src/hotspot/share/jfr/leakprofiler/checkpoint/eventEmitter.cpp

/*
 * Copyright (c) 2019, 2025, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2021, Datadog, 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

#include "jfr/jfrEvents.hpp"
#include "jfr/leakprofiler/chains/edgeStore.hpp"
#include "jfr/leakprofiler/chains/pathToGcRootsOperation.hpp"
#include "jfr/leakprofiler/checkpoint/eventEmitter.hpp"
#include "jfr/leakprofiler/checkpoint/objectSampleCheckpoint.hpp"
#include "jfr/leakprofiler/sampling/objectSample.hpp"
#include "jfr/leakprofiler/sampling/objectSampler.hpp"
#include "jfr/leakprofiler/utilities/unifiedOopRef.inline.hpp"
#include "logging/log.hpp"
#include "memory/resourceArea.hpp"
#include "oops/markWord.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/javaThread.hpp"
#include "runtime/mutexLocker.hpp"
#include "runtime/vmThread.hpp"

EventEmitter::EventEmitter(const JfrTicks& start_time, const JfrTicks& end_time) :
  _start_time(start_time),
  _end_time(end_time),
  _thread(Thread::current()),
  _jfr_thread_local(_thread->jfr_thread_local()) {}

EventEmitter::~EventEmitter() {
  // restore / reset thread local stack trace and thread id
  _jfr_thread_local->clear_cached_stack_trace();
  JfrThreadLocal::stop_impersonating(_thread);
}

void EventEmitter::emit(ObjectSampler* sampler, int64_t cutoff_ticks, bool emit_all, bool skip_bfs) {
  assert(sampler != nullptr, "invariant");
  ResourceMark rm;
  EdgeStore edge_store;
  if (cutoff_ticks <= 0) {
    // no reference chains
    JfrTicks time_stamp = JfrTicks::now();
    EventEmitter emitter(time_stamp, time_stamp);
    emitter.write_events(sampler, &edge_store, emit_all);
    return;
  }
  // events emitted with reference chains require a safepoint operation
  PathToGcRootsOperation op(sampler, &edge_store, cutoff_ticks, emit_all, skip_bfs);
  VMThread::execute(&op);
}

size_t EventEmitter::write_events(ObjectSampler* object_sampler, EdgeStore* edge_store, bool emit_all) {
  assert(_thread == Thread::current(), "invariant");
  assert(_thread->jfr_thread_local() == _jfr_thread_local, "invariant");
  assert(object_sampler != nullptr, "invariant");
  assert(edge_store != nullptr, "invariant");

  const jlong last_sweep = emit_all ? max_jlong : ObjectSampler::last_sweep();
  size_t count = 0;

  // First pass associates a live sample with its immediate edge
  // in preparation for writing checkpoint information.
  const ObjectSample* current = object_sampler->first();
  while (current != nullptr) {
    ObjectSample* prev = current->prev();
    if (current->is_alive_and_older_than(last_sweep)) {
      link_sample_with_edge(current, edge_store);
      ++count;
    }
    current = prev;
  }
  if (count > 0) {
    // We need to serialize the associated checkpoints and potential chains
    // before writing the events to ensure constants are available for resolution
    // at the time old object sample events appear in the stream.
    ObjectSampleCheckpoint::write(object_sampler, edge_store, emit_all, _thread);

    // Now we are ready to write the events
    const ObjectSample* current = object_sampler->first();
    while (current != nullptr) {
      ObjectSample* prev = current->prev();
      if (current->is_alive_and_older_than(last_sweep)) {
        write_event(current, edge_store);
      }
      current = prev;
    }
  }
  return count;
}

static int array_size(const oop object) {
  assert(object != nullptr, "invariant");
  if (object->is_array()) {
    return arrayOop(object)->length();
  }
  return min_jint;
}

void EventEmitter::link_sample_with_edge(const ObjectSample* sample, EdgeStore* edge_store) {
  assert(sample != nullptr, "invariant");
  assert(!sample->is_dead(), "invariant");
  assert(edge_store != nullptr, "invariant");
  if (SafepointSynchronize::is_at_safepoint()) {
    if (edge_store->has_leak_context(sample)) {
      // Associated with an edge (chain) already during heap traversal.
      return;
    }
  }
  // In order to dump out a representation of the event
  // even though the sample object was found not reachable / too long to reach,
  // we need to register a top level edge.
  edge_store->put(UnifiedOopRef::encode_in_native(sample->object_addr()));
}

void EventEmitter::write_event(const ObjectSample* sample, EdgeStore* edge_store) {
  assert(sample != nullptr, "invariant");
  assert(!sample->is_dead(), "invariant");
  assert(edge_store != nullptr, "invariant");
  assert(_jfr_thread_local != nullptr, "invariant");

  const StoredEdge* const edge = edge_store->get(sample);
  assert(edge != nullptr, "invariant");
  assert(edge->pointee() == sample->object(), "invariant");
  const traceid object_id = edge_store->get_id(edge);
  assert(object_id != 0, "invariant");
  const traceid gc_root_id = edge->gc_root_id();

  Tickspan object_age = Ticks(_start_time.value()) - sample->allocation_time();

  EventOldObjectSample e(UNTIMED);
  e.set_starttime(_start_time);
  e.set_endtime(_end_time);
  e.set_allocationTime(sample->allocation_time());
  e.set_objectSize(sample->allocated());
  e.set_objectAge(object_age);
  e.set_lastKnownHeapUsage(sample->heap_used_at_last_gc());
  e.set_object(object_id);
  e.set_arrayElements(array_size(edge->pointee()));
  e.set_root(gc_root_id);

  // Temporarily assigning both the stack trace id and thread id
  // onto the thread local data structure of the emitter thread (for the duration
  // of the commit() call). This trick provides a means to override
  // the event generation mechanism by injecting externally provided id's.
  // At this particular location, it allows us to emit an old object event
  // supplying information from where the actual sampling occurred.
  _jfr_thread_local->set_cached_stack_trace_id(sample->stack_trace_id());
  assert(sample->has_thread(), "invariant");
  JfrThreadLocal::impersonate(_thread, sample->thread_id());
  e.commit();
}