cluster: allow broker configuration update with decreasing core count

src/v/cluster/cluster_utils.cc

@@ -401,18 +401,7 @@ std::optional<ss::sstring> check_result_configuration(
           new_configuration.id());
     }
     auto& current_configuration = it->second.broker;
-    /**
-     * do no allow to decrease node core count
-     */
-    if (
-      current_configuration.properties().cores
-      > new_configuration.properties().cores) {
-        return fmt::format(
-          "core count must not decrease on any broker, currently configured "
-          "core count: {}, requested core count: {}",
-          current_configuration.properties().cores,
-          new_configuration.properties().cores);
-    }
+
     /**
      * When cluster member configuration changes Redpanda by default doesn't
      * allow the change if a new cluster configuration would have two

src/v/cluster/scheduling/allocation_node.cc

@@ -142,15 +142,13 @@ void allocation_node::remove_final_count(partition_allocation_domain domain) {
 }
 
 void allocation_node::update_core_count(uint32_t core_count) {
-    vassert(
-      core_count >= cpus(),
-      "decreasing node core count is not supported, current core count {} > "
-      "requested core count {}",
-      cpus(),
-      core_count);
-    auto current_cpus = cpus();
-    for (auto i = current_cpus; i < core_count; ++i) {
-        _weights.push_back(0);
+    auto old_count = _weights.size();
+    if (core_count < old_count) {
+        _weights.resize(core_count);
+    } else {
+        for (auto i = old_count; i < core_count; ++i) {
+            _weights.push_back(0);
+        }
     }
     _max_capacity = allocation_capacity(
       (core_count * _partitions_per_shard()) - _partitions_reserve_shard0());