HBASE-29262 StochasticLoadBalancer should use the CostFunction epsilon when evaluating whether a move improved costs (#6907) (#6908)

Signed-off-by: Nick Dimiduk <ndimiduk@apache.org>
Co-authored-by: Ray Mattingly <rmattingly@hubspot.com>

Author: rmdmattingly

hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/StochasticLoadBalancer.java

@@ -683,8 +683,12 @@ protected List<RegionPlan> balanceTable(TableName tableName,
 
       newCost = computeCost(cluster, currentCost);
 
+      double costImprovement = currentCost - newCost;
+      double minimumImprovement =
+        Math.max(CostFunction.getCostEpsilon(currentCost), CostFunction.getCostEpsilon(newCost));
+      boolean costsImproved = costImprovement > minimumImprovement;
       boolean conditionalsSimilarCostsImproved =
-        (newCost < currentCost && conditionalViolationsChange == 0 && !isViolatingConditionals);
+        (costsImproved && conditionalViolationsChange == 0 && !isViolatingConditionals);
       // Our first priority is to reduce conditional violations
       // Our second priority is to reduce balancer cost
       // change, regardless of cost change