         fatal("Exception installed and deoptimization is pending");
       }
     }
   }
 }
 //
 //                     Statistics & Instrumentations
 //
 SafepointSynchronize::SafepointStats*  SafepointSynchronize::_safepoint_stats = NULL;
 jlong  SafepointSynchronize::_safepoint_begin_time = 0;
 int    SafepointSynchronize::_cur_stat_index = 0;
 julong SafepointSynchronize::_safepoint_reasons[VM_Operation::VMOp_Terminating];
 julong SafepointSynchronize::_coalesced_vmop_count = 0;
 jlong  SafepointSynchronize::_max_sync_time = 0;
 jlong  SafepointSynchronize::_max_vmop_time = 0;
 float  SafepointSynchronize::_ts_of_current_safepoint = 0.0f;
 static jlong  cleanup_end_time = 0;
 static bool   need_to_track_page_armed_status = false;
 static bool   init_done = false;
 // Helper method to print the header.
 static void print_header() {
   tty->print("         vmop                    "
              "[threads: total initially_running wait_to_block]    ");
   tty->print("[time: spin block sync cleanup vmop] ");
   // no page armed status printed out if it is always armed.
   if (need_to_track_page_armed_status) {
     tty->print("page_armed ");
   }
   tty->print_cr("page_trap_count");
 }
 void SafepointSynchronize::deferred_initialize_stat() {
   if (init_done) return;
   if (PrintSafepointStatisticsCount <= 0) {
     fatal("Wrong PrintSafepointStatisticsCount");
   }
   // If PrintSafepointStatisticsTimeout is specified, the statistics data will
   // be printed right away, in which case, _safepoint_stats will regress to
   // a single element array. Otherwise, it is a circular ring buffer with default
   // size of PrintSafepointStatisticsCount.
   int stats_array_size;
   if (PrintSafepointStatisticsTimeout > 0) {
     stats_array_size = 1;
     PrintSafepointStatistics = true;
   } else {
     stats_array_size = PrintSafepointStatisticsCount;
   }
   _safepoint_stats = (SafepointStats*)os::malloc(stats_array_size
                                                  * sizeof(SafepointStats));
   guarantee(_safepoint_stats != NULL,
             "not enough memory for safepoint instrumentation data");
   if (UseCompilerSafepoints && DeferPollingPageLoopCount >= 0) {
     need_to_track_page_armed_status = true;
   }
   init_done = true;
 }
 void SafepointSynchronize::begin_statistics(int nof_threads, int nof_running) {
   assert(init_done, "safepoint statistics array hasn't been initialized");
   SafepointStats *spstat = &_safepoint_stats[_cur_stat_index];
   spstat->_time_stamp = _ts_of_current_safepoint;
   VM_Operation *op = VMThread::vm_operation();
   spstat->_vmop_type = (op != NULL ? op->type() : -1);
   if (op != NULL) {
     _safepoint_reasons[spstat->_vmop_type]++;
   }
   spstat->_nof_total_threads = nof_threads;
   spstat->_nof_initial_running_threads = nof_running;
   spstat->_nof_threads_hit_page_trap = 0;
   // Records the start time of spinning. The real time spent on spinning
   // will be adjusted when spin is done. Same trick is applied for time
   // spent on waiting for threads to block.
   if (nof_running != 0) {
     spstat->_time_to_spin = os::javaTimeNanos();
   }  else {
     spstat->_time_to_spin = 0;
   }
 }
 void SafepointSynchronize::update_statistics_on_spin_end() {
   SafepointStats *spstat = &_safepoint_stats[_cur_stat_index];
   jlong cur_time = os::javaTimeNanos();
   spstat->_nof_threads_wait_to_block = _waiting_to_block;
   if (spstat->_nof_initial_running_threads != 0) {
     spstat->_time_to_spin = cur_time - spstat->_time_to_spin;
   }
   if (need_to_track_page_armed_status) {
     spstat->_page_armed = (PageArmed == 1);
   }
   // Records the start time of waiting for to block. Updated when block is done.
   if (_waiting_to_block != 0) {
     spstat->_time_to_wait_to_block = cur_time;
   } else {
     spstat->_time_to_wait_to_block = 0;
   }
 }
 void SafepointSynchronize::update_statistics_on_sync_end(jlong end_time) {
   SafepointStats *spstat = &_safepoint_stats[_cur_stat_index];
   if (spstat->_nof_threads_wait_to_block != 0) {
     spstat->_time_to_wait_to_block = end_time -
       spstat->_time_to_wait_to_block;
   }
   // Records the end time of sync which will be used to calculate the total
   // vm operation time. Again, the real time spending in syncing will be deducted
   // from the start of the sync time later when end_statistics is called.
   spstat->_time_to_sync = end_time - _safepoint_begin_time;
   if (spstat->_time_to_sync > _max_sync_time) {
     _max_sync_time = spstat->_time_to_sync;
   }
   spstat->_time_to_do_cleanups = end_time;
 }
 void SafepointSynchronize::update_statistics_on_cleanup_end(jlong end_time) {
   SafepointStats *spstat = &_safepoint_stats[_cur_stat_index];
   // Record how long spent in cleanup tasks.
   spstat->_time_to_do_cleanups = end_time - spstat->_time_to_do_cleanups;
   cleanup_end_time = end_time;
 }
 void SafepointSynchronize::end_statistics(jlong vmop_end_time) {
   SafepointStats *spstat = &_safepoint_stats[_cur_stat_index];
   // Update the vm operation time.
   spstat->_time_to_exec_vmop = vmop_end_time -  cleanup_end_time;
   if (spstat->_time_to_exec_vmop > _max_vmop_time) {
     _max_vmop_time = spstat->_time_to_exec_vmop;
   }
   // Only the sync time longer than the specified
   // PrintSafepointStatisticsTimeout will be printed out right away.
   // By default, it is -1 meaning all samples will be put into the list.
   if ( PrintSafepointStatisticsTimeout > 0) {
     if (spstat->_time_to_sync > PrintSafepointStatisticsTimeout * MICROUNITS) {
       print_statistics();
     }
   } else {
     // The safepoint statistics will be printed out when the _safepoin_stats
     // array fills up.
     if (_cur_stat_index == PrintSafepointStatisticsCount - 1) {
       print_statistics();
       _cur_stat_index = 0;
     } else {
       _cur_stat_index++;
     }
   }
 }
 void SafepointSynchronize::print_statistics() {
   SafepointStats* sstats = _safepoint_stats;
   for (int index = 0; index <= _cur_stat_index; index++) {
     if (index % 30 == 0) {
       print_header();
     }
     sstats = &_safepoint_stats[index];
     tty->print("%.3f: ", sstats->_time_stamp);
     tty->print("%-26s       ["
                INT32_FORMAT_W(8)INT32_FORMAT_W(11)INT32_FORMAT_W(15)
                "    ]    ",
                sstats->_vmop_type == -1 ? "no vm operation" :
                VM_Operation::name(sstats->_vmop_type),
                sstats->_nof_total_threads,
                sstats->_nof_initial_running_threads,
                sstats->_nof_threads_wait_to_block);
     // "/ MICROUNITS " is to convert the unit from nanos to millis.
     tty->print("  ["
                INT64_FORMAT_W(6)INT64_FORMAT_W(6)
                INT64_FORMAT_W(6)INT64_FORMAT_W(6)
                INT64_FORMAT_W(6)"    ]  ",
                sstats->_time_to_spin / MICROUNITS,
                sstats->_time_to_wait_to_block / MICROUNITS,
                sstats->_time_to_sync / MICROUNITS,
                sstats->_time_to_do_cleanups / MICROUNITS,
                sstats->_time_to_exec_vmop / MICROUNITS);
     if (need_to_track_page_armed_status) {
       tty->print(INT32_FORMAT"         ", sstats->_page_armed);
     }
     tty->print_cr(INT32_FORMAT"   ", sstats->_nof_threads_hit_page_trap);
   }
 }
 // This method will be called when VM exits. It will first call
 // print_statistics to print out the rest of the sampling.  Then
 // it tries to summarize the sampling.
 void SafepointSynchronize::print_stat_on_exit() {
   if (_safepoint_stats == NULL) return;
   SafepointStats *spstat = &_safepoint_stats[_cur_stat_index];
   // During VM exit, end_statistics may not get called and in that
   // case, if the sync time is less than PrintSafepointStatisticsTimeout,
   // don't print it out.
   // Approximate the vm op time.
   _safepoint_stats[_cur_stat_index]._time_to_exec_vmop =
     os::javaTimeNanos() - cleanup_end_time;
   if ( PrintSafepointStatisticsTimeout < 0 ||
        spstat->_time_to_sync > PrintSafepointStatisticsTimeout * MICROUNITS) {
     print_statistics();
   }
   tty->print_cr("");
   // Print out polling page sampling status.
   if (!need_to_track_page_armed_status) {
     if (UseCompilerSafepoints) {
       tty->print_cr("Polling page always armed");
     }
   } else {
     tty->print_cr("Defer polling page loop count = %d\n",
                  DeferPollingPageLoopCount);
   }
   for (int index = 0; index < VM_Operation::VMOp_Terminating; index++) {
     if (_safepoint_reasons[index] != 0) {
       tty->print_cr("%-26s"UINT64_FORMAT_W(10), VM_Operation::name(index),
                     _safepoint_reasons[index]);
     }
   }
   tty->print_cr(UINT64_FORMAT_W(5)" VM operations coalesced during safepoint",
                 _coalesced_vmop_count);
   tty->print_cr("Maximum sync time  "INT64_FORMAT_W(5)" ms",
                 _max_sync_time / MICROUNITS);
   tty->print_cr("Maximum vm operation time (except for Exit VM operation)  "
                 INT64_FORMAT_W(5)" ms",
                 _max_vmop_time / MICROUNITS);
 }
 // ------------------------------------------------------------------------------------------------
 // Non-product code
 #ifndef PRODUCT
 void SafepointSynchronize::print_state() {
   if (_state == _not_synchronized) {
     tty->print_cr("not synchronized");
   } else if (_state == _synchronizing || _state == _synchronized) {
     tty->print_cr("State: %s", (_state == _synchronizing) ? "synchronizing" :
                   "synchronized");
     for(JavaThread *cur = Threads::first(); cur; cur = cur->next()) {
        cur->safepoint_state()->print();
     }
   }
 }
 void SafepointSynchronize::safepoint_msg(const char* format, ...) {
   if (ShowSafepointMsgs) {
     va_list ap;
     va_start(ap, format);
     tty->vprint_cr(format, ap);
     va_end(ap);
   }
 }
 #endif // !PRODUCT

Mercurial > jdk8-mips64-public > hotspot / annotate

src/share/vm/runtime/safepoint.cpp@bfe29ec02863 (annotated)

src/share/vm/runtime/safepoint.cpp