1 /*
2 * Copyright (c) 2021, Red Hat, Inc. All rights reserved.
3 * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
4 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
5 *
6 * This code is free software; you can redistribute it and/or modify it
7 * under the terms of the GNU General Public License version 2 only, as
8 * published by the Free Software Foundation.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 *
24 */
25
26 #include "precompiled.hpp"
27
28 #include "gc/shared/collectorCounters.hpp"
29 #include "gc/shenandoah/shenandoahCollectorPolicy.hpp"
30 #include "gc/shenandoah/shenandoahConcurrentMark.hpp"
31 #include "gc/shenandoah/shenandoahDegeneratedGC.hpp"
32 #include "gc/shenandoah/shenandoahFullGC.hpp"
33 #include "gc/shenandoah/shenandoahGeneration.hpp"
34 #include "gc/shenandoah/shenandoahGenerationalHeap.hpp"
35 #include "gc/shenandoah/shenandoahHeap.inline.hpp"
36 #include "gc/shenandoah/shenandoahMetrics.hpp"
37 #include "gc/shenandoah/shenandoahMonitoringSupport.hpp"
38 #include "gc/shenandoah/shenandoahOldGeneration.hpp"
39 #include "gc/shenandoah/shenandoahOopClosures.inline.hpp"
40 #include "gc/shenandoah/shenandoahRootProcessor.inline.hpp"
41 #include "gc/shenandoah/shenandoahSTWMark.hpp"
42 #include "gc/shenandoah/shenandoahUtils.hpp"
43 #include "gc/shenandoah/shenandoahVerifier.hpp"
44 #include "gc/shenandoah/shenandoahYoungGeneration.hpp"
45 #include "gc/shenandoah/shenandoahWorkerPolicy.hpp"
46 #include "gc/shenandoah/shenandoahVMOperations.hpp"
47 #include "runtime/vmThread.hpp"
48 #include "utilities/events.hpp"
49
50 ShenandoahDegenGC::ShenandoahDegenGC(ShenandoahDegenPoint degen_point, ShenandoahGeneration* generation) :
51 ShenandoahGC(),
52 _degen_point(degen_point),
53 _generation(generation),
54 _abbreviated(false) {
55 }
56
57 bool ShenandoahDegenGC::collect(GCCause::Cause cause) {
58 vmop_degenerated();
59 ShenandoahHeap* heap = ShenandoahHeap::heap();
60 if (heap->mode()->is_generational()) {
61 bool is_bootstrap_gc = heap->old_generation()->is_bootstrapping();
62 heap->mmu_tracker()->record_degenerated(GCId::current(), is_bootstrap_gc);
63 const char* msg = is_bootstrap_gc? "At end of Degenerated Bootstrap Old GC": "At end of Degenerated Young GC";
64 heap->log_heap_status(msg);
65 }
66 return true;
67 }
68
69 void ShenandoahDegenGC::vmop_degenerated() {
70 TraceCollectorStats tcs(ShenandoahHeap::heap()->monitoring_support()->full_stw_collection_counters());
71 ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::degen_gc_gross);
72 VM_ShenandoahDegeneratedGC degenerated_gc(this);
73 VMThread::execute(°enerated_gc);
74 }
75
76 void ShenandoahDegenGC::entry_degenerated() {
77 const char* msg = degen_event_message(_degen_point);
78 ShenandoahPausePhase gc_phase(msg, ShenandoahPhaseTimings::degen_gc, true /* log_heap_usage */);
79 EventMark em("%s", msg);
80 ShenandoahHeap* const heap = ShenandoahHeap::heap();
81 ShenandoahWorkerScope scope(heap->workers(),
82 ShenandoahWorkerPolicy::calc_workers_for_stw_degenerated(),
83 "stw degenerated gc");
84
85 heap->set_degenerated_gc_in_progress(true);
86 op_degenerated();
87 heap->set_degenerated_gc_in_progress(false);
88 }
89
90 void ShenandoahDegenGC::op_degenerated() {
91 ShenandoahHeap* const heap = ShenandoahHeap::heap();
92 // Degenerated GC is STW, but it can also fail. Current mechanics communicates
93 // GC failure via cancelled_concgc() flag. So, if we detect the failure after
94 // some phase, we have to upgrade the Degenerate GC to Full GC.
95 heap->clear_cancelled_gc(true /* clear oom handler */);
96
97 #ifdef ASSERT
98 if (heap->mode()->is_generational()) {
99 ShenandoahOldGeneration* old_generation = heap->old_generation();
100 if (!heap->is_concurrent_old_mark_in_progress()) {
101 // If we are not marking the old generation, there should be nothing in the old mark queues
102 assert(old_generation->task_queues()->is_empty(), "Old gen task queues should be empty");
103 }
104
105 if (_generation->is_global()) {
106 // If we are in a global cycle, the old generation should not be marking. It is, however,
107 // allowed to be holding regions for evacuation or coalescing.
108 assert(old_generation->is_idle()
109 || old_generation->is_doing_mixed_evacuations()
110 || old_generation->is_preparing_for_mark(),
111 "Old generation cannot be in state: %s", old_generation->state_name());
112 }
113 }
114 #endif
115
116 ShenandoahMetricsSnapshot metrics;
117 metrics.snap_before();
118
119 switch (_degen_point) {
120 // The cases below form the Duff's-like device: it describes the actual GC cycle,
121 // but enters it at different points, depending on which concurrent phase had
122 // degenerated.
123
124 case _degenerated_outside_cycle:
125 // We have degenerated from outside the cycle, which means something is bad with
126 // the heap, most probably heavy humongous fragmentation, or we are very low on free
127 // space. It makes little sense to wait for Full GC to reclaim as much as it can, when
128 // we can do the most aggressive degen cycle, which includes processing references and
129 // class unloading, unless those features are explicitly disabled.
130
131 // Note that we can only do this for "outside-cycle" degens, otherwise we would risk
132 // changing the cycle parameters mid-cycle during concurrent -> degenerated handover.
133 heap->set_unload_classes(_generation->heuristics()->can_unload_classes() &&
134 (!heap->mode()->is_generational() || _generation->is_global()));
135
136 if (heap->mode()->is_generational() &&
137 (_generation->is_young() || (_generation->is_global() && ShenandoahVerify))) {
138 // Swap remembered sets for young, or if the verifier will run during a global collect
139 // TODO: This path should not depend on ShenandoahVerify
140 _generation->swap_remembered_set();
141 }
142
143 case _degenerated_roots:
144 // Degenerated from concurrent root mark, reset the flag for STW mark
145 if (!heap->mode()->is_generational()) {
146 if (heap->is_concurrent_mark_in_progress()) {
147 heap->cancel_concurrent_mark();
148 }
149 } else {
150 if (_generation->is_concurrent_mark_in_progress()) {
151 // We want to allow old generation marking to be punctuated by young collections
152 // (even if they have degenerated). If this is a global cycle, we'd have cancelled
153 // the entire old gc before coming into this switch. Note that cancel_marking on
154 // the generation does NOT abandon incomplete SATB buffers as cancel_concurrent_mark does.
155 // We need to separate out the old pointers which is done below.
156 _generation->cancel_marking();
157 }
158
159 if (heap->is_concurrent_mark_in_progress()) {
160 // If either old or young marking is in progress, the SATB barrier will be enabled.
161 // The SATB buffer may hold a mix of old and young pointers. The old pointers need to be
162 // transferred to the old generation mark queues and the young pointers are NOT part
163 // of this snapshot, so they must be dropped here. It is safe to drop them here because
164 // we will rescan the roots on this safepoint.
165 heap->old_generation()->transfer_pointers_from_satb();
166 }
167
168 if (_degen_point == ShenandoahDegenPoint::_degenerated_roots) {
169 // We only need this if the concurrent cycle has already swapped the card tables.
170 // Marking will use the 'read' table, but interesting pointers may have been
171 // recorded in the 'write' table in the time between the cancelled concurrent cycle
172 // and this degenerated cycle. These pointers need to be included the 'read' table
173 // used to scan the remembered set during the STW mark which follows here.
174 _generation->merge_write_table();
175 }
176 }
177
178 op_reset();
179
180 // STW mark
181 op_mark();
182
183 case _degenerated_mark:
184 // No fallthrough. Continue mark, handed over from concurrent mark if
185 // concurrent mark has yet completed
186 if (_degen_point == ShenandoahDegenPoint::_degenerated_mark &&
187 heap->is_concurrent_mark_in_progress()) {
188 op_finish_mark();
189 }
190 assert(!heap->cancelled_gc(), "STW mark can not OOM");
191
192 /* Degen select Collection Set. etc. */
193 op_prepare_evacuation();
194
195 op_cleanup_early();
196
197 case _degenerated_evac:
198 // If heuristics thinks we should do the cycle, this flag would be set,
199 // and we can do evacuation. Otherwise, it would be the shortcut cycle.
200 if (heap->is_evacuation_in_progress()) {
201
202 if (_degen_point == _degenerated_evac) {
203 // Degeneration under oom-evac protocol allows the mutator LRB to expose
204 // references to from-space objects. This is okay, in theory, because we
205 // will come to the safepoint here to complete the evacuations and update
206 // the references. However, if the from-space reference is written to a
207 // region that was EC during final mark or was recycled after final mark
208 // it will not have TAMS or UWM updated. Such a region is effectively
209 // skipped during update references which can lead to crashes and corruption
210 // if the from-space reference is accessed.
211 if (UseTLAB) {
212 heap->labs_make_parsable();
213 }
214
215 for (size_t i = 0; i < heap->num_regions(); i++) {
216 ShenandoahHeapRegion* r = heap->get_region(i);
217 if (r->is_active() && r->top() > r->get_update_watermark()) {
218 r->set_update_watermark_at_safepoint(r->top());
219 }
220 }
221 }
222
223 // Degeneration under oom-evac protocol might have left some objects in
224 // collection set un-evacuated. Restart evacuation from the beginning to
225 // capture all objects. For all the objects that are already evacuated,
226 // it would be a simple check, which is supposed to be fast. This is also
227 // safe to do even without degeneration, as CSet iterator is at beginning
228 // in preparation for evacuation anyway.
229 //
230 // Before doing that, we need to make sure we never had any cset-pinned
231 // regions. This may happen if allocation failure happened when evacuating
232 // the about-to-be-pinned object, oom-evac protocol left the object in
233 // the collection set, and then the pin reached the cset region. If we continue
234 // the cycle here, we would trash the cset and alive objects in it. To avoid
235 // it, we fail degeneration right away and slide into Full GC to recover.
236
237 {
238 heap->sync_pinned_region_status();
239 heap->collection_set()->clear_current_index();
240 ShenandoahHeapRegion* r;
241 while ((r = heap->collection_set()->next()) != nullptr) {
242 if (r->is_pinned()) {
243 heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
244 op_degenerated_fail();
245 return;
246 }
247 }
248
249 heap->collection_set()->clear_current_index();
250 }
251 op_evacuate();
252 if (heap->cancelled_gc()) {
253 op_degenerated_fail();
254 return;
255 }
256 }
257
258 // Update collector state regardless of whether or not there are forwarded objects
259 heap->set_evacuation_in_progress(false);
260 heap->set_concurrent_weak_root_in_progress(false);
261 heap->set_concurrent_strong_root_in_progress(false);
262
263 // If heuristics thinks we should do the cycle, this flag would be set,
264 // and we need to do update-refs. Otherwise, it would be the shortcut cycle.
265 if (heap->has_forwarded_objects()) {
266 op_init_updaterefs();
267 assert(!heap->cancelled_gc(), "STW reference update can not OOM");
268 } else {
269 _abbreviated = true;
270 }
271
272 case _degenerated_updaterefs:
273 if (heap->has_forwarded_objects()) {
274 op_updaterefs();
275 op_update_roots();
276 assert(!heap->cancelled_gc(), "STW reference update can not OOM");
277 }
278
279 // Disarm nmethods that armed in concurrent cycle.
280 // In above case, update roots should disarm them
281 ShenandoahCodeRoots::disarm_nmethods();
282
283 op_cleanup_complete();
284
285 if (heap->mode()->is_generational()) {
286 ShenandoahGenerationalHeap::heap()->complete_degenerated_cycle();
287 }
288
289 break;
290 default:
291 ShouldNotReachHere();
292 }
293
294 if (ShenandoahVerify) {
295 heap->verifier()->verify_after_degenerated();
296 }
297
298 if (VerifyAfterGC) {
299 Universe::verify();
300 }
301
302 metrics.snap_after();
303
304 // Check for futility and fail. There is no reason to do several back-to-back Degenerated cycles,
305 // because that probably means the heap is overloaded and/or fragmented.
306 if (!metrics.is_good_progress()) {
307 heap->notify_gc_no_progress();
308 heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
309 op_degenerated_futile();
310 } else {
311 heap->notify_gc_progress();
312 heap->shenandoah_policy()->record_success_degenerated(_generation->is_young(), _abbreviated);
313 _generation->heuristics()->record_success_degenerated();
314 }
315 }
316
317 void ShenandoahDegenGC::op_reset() {
318 _generation->prepare_gc();
319 }
320
321 void ShenandoahDegenGC::op_mark() {
322 assert(!_generation->is_concurrent_mark_in_progress(), "Should be reset");
323 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_mark);
324 ShenandoahSTWMark mark(_generation, false /*full gc*/);
325 mark.mark();
326 }
327
328 void ShenandoahDegenGC::op_finish_mark() {
329 ShenandoahConcurrentMark mark(_generation);
330 mark.finish_mark();
331 }
332
333 void ShenandoahDegenGC::op_prepare_evacuation() {
334 ShenandoahHeap* const heap = ShenandoahHeap::heap();
335 if (ShenandoahVerify) {
336 heap->verifier()->verify_roots_no_forwarded();
337 }
338
339 // STW cleanup weak roots and unload classes
340 heap->parallel_cleaning(false /*full gc*/);
341
342 // Prepare regions and collection set
343 _generation->prepare_regions_and_collection_set(false /*concurrent*/);
344
345 // Retire the TLABs, which will force threads to reacquire their TLABs after the pause.
346 // This is needed for two reasons. Strong one: new allocations would be with new freeset,
347 // which would be outside the collection set, so no cset writes would happen there.
348 // Weaker one: new allocations would happen past update watermark, and so less work would
349 // be needed for reference updates (would update the large filler instead).
350 if (UseTLAB) {
351 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_final_manage_labs);
352 heap->tlabs_retire(false);
353 }
354
355 if (!heap->collection_set()->is_empty() || has_in_place_promotions(heap)) {
356 // Even if the collection set is empty, we need to do evacuation if there are regions to be promoted in place.
357 // Degenerated evacuation takes responsibility for registering objects and setting the remembered set cards to dirty.
358
359 if (ShenandoahVerify) {
360 heap->verifier()->verify_before_evacuation();
361 }
362
363 heap->set_evacuation_in_progress(true);
364
365 if(ShenandoahVerify) {
366 heap->verifier()->verify_during_evacuation();
367 }
368
369 heap->set_has_forwarded_objects(!heap->collection_set()->is_empty());
370 } else {
371 if (ShenandoahVerify) {
372 heap->verifier()->verify_after_concmark();
373 }
374
375 if (VerifyAfterGC) {
376 Universe::verify();
377 }
378 }
379 }
380
381 bool ShenandoahDegenGC::has_in_place_promotions(const ShenandoahHeap* heap) const {
382 return heap->mode()->is_generational() && heap->old_generation()->has_in_place_promotions();
383 }
384
385 void ShenandoahDegenGC::op_cleanup_early() {
386 ShenandoahHeap::heap()->recycle_trash();
387 }
388
389 void ShenandoahDegenGC::op_evacuate() {
390 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_evac);
391 ShenandoahHeap::heap()->evacuate_collection_set(false /* concurrent*/);
392 }
393
394 void ShenandoahDegenGC::op_init_updaterefs() {
395 // Evacuation has completed
396 ShenandoahHeap* const heap = ShenandoahHeap::heap();
397 heap->prepare_update_heap_references(false /*concurrent*/);
398 heap->set_update_refs_in_progress(true);
399 }
400
401 void ShenandoahDegenGC::op_updaterefs() {
402 ShenandoahHeap* const heap = ShenandoahHeap::heap();
403 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_updaterefs);
404 // Handed over from concurrent update references phase
405 heap->update_heap_references(false /*concurrent*/);
406
407 heap->set_update_refs_in_progress(false);
408 heap->set_has_forwarded_objects(false);
409 }
410
411 void ShenandoahDegenGC::op_update_roots() {
412 ShenandoahHeap* const heap = ShenandoahHeap::heap();
413
414 update_roots(false /*full_gc*/);
415
416 heap->update_heap_region_states(false /*concurrent*/);
417
418 if (ShenandoahVerify) {
419 heap->verifier()->verify_after_updaterefs();
420 }
421
422 if (VerifyAfterGC) {
423 Universe::verify();
424 }
425
426 heap->rebuild_free_set(false /*concurrent*/);
427 }
428
429 void ShenandoahDegenGC::op_cleanup_complete() {
430 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_cleanup_complete);
431 ShenandoahHeap::heap()->recycle_trash();
432 }
433
434 void ShenandoahDegenGC::op_degenerated_fail() {
435 upgrade_to_full();
436 }
437
438 void ShenandoahDegenGC::op_degenerated_futile() {
439 upgrade_to_full();
440 }
441
442 const char* ShenandoahDegenGC::degen_event_message(ShenandoahDegenPoint point) const {
443 switch (point) {
444 case _degenerated_unset:
445 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (<UNSET>)");
446 case _degenerated_outside_cycle:
447 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Outside of Cycle)");
448 case _degenerated_roots:
449 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Roots)");
450 case _degenerated_mark:
451 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Mark)");
452 case _degenerated_evac:
453 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Evacuation)");
454 case _degenerated_updaterefs:
455 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Update Refs)");
456 default:
457 ShouldNotReachHere();
458 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (?)");
459 }
460 }
461
462 void ShenandoahDegenGC::upgrade_to_full() {
463 log_info(gc)("Degenerated GC upgrading to Full GC");
464 ShenandoahHeap::heap()->shenandoah_policy()->record_degenerated_upgrade_to_full();
465 ShenandoahFullGC full_gc;
466 full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
467 }