8335356: Shenandoah: Improve concurrent cleanup locking

Reviewed-by: ysr
Backport-of: b32e4a6

Author: William Kemper

src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp

@@ -669,6 +669,7 @@ void ShenandoahRegionPartitions::assert_bounds() {
 ShenandoahFreeSet::ShenandoahFreeSet(ShenandoahHeap* heap, size_t max_regions) :
   _heap(heap),
   _partitions(max_regions, this),
+  _trash_regions(NEW_C_HEAP_ARRAY(ShenandoahHeapRegion*, max_regions, mtGC)),
   _alloc_bias_weight(0)
 {
   clear_internal();
@@ -1217,7 +1218,7 @@ HeapWord* ShenandoahFreeSet::allocate_contiguous(ShenandoahAllocRequest& req) {
   return _heap->get_region(beg)->bottom();
 }
 
-void ShenandoahFreeSet::try_recycle_trashed(ShenandoahHeapRegion *r) {
+void ShenandoahFreeSet::try_recycle_trashed(ShenandoahHeapRegion* r) {
   if (r->is_trash()) {
     r->recycle();
   }
@@ -1226,13 +1227,24 @@ void ShenandoahFreeSet::try_recycle_trashed(ShenandoahHeapRegion *r) {
 void ShenandoahFreeSet::recycle_trash() {
   // lock is not reentrable, check we don't have it
   shenandoah_assert_not_heaplocked();
+  size_t count = 0;
   for (size_t i = 0; i < _heap->num_regions(); i++) {
     ShenandoahHeapRegion* r = _heap->get_region(i);
     if (r->is_trash()) {
-      ShenandoahHeapLocker locker(_heap->lock());
-      try_recycle_trashed(r);
+      _trash_regions[count++] = r;
+    }
+  }
+
+  // Relinquish the lock after this much time passed.
+  static constexpr jlong deadline_ns = 30000; // 30 us
+  size_t idx = 0;
+  while (idx < count) {
+    os::naked_yield(); // Yield to allow allocators to take the lock
+    ShenandoahHeapLocker locker(_heap->lock());
+    const jlong deadline = os::javaTimeNanos() + deadline_ns;
+    while (idx < count && os::javaTimeNanos() < deadline) {
+      try_recycle_trashed(_trash_regions[idx++]);
     }
-    SpinPause(); // allow allocators to take the lock
   }
 }
 

src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp

@@ -287,7 +287,7 @@ class ShenandoahFreeSet : public CHeapObj<mtGC> {
 private:
   ShenandoahHeap* const _heap;
   ShenandoahRegionPartitions _partitions;
-  size_t _retired_old_regions;
+  ShenandoahHeapRegion** _trash_regions;
 
   HeapWord* allocate_aligned_plab(size_t size, ShenandoahAllocRequest& req, ShenandoahHeapRegion* r);