Fix PGC edenSurvivalRate overflow issue

The edenSurvivalRate is representing the number of regions out of those
allocated for this Eden which survived(typically 0.0 <= the Rate <=
1.0).
The edenSurvivalRate overflow (> 1.0) might cause incorrect Heap and
Eden size change(unexpected spike/noise) and for some cases might cause
zero free memory and trigger out of memory exception. the issue is not
regression, but just exposed by recent edenResize change(it might cause
indirect serious GC performance issues, but did not trigger any
crashes/exceptions/assertions).

There are two different overflow cases, one is major, another is minor.
Major issue is incorrect edenCountBeforeCollect.
 During GC, the heap may expand to allow the current collection to
 continue. A heap resize triggers an eden size recalculation without
 allowing further heap expansion (heap reconfiguration). As a result,
 the newly calculatedneden size can differ significantly from the eden
 size at the start of the collection, causing the SurvivalRate
 calculation to become inaccurate. Therefore, set
 copyForwardStats->_edenEvacuateRegionCount (instead of
 getCurrentEdenSizeInRegions(env)) to edenCountBeforeCollect.

Minor issue is _scanBytesEden double-counts.
 When abort is in progress, every Eden object scanned in abort-recovery
 contributes to _scannedBytes. But an object that was partially copied
 before abort (the copy succeeded, the forwarding pointer was installed)
 is also counted again during the abort scan of the source region.
 So _scanBytesEden might be double-counted, then cause edenSurvivorCount
 is bigger than edenCountBeforeCollect.
 For this case, set edenSurvivorCount = edenCountBeforeCollect.

Signed-off-by: lhu <linhu@ca.ibm.com>

Author: LinHu2016

runtime/gc_vlhgc/SchedulingDelegate.cpp

@@ -372,7 +372,15 @@ MM_SchedulingDelegate::partialGarbageCollectCompleted(MM_EnvironmentVLHGC *env,
 	bool globalSweepHappened = _globalSweepRequired;
 	_globalSweepRequired = false;
 	/* copy out the Eden size of the previous interval (between the last PGC and this one) before we recalculate the next one */
-	uintptr_t edenCountBeforeCollect = getCurrentEdenSizeInRegions(env);
+	/* During GC, the heap may expand to allow the current collection to continue.
+	 * A heap resize triggers an eden size recalculation without allowing further
+	 * heap expansion (heap reconfiguration). As a result, the newly calculated
+	 * eden size can differ significantly from the eden size at the start of the
+	 * collection, causing the SurvivalRate calculation to become inaccurate.
+	 * Therefore, set copyForwardStats->_edenEvacuateRegionCount (instead of getCurrentEdenSizeInRegions(env)) to
+	 * edenCountBeforeCollect.
+	 */
+	uintptr_t edenCountBeforeCollect = copyForwardStats->_edenEvacuateRegionCount;
 
 	Trc_MM_SchedulingDelegate_partialGarbageCollectCompleted_stats(env->getLanguageVMThread(),
 			copyForwardStats->_edenEvacuateRegionCount,
@@ -393,11 +401,22 @@ MM_SchedulingDelegate::partialGarbageCollectCompleted(MM_EnvironmentVLHGC *env,
 		Assert_MM_true( (0 == copyForwardStats->_scanBytesEden) || copyForwardStats->_aborted || (0 != copyForwardStats->_nonEvacuateRegionCount));
 		Assert_MM_true( (0 == copyForwardStats->_scanBytesNonEden) || copyForwardStats->_aborted || (0 != copyForwardStats->_nonEvacuateRegionCount));
 		edenSurvivorCount += (copyForwardStats->_scanBytesEden + regionSize - 1) / regionSize;
+		Assert_MM_true(copyForwardStats->_edenSurvivorRegionCount <= edenCountBeforeCollect);
+		if (edenSurvivorCount > edenCountBeforeCollect) {
+			/* when abort is in progress, every Eden object scanned in abort-recovery contributes to _scannedBytes.
+			 * But an object that was partially copied before abort (the copy succeeded,
+			 * the forwarding pointer was installed) is also counted again during the abort scan of the source region.
+			 * So _scanBytesEden might be double-counted, then cause edenSurvivorCount is bigger than edenCountBeforeCollect.
+			 * for this case, set edenSurvivorCount = edenCountBeforeCollect.
+			 */
+			edenSurvivorCount = edenCountBeforeCollect;
+		}
 		nonEdenSurvivorCount += (copyForwardStats->_scanBytesNonEden + regionSize - 1) / regionSize;
 
 		/* Eden count could be 0 in special case, after compaction if there is still no free region for scheduling eden(eden count = 0),
 		   will skip update Survival Rate */
 		if (0 != edenCountBeforeCollect) {
+			/* SurvivalRate should never be over 1.0 */
 			double thisSurvivalRate = (double)edenSurvivorCount / (double)edenCountBeforeCollect;
 			updateSurvivalRatesAfterCopyForward(thisSurvivalRate, nonEdenSurvivorCount);
 		}