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/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/shenandoahOopClosures.inline.hpp"
 39 #include "gc/shenandoah/shenandoahRootProcessor.inline.hpp"
 40 #include "gc/shenandoah/shenandoahSTWMark.hpp"
 41 #include "gc/shenandoah/shenandoahUtils.hpp"
 42 #include "gc/shenandoah/shenandoahVerifier.hpp"
 43 #include "gc/shenandoah/shenandoahYoungGeneration.hpp"
 44 #include "gc/shenandoah/shenandoahWorkerPolicy.hpp"
 45 #include "gc/shenandoah/shenandoahVMOperations.hpp"
 46 #include "runtime/vmThread.hpp"
 47 #include "utilities/events.hpp"
 48 
 49 ShenandoahDegenGC::ShenandoahDegenGC(ShenandoahDegenPoint degen_point, ShenandoahGeneration* generation) :
 50   ShenandoahGC(),
 51   _degen_point(degen_point),
 52   _generation(generation),
 53   _abbreviated(false) {
 54 }
 55 
 56 bool ShenandoahDegenGC::collect(GCCause::Cause cause) {
 57   vmop_degenerated();
 58   ShenandoahHeap* heap = ShenandoahHeap::heap();
 59   if (heap->mode()->is_generational()) {
 60     bool is_bootstrap_gc = heap->old_generation()->state() == ShenandoahOldGeneration::BOOTSTRAPPING;
 61     heap->mmu_tracker()->record_degenerated(GCId::current(), is_bootstrap_gc);
 62     const char* msg = is_bootstrap_gc? "At end of Degenerated Bootstrap Old GC": "At end of Degenerated Young GC";
 63     heap->log_heap_status(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(&degenerated_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 
 89 void ShenandoahDegenGC::op_degenerated() {
 90   ShenandoahHeap* const heap = ShenandoahHeap::heap();
 91   // Degenerated GC is STW, but it can also fail. Current mechanics communicates
 92   // GC failure via cancelled_concgc() flag. So, if we detect the failure after
 93   // some phase, we have to upgrade the Degenerate GC to Full GC.
 94   heap->clear_cancelled_gc(true /* clear oom handler */);
 95 
 96 #ifdef ASSERT
 97   if (heap->mode()->is_generational()) {
 98     ShenandoahOldGeneration* old_generation = heap->old_generation();
 99     if (!heap->is_concurrent_old_mark_in_progress()) {
100       // If we are not marking the old generation, there should be nothing in the old mark queues
101       assert(old_generation->task_queues()->is_empty(), "Old gen task queues should be empty");
102     }
103 
104     if (_generation->is_global()) {
105       // If we are in a global cycle, the old generation should not be marking. It is, however,
106       // allowed to be holding regions for evacuation or coalescing.
107       ShenandoahOldGeneration::State state = old_generation->state();
108       assert(state == ShenandoahOldGeneration::WAITING_FOR_BOOTSTRAP
109              || state == ShenandoahOldGeneration::EVACUATING
110              || state == ShenandoahOldGeneration::FILLING,
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->transfer_old_pointers_from_satb();
166         }
167       }
168 
169       if (_degen_point == ShenandoahDegenPoint::_degenerated_roots) {
170         // We only need this if the concurrent cycle has already swapped the card tables.
171         // Marking will use the 'read' table, but interesting pointers may have been
172         // recorded in the 'write' table in the time between the cancelled concurrent cycle
173         // and this degenerated cycle. These pointers need to be included the 'read' table
174         // used to scan the remembered set during the STW mark which follows here.
175         _generation->merge_write_table();
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       if (heap->mode()->is_generational() && heap->is_concurrent_old_mark_in_progress()) {
284         // This is still necessary for degenerated cycles because the degeneration point may occur
285         // after final mark of the young generation. See ShenandoahConcurrentGC::op_final_updaterefs for
286         // a more detailed explanation.
287         heap->transfer_old_pointers_from_satb();
288       }
289 
290       op_cleanup_complete();
291       // We defer generation resizing actions until after cset regions have been recycled.
292       if (heap->mode()->is_generational()) {
293         size_t old_region_surplus = heap->get_old_region_surplus();
294         size_t old_region_deficit = heap->get_old_region_deficit();
295         bool success;
296         size_t region_xfer;
297         const char* region_destination;
298         if (old_region_surplus) {
299           region_xfer = old_region_surplus;
300           region_destination = "young";
301           success = heap->generation_sizer()->transfer_to_young(old_region_surplus);
302         } else if (old_region_deficit) {
303           region_xfer = old_region_surplus;
304           region_destination = "old";
305           success = heap->generation_sizer()->transfer_to_old(old_region_deficit);
306           if (!success) {
307             heap->old_heuristics()->trigger_cannot_expand();
308           }
309         } else {
310           region_destination = "none";
311           region_xfer = 0;
312           success = true;
313         }
314 
315         size_t young_available = heap->young_generation()->available();
316         size_t old_available = heap->old_generation()->available();
317         log_info(gc, ergo)("After cleanup, %s " SIZE_FORMAT " regions to %s to prepare for next gc, old available: "
318                            SIZE_FORMAT "%s, young_available: " SIZE_FORMAT "%s",
319                            success? "successfully transferred": "failed to transfer", region_xfer, region_destination,
320                            byte_size_in_proper_unit(old_available), proper_unit_for_byte_size(old_available),
321                            byte_size_in_proper_unit(young_available), proper_unit_for_byte_size(young_available));
322 
323         heap->set_old_region_surplus(0);
324         heap->set_old_region_deficit(0);
325       }
326       break;
327     default:
328       ShouldNotReachHere();
329   }
330 
331   if (heap->mode()->is_generational()) {
332     // In case degeneration interrupted concurrent evacuation or update references, we need to clean up transient state.
333     // Otherwise, these actions have no effect.
334     heap->set_young_evac_reserve(0);
335     heap->set_old_evac_reserve(0);
336     heap->set_promoted_reserve(0);
337   }
338 
339   if (ShenandoahVerify) {
340     heap->verifier()->verify_after_degenerated();
341   }
342 
343   if (VerifyAfterGC) {
344     Universe::verify();
345   }
346 
347   metrics.snap_after();
348 
349   // Check for futility and fail. There is no reason to do several back-to-back Degenerated cycles,
350   // because that probably means the heap is overloaded and/or fragmented.
351   if (!metrics.is_good_progress()) {
352     heap->notify_gc_no_progress();
353     heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
354     op_degenerated_futile();
355   } else {
356     heap->notify_gc_progress();
357     heap->shenandoah_policy()->record_success_degenerated(_generation->is_young(), _abbreviated);
358     _generation->heuristics()->record_success_degenerated();
359   }
360 }
361 
362 void ShenandoahDegenGC::op_reset() {
363   _generation->prepare_gc();
364 }
365 
366 void ShenandoahDegenGC::op_mark() {
367   assert(!_generation->is_concurrent_mark_in_progress(), "Should be reset");
368   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_mark);
369   ShenandoahSTWMark mark(_generation, false /*full gc*/);
370   mark.mark();
371 }
372 
373 void ShenandoahDegenGC::op_finish_mark() {
374   ShenandoahConcurrentMark mark(_generation);
375   mark.finish_mark();
376 }
377 
378 void ShenandoahDegenGC::op_prepare_evacuation() {
379   ShenandoahHeap* const heap = ShenandoahHeap::heap();
380   if (ShenandoahVerify) {
381     heap->verifier()->verify_roots_no_forwarded();
382   }
383 
384   // STW cleanup weak roots and unload classes
385   heap->parallel_cleaning(false /*full gc*/);
386 
387   // Prepare regions and collection set
388   _generation->prepare_regions_and_collection_set(false /*concurrent*/);
389 
390   // Retire the TLABs, which will force threads to reacquire their TLABs after the pause.
391   // This is needed for two reasons. Strong one: new allocations would be with new freeset,
392   // which would be outside the collection set, so no cset writes would happen there.
393   // Weaker one: new allocations would happen past update watermark, and so less work would
394   // be needed for reference updates (would update the large filler instead).
395   if (UseTLAB) {
396     ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_final_manage_labs);
397     heap->tlabs_retire(false);
398   }
399 
400   size_t humongous_regions_promoted = heap->get_promotable_humongous_regions();
401   size_t regular_regions_promoted_in_place = heap->get_regular_regions_promoted_in_place();
402   if (!heap->collection_set()->is_empty() || (humongous_regions_promoted + regular_regions_promoted_in_place > 0)) {
403     // Even if the collection set is empty, we need to do evacuation if there are regions to be promoted in place.
404     // Degenerated evacuation takes responsibility for registering objects and setting the remembered set cards to dirty.
405 
406     if (ShenandoahVerify) {
407       heap->verifier()->verify_before_evacuation();
408     }
409 
410     heap->set_evacuation_in_progress(true);
411 
412     if(ShenandoahVerify) {
413       heap->verifier()->verify_during_evacuation();
414     }
415 
416     heap->set_has_forwarded_objects(!heap->collection_set()->is_empty());
417   } else {
418     if (ShenandoahVerify) {
419       heap->verifier()->verify_after_concmark();
420     }
421 
422     if (VerifyAfterGC) {
423       Universe::verify();
424     }
425   }
426 }
427 
428 void ShenandoahDegenGC::op_cleanup_early() {
429   ShenandoahHeap::heap()->recycle_trash();
430 }
431 
432 void ShenandoahDegenGC::op_evacuate() {
433   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_evac);
434   ShenandoahHeap::heap()->evacuate_collection_set(false /* concurrent*/);
435 }
436 
437 void ShenandoahDegenGC::op_init_updaterefs() {
438   // Evacuation has completed
439   ShenandoahHeap* const heap = ShenandoahHeap::heap();
440   heap->prepare_update_heap_references(false /*concurrent*/);
441   heap->set_update_refs_in_progress(true);
442 }
443 
444 void ShenandoahDegenGC::op_updaterefs() {
445   ShenandoahHeap* const heap = ShenandoahHeap::heap();
446   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_updaterefs);
447   // Handed over from concurrent update references phase
448   heap->update_heap_references(false /*concurrent*/);
449 
450   heap->set_update_refs_in_progress(false);
451   heap->set_has_forwarded_objects(false);
452 }
453 
454 void ShenandoahDegenGC::op_update_roots() {
455   ShenandoahHeap* const heap = ShenandoahHeap::heap();
456 
457   update_roots(false /*full_gc*/);
458 
459   heap->update_heap_region_states(false /*concurrent*/);
460 
461   if (ShenandoahVerify) {
462     heap->verifier()->verify_after_updaterefs();
463   }
464 
465   if (VerifyAfterGC) {
466     Universe::verify();
467   }
468 
469   heap->rebuild_free_set(false /*concurrent*/);
470 }
471 
472 void ShenandoahDegenGC::op_cleanup_complete() {
473   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_cleanup_complete);
474   ShenandoahHeap::heap()->recycle_trash();
475 }
476 
477 void ShenandoahDegenGC::op_degenerated_fail() {
478   upgrade_to_full();
479 }
480 
481 void ShenandoahDegenGC::op_degenerated_futile() {
482   upgrade_to_full();
483 }
484 
485 const char* ShenandoahDegenGC::degen_event_message(ShenandoahDegenPoint point) const {
486   switch (point) {
487     case _degenerated_unset:
488       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (<UNSET>)");
489     case _degenerated_outside_cycle:
490       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Outside of Cycle)");
491     case _degenerated_roots:
492       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Roots)");
493     case _degenerated_mark:
494       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Mark)");
495     case _degenerated_evac:
496       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Evacuation)");
497     case _degenerated_updaterefs:
498       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Update Refs)");
499     default:
500       ShouldNotReachHere();
501       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (?)");
502   }
503 }
504 
505 void ShenandoahDegenGC::upgrade_to_full() {
506   log_info(gc)("Degenerate GC upgrading to Full GC");
507   ShenandoahHeap::heap()->shenandoah_policy()->record_degenerated_upgrade_to_full();
508   ShenandoahFullGC full_gc;
509   full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
510 }