--- a/src/hotspot/share/runtime/safepoint.cpp Wed Nov 22 14:31:48 2017 -0500
+++ b/src/hotspot/share/runtime/safepoint.cpp Wed Nov 22 17:54:50 2017 -0800
@@ -59,6 +59,7 @@
#include "runtime/sweeper.hpp"
#include "runtime/synchronizer.hpp"
#include "runtime/thread.inline.hpp"
+#include "runtime/threadSMR.hpp"
#include "runtime/timerTrace.hpp"
#include "services/runtimeService.hpp"
#include "trace/tracing.hpp"
@@ -174,7 +175,7 @@
if (SafepointMechanism::uses_thread_local_poll()) {
// Arming the per thread poll while having _state != _not_synchronized means safepointing
log_trace(safepoint)("Setting thread local yield flag for threads");
- for (JavaThread *cur = Threads::first(); cur != NULL; cur = cur->next()) {
+ for (JavaThreadIteratorWithHandle jtiwh; JavaThread *cur = jtiwh.next(); ) {
// Make sure the threads start polling, it is time to yield.
SafepointMechanism::arm_local_poll(cur); // release store, global state -> local state
}
@@ -200,133 +201,137 @@
// Consider using active_processor_count() ... but that call is expensive.
int ncpus = os::processor_count() ;
+ unsigned int iterations = 0;
+ {
+ JavaThreadIteratorWithHandle jtiwh;
#ifdef ASSERT
- for (JavaThread *cur = Threads::first(); cur != NULL; cur = cur->next()) {
- assert(cur->safepoint_state()->is_running(), "Illegal initial state");
- // Clear the visited flag to ensure that the critical counts are collected properly.
- cur->set_visited_for_critical_count(false);
- }
+ for (; JavaThread *cur = jtiwh.next(); ) {
+ assert(cur->safepoint_state()->is_running(), "Illegal initial state");
+ // Clear the visited flag to ensure that the critical counts are collected properly.
+ cur->set_visited_for_critical_count(false);
+ }
#endif // ASSERT
- if (SafepointTimeout)
- safepoint_limit_time = os::javaTimeNanos() + (jlong)SafepointTimeoutDelay * MICROUNITS;
+ 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
- }
- LogTarget(Trace, safepoint) lt;
- if (lt.is_enabled()) {
- ResourceMark rm;
- LogStream ls(lt);
- cur_state->print_on(&ls);
+ // Iterate through all threads until it have been determined how to stop them all at a safepoint
+ int steps = 0 ;
+ while(still_running > 0) {
+ jtiwh.rewind();
+ for (; JavaThread *cur = jtiwh.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
+ }
+ LogTarget(Trace, safepoint) lt;
+ if (lt.is_enabled()) {
+ ResourceMark rm;
+ LogStream ls(lt);
+ cur_state->print_on(&ls);
+ }
}
}
- }
- if (iterations == 0) {
- initial_running = still_running;
- if (PrintSafepointStatistics) {
- 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);
+ if (iterations == 0) {
+ initial_running = still_running;
+ if (PrintSafepointStatistics) {
+ begin_statistics(nof_threads, still_running);
+ }
}
- // 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 to naked_short_sleep() after some number of iterations.
- // nakes_short_sleep() is implemented as a short unconditional sleep.
- // 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 potentially 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 (SafepointMechanism::uses_global_page_poll() && 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::naked_yield() ;
- } else {
- os::naked_short_sleep(1);
+ if (still_running > 0) {
+ // Check for if it takes to long
+ if (SafepointTimeout && safepoint_limit_time < os::javaTimeNanos()) {
+ print_safepoint_timeout(_spinning_timeout);
}
- iterations ++ ;
+ // 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 to naked_short_sleep() after some number of iterations.
+ // nakes_short_sleep() is implemented as a short unconditional sleep.
+ // 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 potentially 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 (SafepointMechanism::uses_global_page_poll() && 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::naked_yield() ;
+ } else {
+ os::naked_short_sleep(1);
+ }
+
+ iterations ++ ;
+ }
+ assert(iterations < (uint)max_jint, "We have been iterating in the safepoint loop too long");
}
- assert(iterations < (uint)max_jint, "We have been iterating in the safepoint loop too long");
- }
+ } // ThreadsListHandle destroyed here.
assert(still_running == 0, "sanity check");
if (PrintSafepointStatistics) {
@@ -341,7 +346,7 @@
sync_event.set_iterations(iterations);
sync_event.commit();
}
- } //EventSafepointStateSync
+ } // EventSafepointStateSynchronization destroyed here.
// wait until all threads are stopped
{
@@ -393,8 +398,8 @@
} // EventSafepointWaitBlocked
#ifdef ASSERT
- for (JavaThread *cur = Threads::first(); cur != NULL; cur = cur->next()) {
- // make sure all the threads were visited
+ // Make sure all the threads were visited.
+ for (JavaThreadIteratorWithHandle jtiwh; JavaThread *cur = jtiwh.next(); ) {
assert(cur->was_visited_for_critical_count(), "missed a thread");
}
#endif // ASSERT
@@ -452,81 +457,86 @@
end_statistics(os::javaTimeNanos());
}
+ {
+ JavaThreadIteratorWithHandle jtiwh;
#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");
- }
+ // 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 = jtiwh.next(); ) {
+ assert (!(cur->has_pending_exception() &&
+ cur->safepoint_state()->is_at_poll_safepoint()),
+ "safepoint installed a pending exception");
+ }
#endif // ASSERT
- if (PageArmed) {
- assert(SafepointMechanism::uses_global_page_poll(), "sanity");
- // Make polling safepoint aware
- os::make_polling_page_readable();
- PageArmed = 0 ;
- }
-
- if (SafepointMechanism::uses_global_page_poll()) {
- // Remove safepoint check from interpreter
- Interpreter::ignore_safepoints();
- }
-
- {
- MutexLocker mu(Safepoint_lock);
-
- assert(_state == _synchronized, "must be synchronized before ending safepoint synchronization");
+ if (PageArmed) {
+ assert(SafepointMechanism::uses_global_page_poll(), "sanity");
+ // Make polling safepoint aware
+ os::make_polling_page_readable();
+ PageArmed = 0 ;
+ }
- if (SafepointMechanism::uses_thread_local_poll()) {
- _state = _not_synchronized;
- OrderAccess::storestore(); // global state -> local state
- for (JavaThread *current = Threads::first(); current; current = current->next()) {
- ThreadSafepointState* cur_state = current->safepoint_state();
- cur_state->restart(); // TSS _running
- SafepointMechanism::disarm_local_poll(current); // release store, local state -> polling page
- }
- log_debug(safepoint)("Leaving safepoint region");
- } else {
- // 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();
-
- log_debug(safepoint)("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");
- }
+ if (SafepointMechanism::uses_global_page_poll()) {
+ // Remove safepoint check from interpreter
+ Interpreter::ignore_safepoints();
}
- RuntimeService::record_safepoint_end();
+ {
+ MutexLocker mu(Safepoint_lock);
+
+ assert(_state == _synchronized, "must be synchronized before ending safepoint synchronization");
+
+ if (SafepointMechanism::uses_thread_local_poll()) {
+ _state = _not_synchronized;
+ OrderAccess::storestore(); // global state -> local state
+ jtiwh.rewind();
+ for (; JavaThread *current = jtiwh.next(); ) {
+ ThreadSafepointState* cur_state = current->safepoint_state();
+ cur_state->restart(); // TSS _running
+ SafepointMechanism::disarm_local_poll(current); // release store, local state -> polling page
+ }
+ log_debug(safepoint)("Leaving safepoint region");
+ } else {
+ // 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();
+
+ log_debug(safepoint)("Leaving safepoint region");
- // Release threads lock, so threads can be created/destroyed again. It will also starts all threads
- // blocked in signal_thread_blocked
- Threads_lock->unlock();
+ // Start suspended threads
+ jtiwh.rewind();
+ for (; JavaThread *current = jtiwh.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 release all threads blocked in signal_thread_blocked.
+ Threads_lock->unlock();
+ }
+ } // ThreadsListHandle destroyed here.
+
Universe::heap()->safepoint_synchronize_end();
// record this time so VMThread can keep track how much time has elapsed
// since last safepoint.
@@ -915,12 +925,11 @@
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()) {
+ for (JavaThreadIteratorWithHandle jtiwh; JavaThread *cur_thread = jtiwh.next(); ) {
cur_state = cur_thread->safepoint_state();
if (cur_thread->thread_state() != _thread_blocked &&
- ((reason == _spinning_timeout && cur_state->is_running()) ||
+ ((reason == _spinning_timeout && cur_state->is_running()) ||
(reason == _blocking_timeout && !cur_state->has_called_back()))) {
tty->print("# ");
cur_thread->print();
@@ -1427,7 +1436,7 @@
tty->print_cr("State: %s", (_state == _synchronizing) ? "synchronizing" :
"synchronized");
- for (JavaThread *cur = Threads::first(); cur; cur = cur->next()) {
+ for (JavaThreadIteratorWithHandle jtiwh; JavaThread *cur = jtiwh.next(); ) {
cur->safepoint_state()->print();
}
}