HBASE-26237 Improve computation complexity for primaryRegionCountSkewCostFunction (#3657)

Signed-off-by: Duo Zhang <zhangduo@apache.org>
Signed-off-by: Huaxiang Sun <huaxiangsun@apache.org>

Author: clarax

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

@@ -66,8 +66,12 @@ void prepare(BalancerClusterState cluster) {
   @Override
   protected void regionMoved(int region, int oldServer, int newServer) {
     cost.applyCostsChange(costs -> {
-      costs[oldServer] = computeCostForRegionServer(oldServer);
-      costs[newServer] = computeCostForRegionServer(newServer);
+      if (region == cluster.regionIndexToPrimaryIndex[region]) {
+        if (oldServer >= 0) {
+          costs[oldServer]--;
+        }
+        costs[newServer]++;
+      }
     });
   }
 
@@ -80,4 +84,4 @@ boolean isNeeded() {
   protected double cost() {
     return cost.cost();
   }
-}
+}