1 /*
2 * Copyright (c) 2021, Red Hat, Inc. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
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/shenandoahHeap.inline.hpp"
33 #include "gc/shenandoah/shenandoahMetrics.hpp"
34 #include "gc/shenandoah/shenandoahMonitoringSupport.hpp"
35 #include "gc/shenandoah/shenandoahRootProcessor.inline.hpp"
36 #include "gc/shenandoah/shenandoahSTWMark.hpp"
37 #include "gc/shenandoah/shenandoahUtils.hpp"
38 #include "gc/shenandoah/shenandoahVerifier.hpp"
39 #include "gc/shenandoah/shenandoahWorkerPolicy.hpp"
40 #include "gc/shenandoah/shenandoahVMOperations.hpp"
41 #include "runtime/vmThread.hpp"
42 #include "utilities/events.hpp"
43
44 ShenandoahDegenGC::ShenandoahDegenGC(ShenandoahDegenPoint degen_point) :
45 ShenandoahGC(),
46 _degen_point(degen_point),
47 _abbreviated(false) {
48 }
49
50 bool ShenandoahDegenGC::collect(GCCause::Cause cause) {
51 vmop_degenerated();
52 return true;
53 }
54
55 void ShenandoahDegenGC::vmop_degenerated() {
56 TraceCollectorStats tcs(ShenandoahHeap::heap()->monitoring_support()->full_stw_collection_counters());
57 ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::degen_gc_gross);
58 VM_ShenandoahDegeneratedGC degenerated_gc(this);
59 VMThread::execute(°enerated_gc);
60 }
61
62 void ShenandoahDegenGC::entry_degenerated() {
63 const char* msg = degen_event_message(_degen_point);
64 ShenandoahPausePhase gc_phase(msg, ShenandoahPhaseTimings::degen_gc, true /* log_heap_usage */);
65 EventMark em("%s", msg);
66 ShenandoahHeap* const heap = ShenandoahHeap::heap();
67
68 ShenandoahWorkerScope scope(heap->workers(),
69 ShenandoahWorkerPolicy::calc_workers_for_stw_degenerated(),
70 "stw degenerated gc");
71
72 heap->set_degenerated_gc_in_progress(true);
73 op_degenerated();
74 heap->set_degenerated_gc_in_progress(false);
75 }
76
77 void ShenandoahDegenGC::op_degenerated() {
78 ShenandoahHeap* const heap = ShenandoahHeap::heap();
79 // Degenerated GC is STW, but it can also fail. Current mechanics communicates
80 // GC failure via cancelled_concgc() flag. So, if we detect the failure after
81 // some phase, we have to upgrade the Degenerate GC to Full GC.
82 heap->clear_cancelled_gc();
83
84 ShenandoahMetricsSnapshot metrics;
85 metrics.snap_before();
86
87 switch (_degen_point) {
88 // The cases below form the Duff's-like device: it describes the actual GC cycle,
89 // but enters it at different points, depending on which concurrent phase had
90 // degenerated.
91
92 case _degenerated_outside_cycle:
93 // We have degenerated from outside the cycle, which means something is bad with
94 // the heap, most probably heavy humongous fragmentation, or we are very low on free
95 // space. It makes little sense to wait for Full GC to reclaim as much as it can, when
96 // we can do the most aggressive degen cycle, which includes processing references and
97 // class unloading, unless those features are explicitly disabled.
98 //
99
100 // Degenerated from concurrent root mark, reset the flag for STW mark
101 if (heap->is_concurrent_mark_in_progress()) {
102 ShenandoahConcurrentMark::cancel();
103 heap->set_concurrent_mark_in_progress(false);
104 }
105
106 // Note that we can only do this for "outside-cycle" degens, otherwise we would risk
107 // changing the cycle parameters mid-cycle during concurrent -> degenerated handover.
108 heap->set_unload_classes(heap->heuristics()->can_unload_classes());
109
110 op_reset();
111
112 // STW mark
113 op_mark();
114
115 case _degenerated_mark:
116 // No fallthrough. Continue mark, handed over from concurrent mark if
117 // concurrent mark has yet completed
118 if (_degen_point == ShenandoahDegenPoint::_degenerated_mark &&
119 heap->is_concurrent_mark_in_progress()) {
120 op_finish_mark();
121 }
122 assert(!heap->cancelled_gc(), "STW mark can not OOM");
123
124 /* Degen select Collection Set. etc. */
125 op_prepare_evacuation();
126
127 op_cleanup_early();
128
129 case _degenerated_evac:
130 // If heuristics thinks we should do the cycle, this flag would be set,
131 // and we can do evacuation. Otherwise, it would be the shortcut cycle.
132 if (heap->is_evacuation_in_progress()) {
133
134 if (_degen_point == _degenerated_evac) {
135 // Degeneration under oom-evac protocol allows the mutator LRB to expose
136 // references to from-space objects. This is okay, in theory, because we
137 // will come to the safepoint here to complete the evacuations and update
138 // the references. However, if the from-space reference is written to a
139 // region that was EC during final mark or was recycled after final mark
140 // it will not have TAMS or UWM updated. Such a region is effectively
141 // skipped during update references which can lead to crashes and corruption
142 // if the from-space reference is accessed.
143 if (UseTLAB) {
144 heap->labs_make_parsable();
145 }
146
147 for (size_t i = 0; i < heap->num_regions(); i++) {
148 ShenandoahHeapRegion* r = heap->get_region(i);
149 if (r->is_active() && r->top() > r->get_update_watermark()) {
150 r->set_update_watermark_at_safepoint(r->top());
151 }
152 }
153 }
154
155 // Degeneration under oom-evac protocol might have left some objects in
156 // collection set un-evacuated. Restart evacuation from the beginning to
157 // capture all objects. For all the objects that are already evacuated,
158 // it would be a simple check, which is supposed to be fast. This is also
159 // safe to do even without degeneration, as CSet iterator is at beginning
160 // in preparation for evacuation anyway.
161 //
162 // Before doing that, we need to make sure we never had any cset-pinned
163 // regions. This may happen if allocation failure happened when evacuating
164 // the about-to-be-pinned object, oom-evac protocol left the object in
165 // the collection set, and then the pin reached the cset region. If we continue
166 // the cycle here, we would trash the cset and alive objects in it. To avoid
167 // it, we fail degeneration right away and slide into Full GC to recover.
168
169 {
170 heap->sync_pinned_region_status();
171 heap->collection_set()->clear_current_index();
172
173 ShenandoahHeapRegion* r;
174 while ((r = heap->collection_set()->next()) != nullptr) {
175 if (r->is_pinned()) {
176 heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
177 op_degenerated_fail();
178 return;
179 }
180 }
181
182 heap->collection_set()->clear_current_index();
183 }
184 op_evacuate();
185 if (heap->cancelled_gc()) {
186 op_degenerated_fail();
187 return;
188 }
189 }
190
191 // If heuristics thinks we should do the cycle, this flag would be set,
192 // and we need to do update-refs. Otherwise, it would be the shortcut cycle.
193 if (heap->has_forwarded_objects()) {
194 op_init_updaterefs();
195 assert(!heap->cancelled_gc(), "STW reference update can not OOM");
196 } else {
197 _abbreviated = true;
198 }
199
200 case _degenerated_updaterefs:
201 if (heap->has_forwarded_objects()) {
202 op_updaterefs();
203 op_update_roots();
204 assert(!heap->cancelled_gc(), "STW reference update can not OOM");
205 }
206
207 // Disarm nmethods that armed in concurrent cycle.
208 // In above case, update roots should disarm them
209 ShenandoahCodeRoots::disarm_nmethods();
210
211 op_cleanup_complete();
212 break;
213 default:
214 ShouldNotReachHere();
215 }
216
217 if (ShenandoahVerify) {
218 heap->verifier()->verify_after_degenerated();
219 }
220
221 if (VerifyAfterGC) {
222 Universe::verify();
223 }
224
225 metrics.snap_after();
226
227 // Check for futility and fail. There is no reason to do several back-to-back Degenerated cycles,
228 // because that probably means the heap is overloaded and/or fragmented.
229 if (!metrics.is_good_progress()) {
230 heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
231 op_degenerated_futile();
232 } else {
233 heap->notify_gc_progress();
234 heap->shenandoah_policy()->record_success_degenerated(_abbreviated);
235 heap->heuristics()->record_success_degenerated();
236 }
237 }
238
239 void ShenandoahDegenGC::op_reset() {
240 ShenandoahHeap::heap()->prepare_gc();
241 }
242
243 void ShenandoahDegenGC::op_mark() {
244 assert(!ShenandoahHeap::heap()->is_concurrent_mark_in_progress(), "Should be reset");
245 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_mark);
246 ShenandoahSTWMark mark(false /*full gc*/);
247 mark.clear();
248 mark.mark();
249 }
250
251 void ShenandoahDegenGC::op_finish_mark() {
252 ShenandoahConcurrentMark mark;
253 mark.finish_mark();
254 }
255
256 void ShenandoahDegenGC::op_prepare_evacuation() {
257 ShenandoahHeap* const heap = ShenandoahHeap::heap();
258 if (ShenandoahVerify) {
259 heap->verifier()->verify_roots_no_forwarded();
260 }
261
262 // STW cleanup weak roots and unload classes
263 heap->parallel_cleaning(false /*full gc*/);
264 // Prepare regions and collection set
265 heap->prepare_regions_and_collection_set(false /*concurrent*/);
266
267 // Retire the TLABs, which will force threads to reacquire their TLABs after the pause.
268 // This is needed for two reasons. Strong one: new allocations would be with new freeset,
269 // which would be outside the collection set, so no cset writes would happen there.
270 // Weaker one: new allocations would happen past update watermark, and so less work would
271 // be needed for reference updates (would update the large filler instead).
272 if (UseTLAB) {
273 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_final_manage_labs);
274 heap->tlabs_retire(false);
275 }
276
277 if (!heap->collection_set()->is_empty()) {
278 if (ShenandoahVerify) {
279 heap->verifier()->verify_before_evacuation();
280 }
281
282 heap->set_evacuation_in_progress(true);
283 heap->set_has_forwarded_objects(true);
284 } else {
285 if (ShenandoahVerify) {
286 heap->verifier()->verify_after_concmark();
287 }
288
289 if (VerifyAfterGC) {
290 Universe::verify();
291 }
292 }
293 }
294
295 void ShenandoahDegenGC::op_cleanup_early() {
296 ShenandoahHeap::heap()->recycle_trash();
297 }
298
299 void ShenandoahDegenGC::op_evacuate() {
300 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_evac);
301 ShenandoahHeap::heap()->evacuate_collection_set(false /* concurrent*/);
302 }
303
304 void ShenandoahDegenGC::op_init_updaterefs() {
305 // Evacuation has completed
306 ShenandoahHeap* const heap = ShenandoahHeap::heap();
307 heap->set_evacuation_in_progress(false);
308 heap->set_concurrent_weak_root_in_progress(false);
309 heap->set_concurrent_strong_root_in_progress(false);
310
311 heap->prepare_update_heap_references(false /*concurrent*/);
312 heap->set_update_refs_in_progress(true);
313 }
314
315 void ShenandoahDegenGC::op_updaterefs() {
316 ShenandoahHeap* const heap = ShenandoahHeap::heap();
317 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_updaterefs);
318 // Handed over from concurrent update references phase
319 heap->update_heap_references(false /*concurrent*/);
320
321 heap->set_update_refs_in_progress(false);
322 heap->set_has_forwarded_objects(false);
323 }
324
325 void ShenandoahDegenGC::op_update_roots() {
326 ShenandoahHeap* const heap = ShenandoahHeap::heap();
327
328 update_roots(false /*full_gc*/);
329
330 heap->update_heap_region_states(false /*concurrent*/);
331
332 if (ShenandoahVerify) {
333 heap->verifier()->verify_after_updaterefs();
334 }
335
336 if (VerifyAfterGC) {
337 Universe::verify();
338 }
339
340 heap->rebuild_free_set(false /*concurrent*/);
341 }
342
343 void ShenandoahDegenGC::op_cleanup_complete() {
344 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_cleanup_complete);
345 ShenandoahHeap::heap()->recycle_trash();
346 }
347
348 void ShenandoahDegenGC::op_degenerated_fail() {
349 upgrade_to_full();
350 }
351
352 void ShenandoahDegenGC::op_degenerated_futile() {
353 upgrade_to_full();
354 }
355
356 const char* ShenandoahDegenGC::degen_event_message(ShenandoahDegenPoint point) const {
357 switch (point) {
358 case _degenerated_unset:
359 return "Pause Degenerated GC (<UNSET>)";
360 case _degenerated_outside_cycle:
361 return "Pause Degenerated GC (Outside of Cycle)";
362 case _degenerated_mark:
363 return "Pause Degenerated GC (Mark)";
364 case _degenerated_evac:
365 return "Pause Degenerated GC (Evacuation)";
366 case _degenerated_updaterefs:
367 return "Pause Degenerated GC (Update Refs)";
368 default:
369 ShouldNotReachHere();
370 return "ERROR";
371 }
372 }
373
374 void ShenandoahDegenGC::upgrade_to_full() {
375 log_info(gc)("Degenerated GC upgrading to Full GC");
376 ShenandoahHeap::heap()->shenandoah_policy()->record_degenerated_upgrade_to_full();
377 ShenandoahFullGC full_gc;
378 full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
379 }