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
27 #include "gc/shared/collectorCounters.hpp"
28 #include "gc/shenandoah/shenandoahCollectorPolicy.hpp"
29 #include "gc/shenandoah/shenandoahConcurrentMark.hpp"
30 #include "gc/shenandoah/shenandoahDegeneratedGC.hpp"
31 #include "gc/shenandoah/shenandoahFullGC.hpp"
32 #include "gc/shenandoah/shenandoahGeneration.hpp"
33 #include "gc/shenandoah/shenandoahGenerationalHeap.hpp"
34 #include "gc/shenandoah/shenandoahHeap.inline.hpp"
35 #include "gc/shenandoah/shenandoahMetrics.hpp"
36 #include "gc/shenandoah/shenandoahMonitoringSupport.hpp"
37 #include "gc/shenandoah/shenandoahOldGeneration.hpp"
38 #include "gc/shenandoah/shenandoahRootProcessor.inline.hpp"
39 #include "gc/shenandoah/shenandoahStackWatermark.hpp"
40 #include "gc/shenandoah/shenandoahSTWMark.hpp"
41 #include "gc/shenandoah/shenandoahUtils.hpp"
42 #include "gc/shenandoah/shenandoahVerifier.hpp"
43 #include "gc/shenandoah/shenandoahVMOperations.hpp"
44 #include "gc/shenandoah/shenandoahWorkerPolicy.hpp"
45 #include "gc/shenandoah/shenandoahYoungGeneration.hpp"
46 #include "runtime/vmThread.hpp"
47 #include "utilities/events.hpp"
48
49 ShenandoahDegenGC::ShenandoahDegenGC(ShenandoahDegenPoint degen_point, ShenandoahGeneration* generation, bool do_old_gc_bootstrap) :
50 ShenandoahGC(generation),
51 _degen_point(degen_point),
52 _abbreviated(false),
53 _do_old_gc_bootstrap(do_old_gc_bootstrap) {
54 }
55
56 bool ShenandoahDegenGC::collect(GCCause::Cause cause) {
57 vmop_degenerated();
58 ShenandoahHeap* heap = ShenandoahHeap::heap();
59 if (heap->mode()->is_generational()) {
60 FormatBuffer<32> buf("Degenerated %s GC", _generation->name());
61 const char* msg = _do_old_gc_bootstrap ? "Degenerated Bootstrap Old GC" : buf.buffer();
62 heap->mmu_tracker()->record_degenerated(GCId::current(), msg);
63 heap->log_heap_status(FormatBuffer<64>("At end of %s", msg));
64 }
65 return true;
66 }
67
68 void ShenandoahDegenGC::vmop_degenerated() {
69 TraceCollectorStats tcs(ShenandoahHeap::heap()->monitoring_support()->full_stw_collection_counters());
70 ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::degen_gc_gross);
71 VM_ShenandoahDegeneratedGC degenerated_gc(this);
72 VMThread::execute(°enerated_gc);
73 }
74
75 void ShenandoahDegenGC::entry_degenerated() {
76 const char* msg = degen_event_message(_degen_point);
77 ShenandoahPausePhase gc_phase(msg, ShenandoahPhaseTimings::degen_gc, true /* log_heap_usage */);
78 EventMark em("%s", msg);
79 ShenandoahHeap* const heap = ShenandoahHeap::heap();
80 ShenandoahWorkerScope scope(heap->workers(),
81 ShenandoahWorkerPolicy::calc_workers_for_stw_degenerated(),
82 "stw degenerated gc");
83
84 heap->set_degenerated_gc_in_progress(true);
85 op_degenerated();
86 heap->set_degenerated_gc_in_progress(false);
87 {
88 ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::degen_gc_propagate_gc_state);
89 heap->propagate_gc_state_to_all_threads();
90 }
91 }
92
93 void ShenandoahDegenGC::op_degenerated() {
94 ShenandoahHeap* const heap = ShenandoahHeap::heap();
95 // Degenerated GC is STW, but it can also fail. Current mechanics communicates
96 // GC failure via cancelled_concgc() flag. So, if we detect the failure after
97 // some phase, we have to upgrade the Degenerate GC to Full GC.
98 heap->clear_cancelled_gc();
99
100 // If we degenerated from evacuation or update-refs, some objects in cset may
101 // have been self-forwarded by the failing thread. Clear those marks now so
102 // the remainder of this cycle (re-evac, update-refs, verification) sees a
103 // clean forwarding state.
104 if (_degen_point == ShenandoahDegenPoint::_degenerated_evac ||
105 _degen_point == ShenandoahDegenPoint::_degenerated_update_refs) {
106 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_un_self_forward);
107 heap->un_self_forward_cset_regions();
108 }
109
110 // If it's passive mode with ShenandoahCardBarrier turned on: clean the write table
111 // without swapping the tables since no scan happens in passive mode anyway
112 if (ShenandoahCardBarrier && !heap->mode()->is_generational()) {
113 heap->old_generation()->card_scan()->mark_write_table_as_clean();
114 }
115
116 if (heap->mode()->is_generational()) {
117 const ShenandoahOldGeneration* old_generation = heap->old_generation();
118 if (!heap->is_concurrent_old_mark_in_progress()) {
119 // If we are not marking the old generation, there should be nothing in the old mark queues
120 assert(old_generation->task_queues()->is_empty(), "Old gen task queues should be empty");
121 } else {
122 // This is still necessary for degenerated cycles because the degeneration point may occur
123 // after final mark of the young generation. See ShenandoahConcurrentGC::op_final_update_refs for
124 // a more detailed explanation.
125 old_generation->transfer_pointers_from_satb();
126 }
127
128 if (_generation->is_global()) {
129 // If we are in a global cycle, the old generation should not be marking. It is, however,
130 // allowed to be holding regions for evacuation or coalescing.
131 assert(old_generation->is_idle()
132 || old_generation->is_doing_mixed_evacuations()
133 || old_generation->is_preparing_for_mark(),
134 "Old generation cannot be in state: %s", old_generation->state_name());
135 }
136 }
137
138 ShenandoahMetricsSnapshot metrics(heap->free_set());
139
140 switch (_degen_point) {
141 // The cases below form the Duff's-like device: it describes the actual GC cycle,
142 // but enters it at different points, depending on which concurrent phase had
143 // degenerated.
144
145 case _degenerated_outside_cycle:
146 // We have degenerated from outside the cycle, which means something is bad with
147 // the heap, most probably heavy humongous fragmentation, or we are very low on free
148 // space. It makes little sense to wait for Full GC to reclaim as much as it can, when
149 // we can do the most aggressive degen cycle, which includes processing references and
150 // class unloading, unless those features are explicitly disabled.
151
152 // Note that we can only do this for "outside-cycle" degens, otherwise we would risk
153 // changing the cycle parameters mid-cycle during concurrent -> degenerated handover.
154 heap->set_unload_classes(_generation->heuristics()->can_unload_classes() &&
155 (!heap->mode()->is_generational() || _generation->is_global()));
156
157 if (heap->mode()->is_generational()) {
158 // Clean the read table before swapping it. The end goal here is to have a clean
159 // write table, and to have the read table updated with the previous write table.
160 heap->old_generation()->card_scan()->mark_read_table_as_clean();
161
162 if (_generation->is_young()) {
163 // Swap remembered sets for young
164 _generation->swap_card_tables();
165 }
166 }
167
168 case _degenerated_roots:
169 // Degenerated from concurrent root mark, reset the flag for STW mark
170 if (!heap->mode()->is_generational()) {
171 if (heap->is_concurrent_mark_in_progress()) {
172 heap->cancel_concurrent_mark();
173 }
174 } else {
175 if (_generation->is_concurrent_mark_in_progress()) {
176 // We want to allow old generation marking to be punctuated by young collections
177 // (even if they have degenerated). If this is a global cycle, we'd have cancelled
178 // the entire old gc before coming into this switch. Note that cancel_marking on
179 // the generation does NOT abandon incomplete SATB buffers as cancel_concurrent_mark does.
180 // We need to separate out the old pointers which is done below.
181 _generation->cancel_marking();
182 }
183
184 if (_degen_point == ShenandoahDegenPoint::_degenerated_roots) {
185 // We only need this if the concurrent cycle has already swapped the card tables.
186 // Marking will use the 'read' table, but interesting pointers may have been
187 // recorded in the 'write' table in the time between the cancelled concurrent cycle
188 // and this degenerated cycle. These pointers need to be included in the 'read' table
189 // used to scan the remembered set during the STW mark which follows here.
190 _generation->merge_write_table();
191 }
192 }
193
194 op_reset();
195
196 // STW mark
197 op_mark();
198
199 case _degenerated_mark:
200 // No fallthrough. Continue mark, handed over from concurrent mark if
201 // concurrent mark has yet completed
202 if (_degen_point == ShenandoahDegenPoint::_degenerated_mark && heap->is_concurrent_mark_in_progress()) {
203 assert(!ShenandoahBarrierSet::satb_mark_queue_set().get_filter_out_young(),
204 "Should not be filtering out young pointers when concurrent mark degenerates");
205 op_finish_mark();
206 }
207 assert(!heap->cancelled_gc(), "STW mark can not OOM");
208
209 /* Degen select Collection Set. etc. */
210 op_prepare_evacuation();
211
212 op_cleanup_early();
213
214 case _degenerated_evac:
215 // If heuristics thinks we should do the cycle, this flag would be set,
216 // and we can do evacuation. Otherwise, it would be the shortcut cycle.
217 if (heap->is_evacuation_in_progress()) {
218
219 if (_degen_point == _degenerated_evac) {
220 // Degeneration under oom-evac protocol allows the mutator LRB to expose
221 // references to from-space objects. This is okay, in theory, because we
222 // will come to the safepoint here to complete the evacuations and update
223 // the references. However, if the from-space reference is written to a
224 // region that was EC during final mark or was recycled after final mark
225 // it will not have TAMS or UWM updated. Such a region is effectively
226 // skipped during update references which can lead to crashes and corruption
227 // if the from-space reference is accessed.
228 if (UseTLAB) {
229 heap->labs_make_parsable();
230 }
231
232 for (size_t i = 0; i < heap->num_regions(); i++) {
233 ShenandoahHeapRegion* r = heap->get_region(i);
234 if (r->is_active() && r->top() > r->get_update_watermark()) {
235 r->set_update_watermark_at_safepoint(r->top());
236 }
237 }
238 }
239
240 // Degeneration under oom-evac protocol might have left some objects in
241 // collection set un-evacuated. Restart evacuation from the beginning to
242 // capture all objects. For all the objects that are already evacuated,
243 // it would be a simple check, which is supposed to be fast. This is also
244 // safe to do even without degeneration, as CSet iterator is at beginning
245 // in preparation for evacuation anyway.
246 //
247 // Before doing that, we need to make sure we never had any cset-pinned
248 // regions. This may happen if allocation failure happened when evacuating
249 // the about-to-be-pinned object, oom-evac protocol left the object in
250 // the collection set, and then the pin reached the cset region. If we continue
251 // the cycle here, we would trash the cset and alive objects in it. To avoid
252 // it, we fail degeneration right away and slide into Full GC to recover.
253
254 {
255 heap->sync_pinned_region_status();
256 heap->collection_set()->clear_current_index();
257 ShenandoahHeapRegion* r;
258 while ((r = heap->collection_set()->next()) != nullptr) {
259 if (r->is_pinned()) {
260 op_degenerated_fail();
261 return;
262 }
263 }
264
265 heap->collection_set()->clear_current_index();
266 }
267 op_evacuate();
268 if (heap->cancelled_gc()) {
269 op_degenerated_fail();
270 return;
271 }
272 } else if (has_in_place_promotions(heap)) {
273 // We have nothing to evacuate, but there are still regions to promote in place.
274 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_promote_regions);
275 ShenandoahGenerationalHeap::heap()->promote_regions_in_place(_generation, false /* concurrent*/);
276 }
277
278 // Update collector state regardless of whether there are forwarded objects
279 heap->set_evacuation_in_progress(false);
280 heap->set_concurrent_weak_root_in_progress(false);
281 heap->set_concurrent_strong_root_in_progress(false);
282
283 // If heuristics thinks we should do the cycle, this flag would be set,
284 // and we need to do update-refs. Otherwise, it would be the shortcut cycle.
285 if (heap->has_forwarded_objects()) {
286 op_init_update_refs();
287 assert(!heap->cancelled_gc(), "STW reference update can not OOM");
288 } else {
289 _abbreviated = true;
290 }
291
292 // labs are retired, walk the old regions and update remembered set
293 if (ShenandoahHeap::heap()->mode()->is_generational()) {
294 ShenandoahGenerationalHeap::heap()->old_generation()->update_card_table();
295 }
296
297 case _degenerated_update_refs:
298 if (heap->has_forwarded_objects()) {
299 op_update_refs();
300 op_update_roots();
301 assert(!heap->cancelled_gc(), "STW reference update can not OOM");
302 }
303
304 op_cleanup_complete();
305
306 if (heap->mode()->is_generational()) {
307 ShenandoahGenerationalHeap::heap()->complete_degenerated_cycle();
308 }
309
310 break;
311 default:
312 ShouldNotReachHere();
313 }
314
315 DEBUG_ONLY(heap->assert_no_self_forwards());
316
317 // Leaving degenerated GC, we need to flip barriers back to idle.
318 ShenandoahCodeRoots::arm_nmethods();
319
320 // There is no operation that follows degenerated GC, so run all nmethod barriers now.
321 // We will take additional time during the safepoint, but we would also incur no
322 // additional latency on mutators once we return from here.
323 ShenandoahCodeRoots::disarm_nmethods();
324
325 if (ShenandoahVerify) {
326 heap->verifier()->verify_after_degenerated(_generation);
327 }
328
329 if (VerifyAfterGC) {
330 Universe::verify();
331 }
332
333 // Decide if this cycle made good progress, and, if not, should it upgrade to a full GC.
334 const bool progress = metrics.is_good_progress();
335 ShenandoahCollectorPolicy* policy = heap->shenandoah_policy();
336 policy->record_degenerated(_generation->is_young(), _abbreviated, progress);
337 if (progress) {
338 heap->notify_gc_progress();
339 _generation->heuristics()->record_degenerated(heap->mode()->is_generational() && _generation->is_global());
340 heap->start_idle_span();
341 } else if (policy->should_upgrade_degenerated_gc()) {
342 // Upgrade to full GC, register full-GC impact on heuristics.
343 op_degenerated_futile();
344 } else {
345 _generation->heuristics()->record_degenerated(heap->mode()->is_generational() && _generation->is_global());
346 }
347 }
348
349 void ShenandoahDegenGC::op_reset() {
350 _generation->prepare_gc();
351 }
352
353 void ShenandoahDegenGC::op_mark() {
354 assert(!_generation->is_concurrent_mark_in_progress(), "Should be reset");
355 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_mark);
356 ShenandoahSTWMark mark(_generation, false /*full gc*/);
357 mark.mark();
358 }
359
360 void ShenandoahDegenGC::op_finish_mark() {
361 ShenandoahConcurrentMark mark(_generation);
362 mark.finish_mark();
363 }
364
365 void ShenandoahDegenGC::op_prepare_evacuation() {
366 ShenandoahHeap* const heap = ShenandoahHeap::heap();
367 if (ShenandoahVerify) {
368 heap->verifier()->verify_roots_no_forwarded(_generation);
369 }
370
371 // STW cleanup weak roots and unload classes
372 heap->parallel_cleaning(_generation, false /*full gc*/);
373
374 // Prepare regions and collection set
375 _generation->prepare_regions_and_collection_set(false /*concurrent*/);
376
377 // Retire the TLABs, which will force threads to reacquire their TLABs after the pause.
378 // This is needed for two reasons. Strong one: new allocations would be with new freeset,
379 // which would be outside the collection set, so no cset writes would happen there.
380 // Weaker one: new allocations would happen past update watermark, and so less work would
381 // be needed for reference updates (would update the large filler instead).
382 if (UseTLAB) {
383 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_final_manage_labs);
384 heap->tlabs_retire(false);
385 }
386
387 if (!heap->collection_set()->is_empty()) {
388 if (ShenandoahVerify) {
389 heap->verifier()->verify_before_evacuation(_generation);
390 }
391
392 heap->set_evacuation_in_progress(true);
393 heap->set_has_forwarded_objects(true);
394 } else {
395 if (ShenandoahVerify) {
396 if (has_in_place_promotions(heap)) {
397 heap->verifier()->verify_after_concmark_with_promotions(_generation);
398 } else {
399 heap->verifier()->verify_after_concmark(_generation);
400 }
401 }
402
403 if (VerifyAfterGC) {
404 Universe::verify();
405 }
406 }
407 }
408
409 bool ShenandoahDegenGC::has_in_place_promotions(const ShenandoahHeap* heap) const {
410 return heap->mode()->is_generational() && heap->old_generation()->has_in_place_promotions();
411 }
412
413 void ShenandoahDegenGC::op_cleanup_early() {
414 ShenandoahHeap::heap()->recycle_trash();
415 }
416
417 void ShenandoahDegenGC::op_evacuate() {
418 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_evac);
419 ShenandoahHeap::heap()->evacuate_collection_set(_generation, false /* concurrent*/);
420 }
421
422 void ShenandoahDegenGC::op_init_update_refs() {
423 // Evacuation has completed
424 ShenandoahHeap* const heap = ShenandoahHeap::heap();
425 heap->prepare_update_heap_references();
426 heap->set_update_refs_in_progress(true);
427 }
428
429 void ShenandoahDegenGC::op_update_refs() {
430 ShenandoahHeap* const heap = ShenandoahHeap::heap();
431 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_update_refs);
432 // Handed over from concurrent update references phase
433 heap->update_heap_references(_generation, false /*concurrent*/);
434
435 heap->set_update_refs_in_progress(false);
436 heap->set_has_forwarded_objects(false);
437 }
438
439 void ShenandoahDegenGC::op_update_roots() {
440 ShenandoahHeap* const heap = ShenandoahHeap::heap();
441
442 update_roots(false /*full_gc*/);
443
444 heap->update_heap_region_states(false /*concurrent*/);
445
446 if (ShenandoahVerify) {
447 heap->verifier()->verify_after_update_refs(_generation);
448 }
449
450 if (VerifyAfterGC) {
451 Universe::verify();
452 }
453
454 heap->rebuild_free_set(false /*concurrent*/);
455 }
456
457 void ShenandoahDegenGC::op_cleanup_complete() {
458 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_cleanup_complete);
459 ShenandoahHeap::heap()->recycle_trash();
460 }
461
462 void ShenandoahDegenGC::op_degenerated_fail() {
463 upgrade_to_full();
464 }
465
466 void ShenandoahDegenGC::op_degenerated_futile() {
467 upgrade_to_full();
468 }
469
470 const char* ShenandoahDegenGC::degen_event_message(ShenandoahDegenPoint point) const {
471 switch (point) {
472 case _degenerated_unset:
473 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (<UNSET>)");
474 case _degenerated_outside_cycle:
475 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Outside of Cycle)");
476 case _degenerated_roots:
477 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Roots)");
478 case _degenerated_mark:
479 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Mark)");
480 case _degenerated_evac:
481 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Evacuation)");
482 case _degenerated_update_refs:
483 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Update Refs)");
484 default:
485 ShouldNotReachHere();
486 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (?)");
487 }
488 }
489
490 void ShenandoahDegenGC::upgrade_to_full() {
491 log_info(gc)("Degenerated GC upgrading to Full GC");
492 ShenandoahHeap* heap = ShenandoahHeap::heap();
493 heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
494 heap->increment_total_collections(true);
495 heap->shenandoah_policy()->record_degenerated_upgrade_to_full();
496 ShenandoahFullGC full_gc;
497 full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
498 }