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src/hotspot/share/gc/g1/g1DirtyCardQueue.cpp
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rev 57716 : [mq]: remove_cbl_mon
@@ -1,7 +1,7 @@
/*
- * Copyright (c) 2001, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2020, Oracle and/or its affiliates. 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.
@@ -24,10 +24,11 @@
#include "precompiled.hpp"
#include "gc/g1/g1BufferNodeList.hpp"
#include "gc/g1/g1CardTableEntryClosure.hpp"
#include "gc/g1/g1CollectedHeap.inline.hpp"
+#include "gc/g1/g1ConcurrentRefineThread.hpp"
#include "gc/g1/g1DirtyCardQueue.hpp"
#include "gc/g1/g1FreeIdSet.hpp"
#include "gc/g1/g1RedirtyCardsQueue.hpp"
#include "gc/g1/g1RemSet.hpp"
#include "gc/g1/g1ThreadLocalData.hpp"
@@ -40,11 +41,14 @@
#include "runtime/orderAccess.hpp"
#include "runtime/os.hpp"
#include "runtime/safepoint.hpp"
#include "runtime/thread.inline.hpp"
#include "runtime/threadSMR.hpp"
+#include "utilities/globalCounter.inline.hpp"
+#include "utilities/macros.hpp"
#include "utilities/quickSort.hpp"
+#include <new>
G1DirtyCardQueue::G1DirtyCardQueue(G1DirtyCardQueueSet* qset) :
// Dirty card queues are always active, so we create them with their
// active field set to true.
PtrQueue(qset, true /* active */)
@@ -52,10 +56,12 @@
G1DirtyCardQueue::~G1DirtyCardQueue() {
flush();
}
+BufferNode* const NULL_buffer = NULL;
+
void G1DirtyCardQueue::handle_completed_buffer() {
assert(_buf != NULL, "precondition");
BufferNode* node = BufferNode::make_node_from_buffer(_buf, index());
G1DirtyCardQueueSet* dcqs = dirty_card_qset();
if (dcqs->process_or_enqueue_completed_buffer(node)) {
@@ -66,19 +72,19 @@
}
// Assumed to be zero by concurrent threads.
static uint par_ids_start() { return 0; }
-G1DirtyCardQueueSet::G1DirtyCardQueueSet(Monitor* cbl_mon,
- BufferNode::Allocator* allocator) :
+G1DirtyCardQueueSet::G1DirtyCardQueueSet(BufferNode::Allocator* allocator) :
PtrQueueSet(allocator),
- _cbl_mon(cbl_mon),
- _completed_buffers_head(NULL),
- _completed_buffers_tail(NULL),
+ _primary_refinement_thread(NULL),
+ _completed_buffers_head(NULL_buffer),
+ _completed_buffers_tail(NULL_buffer),
_num_cards(0),
+ DEBUG_ONLY(_concurrency(0) COMMA)
+ _paused(),
_process_cards_threshold(ProcessCardsThresholdNever),
- _process_completed_buffers(false),
_max_cards(MaxCardsUnlimited),
_max_cards_padding(0),
_free_ids(par_ids_start(), num_par_ids()),
_mutator_refined_cards_counters(NEW_C_HEAP_ARRAY(size_t, num_par_ids(), mtGC))
{
@@ -106,127 +112,328 @@
void G1DirtyCardQueueSet::handle_zero_index_for_thread(Thread* t) {
G1ThreadLocalData::dirty_card_queue(t).handle_zero_index();
}
-void G1DirtyCardQueueSet::enqueue_completed_buffer(BufferNode* cbn) {
- MonitorLocker ml(_cbl_mon, Mutex::_no_safepoint_check_flag);
- cbn->set_next(NULL);
- if (_completed_buffers_tail == NULL) {
- assert(_completed_buffers_head == NULL, "Well-formedness");
- _completed_buffers_head = cbn;
- _completed_buffers_tail = cbn;
+// _concurrency is an int that is used in debug-only context to verify
+// we're not overlapping queue operations that support concurrency with
+// those which don't. The value is initially zero, meaning there are no
+// relevant operations in progress. A "no concurrency" context is entered
+// by atomically changing the value from 0 to -1, with an assert on failure.
+// It is similarly exited by reverting the value back to 0. A "concurrent"
+// context is entered by atomically incrementing the value and verifying the
+// result is greater than zero (so we weren't in a "no concurrency" context).
+// It is similarly exited by atomically decrementing the value and verifying
+// the result is at least zero (so no mismatches).
+//
+// ConcurrentVerifier and NonconcurrentVerifier are helper classes to
+// establish and remove such contexts.
+
+class G1DirtyCardQueueSet::ConcurrentVerifier : public StackObj {
+#ifdef ASSERT
+ const G1DirtyCardQueueSet* _dcqs;
+
+public:
+ ~ConcurrentVerifier() {
+ assert(Atomic::sub(&_dcqs->_concurrency, 1) >= 0, "invariant");
+ }
+#endif // ASSERT
+
+public:
+ ConcurrentVerifier(const G1DirtyCardQueueSet* dcqs) DEBUG_ONLY(: _dcqs(dcqs)) {
+ assert(Atomic::add(&_dcqs->_concurrency, 1) > 0, "invariant");
+ }
+};
+
+class G1DirtyCardQueueSet::NonconcurrentVerifier : public StackObj {
+#ifdef ASSERT
+ const G1DirtyCardQueueSet* _dcqs;
+
+public:
+ ~NonconcurrentVerifier() {
+ assert(Atomic::cmpxchg(&_dcqs->_concurrency, -1, 0) == -1, "invariant");
+ }
+#endif // ASSERT
+
+public:
+ NonconcurrentVerifier(const G1DirtyCardQueueSet* dcqs) DEBUG_ONLY(: _dcqs(dcqs)) {
+ assert(Atomic::cmpxchg(&_dcqs->_concurrency, 0, -1) == 0, "invariant");
+ }
+};
+
+// _completed_buffers_{head,tail} and _num_cards provide a lock-free FIFO
+// of buffers, linked through their next() fields.
+//
+// The key idea to make this work is that pop (get_completed_buffer) never
+// returns an element of the queue if it is the only accessible element,
+// e.g. its "next" value is NULL. It is expected that there will be a
+// later push/append that will make that element available to a future pop,
+// or there will eventually be a complete transfer (take_all_completed_buffers).
+//
+// An append operation atomically exchanges the new tail with the queue tail.
+// It then sets the "next" value of the old tail to the head of the list being
+// appended. (It is an invariant that the old tail's "next" value is NULL.)
+// But if the old tail is NULL then the queue was empty. In this case the
+// head of the list being appended is instead stored in the queue head (which
+// must be NULL).
+//
+// A push operation is just a degenerate append, where the buffer being pushed
+// is both the head and the tail of the list being appended.
+//
+// This means there is a period between the exchange and the old tail update
+// where the queue sequence is split into two parts, the list from the queue
+// head to the old tail, and the list being appended. If there are concurrent
+// push/append operations, each may introduce another such segment. But they
+// all eventually get resolved by their respective updates of their old tail's
+// "next" value.
+//
+// pop gets the queue head as the candidate result (returning NULL if the
+// queue head was NULL), and then gets that result node's "next" value. If
+// that "next" value is NULL and the queue head hasn't changed, then there
+// is only one element in the (accessible) list. We can't return that
+// element, because it may be the old tail of a concurrent push/append. So
+// return NULL in this case. Otherwise, attempt to cmpxchg that "next"
+// value into the queue head, retrying the whole operation if that fails.
+// This is the "usual" lock-free pop from head of slist, with the additional
+// restriction on taking the last element.
+//
+// In order to address the ABA problem for pop, a pop operation protects its
+// access to the head of the list with a GlobalCounter critical section. This
+// works with the buffer allocator's use of GlobalCounter synchronization to
+// prevent ABA from arising in the normal buffer usage cycle. The paused
+// buffer handling prevents another ABA source (see record_paused_buffer and
+// enqueue_previous_paused_buffers).
+
+size_t G1DirtyCardQueueSet::append_buffers(BufferNode* first,
+ BufferNode* last,
+ size_t card_count) {
+ assert(last->next() == NULL_buffer, "precondition");
+ ConcurrentVerifier cv(this);
+ // Increment _num_cards before adding to queue, so queue removal doesn't
+ // need to deal with _num_cards possibly going negative.
+ size_t new_num_cards = Atomic::add(&_num_cards, card_count);
+ BufferNode* old_tail = Atomic::xchg(&_completed_buffers_tail, last);
+ if (old_tail == NULL_buffer) { // Empty list.
+ assert(Atomic::load(&_completed_buffers_head) == NULL_buffer, "invariant");
+ Atomic::store(&_completed_buffers_head, first);
} else {
- _completed_buffers_tail->set_next(cbn);
- _completed_buffers_tail = cbn;
+ assert(old_tail->next() == NULL_buffer, "invariant");
+ old_tail->set_next(first);
}
- _num_cards += buffer_size() - cbn->index();
+ return new_num_cards;
+}
- if (!process_completed_buffers() &&
- (num_cards() > process_cards_threshold())) {
- set_process_completed_buffers(true);
- ml.notify_all();
+void G1DirtyCardQueueSet::enqueue_completed_buffer(BufferNode* cbn) {
+ assert(cbn != NULL_buffer, "precondition");
+ size_t new_num_cards = append_buffers(cbn, cbn, buffer_size() - cbn->index());
+ if ((_primary_refinement_thread != NULL) &&
+ (new_num_cards > process_cards_threshold())) {
+ _primary_refinement_thread->activate();
}
- verify_num_cards();
}
BufferNode* G1DirtyCardQueueSet::get_completed_buffer(size_t stop_at) {
- MutexLocker x(_cbl_mon, Mutex::_no_safepoint_check_flag);
+ enqueue_previous_paused_buffers();
- if (num_cards() <= stop_at) {
- return NULL;
- }
+ ConcurrentVerifier cv(this);
- assert(num_cards() > 0, "invariant");
- assert(_completed_buffers_head != NULL, "invariant");
- assert(_completed_buffers_tail != NULL, "invariant");
-
- BufferNode* bn = _completed_buffers_head;
- _num_cards -= buffer_size() - bn->index();
- _completed_buffers_head = bn->next();
- if (_completed_buffers_head == NULL) {
- assert(num_cards() == 0, "invariant");
- _completed_buffers_tail = NULL;
- set_process_completed_buffers(false);
+ // Check for unsufficient cards to satisfy request. We only do this once,
+ // up front, rather than on each iteration below, since the test is racy
+ // regardless of when we do it.
+ if (Atomic::load_acquire(&_num_cards) <= stop_at) {
+ return NULL_buffer;
+ }
+
+ Thread* current_thread = Thread::current();
+
+ while (true) {
+ // Use a critical section per iteration, rather than over the whole
+ // operation. We're not guaranteed to make progress, because of possible
+ // contention on the queue head. Lingering in one CS the whole time could
+ // lead to excessive allocation of buffers, because the CS blocks return
+ // of released buffers to the free list for reuse.
+ GlobalCounter::CriticalSection cs(current_thread);
+
+ BufferNode* result = Atomic::load_acquire(&_completed_buffers_head);
+ // Check for empty queue. Only needs to be done on first iteration,
+ // since we never take the last element, but it's messy to make use
+ // of that and we expect one iteration to be the common case.
+ if (result == NULL_buffer) return result;
+
+ BufferNode* next = Atomic::load_acquire(BufferNode::next_ptr(*result));
+ if (next != NULL_buffer) {
+ next = Atomic::cmpxchg(&_completed_buffers_head, result, next);
+ if (next == result) {
+ // Former head successfully taken; it is not the last.
+ assert(Atomic::load(&_completed_buffers_tail) != result, "invariant");
+ assert(result->next() != NULL_buffer, "invariant");
+ result->set_next(NULL_buffer);
+ Atomic::sub(&_num_cards, buffer_size() - result->index());
+ return result;
}
- verify_num_cards();
- bn->set_next(NULL);
- return bn;
+ // cmpxchg failed; try again.
+ } else if (result == Atomic::load_acquire(&_completed_buffers_head)) {
+ // If follower of head is NULL and head hasn't changed, then only
+ // the one element is currently accessible. We don't take the last
+ // accessible element, because there may be a concurrent add using it.
+ // The check for unchanged head isn't needed for correctness, but the
+ // retry on change may sometimes let us get a buffer after all.
+ return NULL_buffer;
+ }
+ // Head changed; try again.
+ }
+ // Unreachable
}
#ifdef ASSERT
void G1DirtyCardQueueSet::verify_num_cards() const {
+ NonconcurrentVerifier ncv(this);
size_t actual = 0;
- BufferNode* cur = _completed_buffers_head;
- while (cur != NULL) {
+ BufferNode* cur = Atomic::load(&_completed_buffers_head);
+ for ( ; cur != NULL_buffer; cur = cur->next()) {
actual += buffer_size() - cur->index();
- cur = cur->next();
}
- assert(actual == _num_cards,
+ assert(actual == Atomic::load(&_num_cards),
"Num entries in completed buffers should be " SIZE_FORMAT " but are " SIZE_FORMAT,
- _num_cards, actual);
+ Atomic::load(&_num_cards), actual);
}
-#endif
+#endif // ASSERT
-void G1DirtyCardQueueSet::abandon_completed_buffers() {
- BufferNode* buffers_to_delete = NULL;
- {
- MutexLocker x(_cbl_mon, Mutex::_no_safepoint_check_flag);
- buffers_to_delete = _completed_buffers_head;
- _completed_buffers_head = NULL;
- _completed_buffers_tail = NULL;
- _num_cards = 0;
- set_process_completed_buffers(false);
+// Refinement processing stops early if there is a pending safepoint, to
+// avoid long delays to safepoint. We need to record the partially
+// processed buffer for later continued processing. However, we can't
+// simply add it back to the completed buffer queue, as that would introduce
+// a new source of ABA for the queue. Instead, we have a pair of buffer
+// lists (with each list represented by head and tail), one for each of the
+// previous and next safepoints (*). When pausing the processing of a
+// buffer for a safepoint, we add the buffer (lock free) to the list for the
+// next safepoint. Before attempting to obtain a buffer from the queue we
+// first transfer any buffers in the previous safepoint list to the queue.
+// This is safe (doesn't introduce ABA) because threads cannot be in the
+// midst of a queue pop across a safepoint.
+//
+// These paused buffer lists are conceptually an extension of the queue, and
+// operations which need to deal with all of the queued buffers (such as
+// concatenate_logs) also need to deal with any paused buffers. In general,
+// if the safepoint performs a GC then the paused buffers will be processed
+// as part of it and both lists will be empty afterward.
+//
+// An alternative would be to directly reenqueue a paused buffer, but only
+// after first calling GlobalCounter::write_synchronize. However, that
+// might noticeably delay the pending safepoint.
+//
+// A single paused list and a safepoint cleanup action to perform the transfer
+// doesn't work because cleanup actions are not invoked for every safepoint.
+//
+// (*) If the safepoint does not perform a GC, the next list becomes the
+// previous list after the safepoint. Since buffers are only added to the
+// next list if there were threads performing refinement work, there will
+// likely be refinement work done after the safepoint, which will transfer
+// those buffers to the queue. However, multiple non-GC safepoints in
+// succession, without intervening refinement work to perform a transfer
+// (possibly through lack of time), can result in old buffers being present
+// and inaccessible in the next list. This doesn't affect correctness, but
+// might affect performance. The alternatives discussed above don't have
+// this problem, but have problems of their own.
+
+static size_t next_paused_buffer_list_index() {
+ return SafepointSynchronize::safepoint_id() & 1;
+}
+
+static size_t previous_paused_buffer_list_index() {
+ return next_paused_buffer_list_index() ^ 1;
+}
+
+void G1DirtyCardQueueSet::record_paused_buffer(BufferNode* node) {
+ assert_not_at_safepoint();
+ assert(node->next() == NULL_buffer, "precondition");
+ size_t next_index = next_paused_buffer_list_index();
+ // Cards for paused buffers are included in count, to contribute to
+ // notification checking after the coming safepoint if it doesn't GC.
+ Atomic::add(&_num_cards, buffer_size() - node->index());
+ BufferNode* old_head = Atomic::xchg(&_paused[next_index]._head, node);
+ if (old_head == NULL_buffer) {
+ assert(_paused[next_index]._tail == NULL, "invariant");
+ _paused[next_index]._tail = node;
+ } else {
+ node->set_next(old_head);
+ }
+}
+
+void G1DirtyCardQueueSet::enqueue_paused_buffers_aux(size_t index) {
+ if (Atomic::load(&_paused[index]._head) != NULL_buffer) {
+ BufferNode* paused = Atomic::xchg(&_paused[index]._head, NULL_buffer);
+ if (paused != NULL_buffer) {
+ BufferNode* tail = _paused[index]._tail;
+ assert(tail != NULL, "invariant");
+ _paused[index]._tail = NULL_buffer;
+ append_buffers(paused, tail, 0); // Cards already counted when recorded.
}
- while (buffers_to_delete != NULL) {
+ }
+}
+
+void G1DirtyCardQueueSet::enqueue_previous_paused_buffers() {
+ size_t previous_index = previous_paused_buffer_list_index();
+ enqueue_paused_buffers_aux(previous_index);
+}
+
+void G1DirtyCardQueueSet::enqueue_all_paused_buffers() {
+ assert_at_safepoint();
+ for (size_t i = 0; i < ARRAY_SIZE(_paused); ++i) {
+ enqueue_paused_buffers_aux(i);
+ }
+}
+
+void G1DirtyCardQueueSet::clear_completed_buffers() {
+ Atomic::store(&_completed_buffers_head, NULL_buffer);
+ Atomic::store(&_completed_buffers_tail, NULL_buffer);
+ Atomic::store(&_num_cards, size_t(0));
+}
+
+void G1DirtyCardQueueSet::abandon_completed_buffers() {
+ enqueue_all_paused_buffers();
+ verify_num_cards();
+ NonconcurrentVerifier ncv(this);
+ BufferNode* buffers_to_delete = Atomic::load(&_completed_buffers_head);
+ clear_completed_buffers();
+ while (buffers_to_delete != NULL_buffer) {
BufferNode* bn = buffers_to_delete;
buffers_to_delete = bn->next();
- bn->set_next(NULL);
+ bn->set_next(NULL_buffer);
deallocate_buffer(bn);
}
}
void G1DirtyCardQueueSet::notify_if_necessary() {
- MonitorLocker ml(_cbl_mon, Mutex::_no_safepoint_check_flag);
- if (num_cards() > process_cards_threshold()) {
- set_process_completed_buffers(true);
- ml.notify_all();
+ if ((_primary_refinement_thread != NULL) &&
+ (num_cards() > process_cards_threshold())) {
+ _primary_refinement_thread->activate();
}
}
-// Merge lists of buffers. Notify the processing threads.
-// The source queue is emptied as a result. The queues
-// must share the monitor.
+// Merge lists of buffers. The source queue set is emptied as a
+// result. The queue sets must share the same allocator.
void G1DirtyCardQueueSet::merge_bufferlists(G1RedirtyCardsQueueSet* src) {
assert(allocator() == src->allocator(), "precondition");
const G1BufferNodeList from = src->take_all_completed_buffers();
- if (from._head == NULL) return;
-
- MutexLocker x(_cbl_mon, Mutex::_no_safepoint_check_flag);
- if (_completed_buffers_tail == NULL) {
- assert(_completed_buffers_head == NULL, "Well-formedness");
- _completed_buffers_head = from._head;
- _completed_buffers_tail = from._tail;
- } else {
- assert(_completed_buffers_head != NULL, "Well formedness");
- _completed_buffers_tail->set_next(from._head);
- _completed_buffers_tail = from._tail;
- }
- _num_cards += from._entry_count;
-
- assert(_completed_buffers_head == NULL && _completed_buffers_tail == NULL ||
- _completed_buffers_head != NULL && _completed_buffers_tail != NULL,
- "Sanity");
- verify_num_cards();
+ if (from._head == NULL_buffer) return;
+ append_buffers(from._head, from._tail, from._entry_count);
}
G1BufferNodeList G1DirtyCardQueueSet::take_all_completed_buffers() {
- MutexLocker x(_cbl_mon, Mutex::_no_safepoint_check_flag);
- G1BufferNodeList result(_completed_buffers_head, _completed_buffers_tail, _num_cards);
- _completed_buffers_head = NULL;
- _completed_buffers_tail = NULL;
- _num_cards = 0;
+#ifdef ASSERT
+ for (size_t i = 0; i < ARRAY_SIZE(_paused); ++i) {
+ assert(Atomic::load(&_paused[i]._head) == NULL_buffer, "precondition");
+ assert(Atomic::load(&_paused[i]._tail) == NULL_buffer, "precondition");
+ }
+#endif // ASSERT
+ verify_num_cards();
+ NonconcurrentVerifier ncv(this);
+ G1BufferNodeList result(Atomic::load(&_completed_buffers_head),
+ Atomic::load(&_completed_buffers_tail),
+ Atomic::load(&_num_cards));
+ clear_completed_buffers();
return result;
}
class G1RefineBufferedCards : public StackObj {
BufferNode* const _node;
@@ -397,26 +604,27 @@
bool G1DirtyCardQueueSet::refine_completed_buffer_concurrently(uint worker_id,
size_t stop_at,
size_t* total_refined_cards) {
BufferNode* node = get_completed_buffer(stop_at);
- if (node == NULL) {
+ if (node == NULL_buffer) {
return false;
} else if (refine_buffer(node, worker_id, total_refined_cards)) {
assert_fully_consumed(node, buffer_size());
// Done with fully processed buffer.
deallocate_buffer(node);
return true;
} else {
- // Return partially processed buffer to the queue.
- enqueue_completed_buffer(node);
+ // Buffer incompletely processed because there is a pending safepoint.
+ // Record partially processed buffer, to be finished later.
+ record_paused_buffer(node);
return true;
}
}
void G1DirtyCardQueueSet::abandon_logs() {
- assert(SafepointSynchronize::is_at_safepoint(), "Must be at safepoint.");
+ assert_at_safepoint();
abandon_completed_buffers();
// Since abandon is done only at safepoints, we can safely manipulate
// these queues.
struct AbandonThreadLogClosure : public ThreadClosure {
@@ -431,11 +639,11 @@
void G1DirtyCardQueueSet::concatenate_logs() {
// Iterate over all the threads, if we find a partial log add it to
// the global list of logs. Temporarily turn off the limit on the number
// of outstanding buffers.
- assert(SafepointSynchronize::is_at_safepoint(), "Must be at safepoint.");
+ assert_at_safepoint();
size_t old_limit = max_cards();
set_max_cards(MaxCardsUnlimited);
struct ConcatenateThreadLogClosure : public ThreadClosure {
virtual void do_thread(Thread* t) {
@@ -446,7 +654,9 @@
}
} closure;
Threads::threads_do(&closure);
G1BarrierSet::shared_dirty_card_queue().flush();
+ enqueue_all_paused_buffers();
+ verify_num_cards();
set_max_cards(old_limit);
}
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