[v24.2.x] CORE-8394 cluster: consider shard0 reserve in check_cluster_limits

src/v/cluster/scheduling/allocation_node.cc

@@ -50,24 +50,26 @@ allocation_node::allocation_node(
     });
 }
 
-bool allocation_node::is_full(
-  const model::ntp& ntp, bool will_add_allocation) const {
-    // Internal topics are excluded from checks to prevent allocation failures
-    // when creating them. This is okay because they are fairly small in number
-    // compared to kafka user topic partitions.
+bool allocation_node::is_internal_topic(
+  const config::binding<std::vector<ss::sstring>>& internal_kafka_topics,
+  model::topic_namespace_view ntp) {
     auto is_internal_ns = ntp.ns == model::redpanda_ns
                           || ntp.ns == model::kafka_internal_namespace;
     if (is_internal_ns) {
-        return false;
+        return true;
     }
-    const auto& internal_topics = _internal_kafka_topics();
-    auto is_internal_topic = ntp.ns == model::kafka_namespace
-                             && std::any_of(
-                               internal_topics.cbegin(),
-                               internal_topics.cend(),
-                               [&ntp](const ss::sstring& topic) {
-                                   return topic == ntp.tp.topic();
-                               });
+    const auto& internal_topics = internal_kafka_topics();
+    return ntp.ns == model::kafka_namespace
+           && std::any_of(
+             internal_topics.cbegin(),
+             internal_topics.cend(),
+             [&ntp](const ss::sstring& topic) { return topic == ntp.tp; });
+}
+
+bool allocation_node::is_full(
+  const model::ntp& ntp, bool will_add_allocation) const {
+    auto is_internal_topic = allocation_node::is_internal_topic(
+      _internal_kafka_topics, model::topic_namespace_view{ntp});
 
     auto count = _allocated_partitions;
     if (will_add_allocation) {

src/v/cluster/scheduling/allocation_node.h

@@ -126,6 +126,13 @@ class allocation_node {
     }
     bool is_full(const model::ntp&, bool will_add_allocation) const;
 
+    // Internal topics are excluded from checks to prevent allocation failures
+    // when creating them. This is okay because they are fairly small in number
+    // compared to kafka user topic partitions.
+    static bool is_internal_topic(
+      const config::binding<std::vector<ss::sstring>>& internal_kafka_topics,
+      model::topic_namespace_view ntp);
+
 private:
     friend allocation_state;
 

src/v/cluster/scheduling/partition_allocator.cc

@@ -109,11 +109,16 @@ allocation_constraints partition_allocator::default_constraints() {
  * with partitions that cannot be re-accommodated on smaller peers).
  */
 std::error_code partition_allocator::check_cluster_limits(
-  const uint64_t new_partitions_replicas_requested) const {
+  const uint64_t new_partitions_replicas_requested,
+  const model::topic_namespace& topic) const {
     if (_members.local().nodes().empty()) {
         // Empty members table, we're probably running in a unit test
         return errc::success;
     }
+    if (allocation_node::is_internal_topic(_internal_kafka_topics, topic)) {
+        return errc::success;
+    }
+
     // Calculate how many partition-replicas already exist, so that we can
     // check if the new topic would take us past any limits.
     uint64_t existent_partitions{0};
@@ -169,15 +174,19 @@ std::error_code partition_allocator::check_cluster_limits(
 
     // Refuse to create a partition count that would violate the per-core
     // limit.
-    const uint64_t core_limit = (effective_cpu_count * _partitions_per_shard());
+    const uint64_t core_limit = (effective_cpu_count * _partitions_per_shard())
+                                - (broker_count * _partitions_reserve_shard0());
     if (proposed_total_partitions > core_limit) {
         vlog(
           clusterlog.warn,
           "Refusing to create {} partitions as total partition count {} would "
-          "exceed core limit {}",
+          "exceed the core-based limit {} (per-shard limit: {}, shard0 "
+          "reservation: {})",
           new_partitions_replicas_requested,
           proposed_total_partitions,
-          effective_cpu_count * _partitions_per_shard());
+          core_limit,
+          _partitions_per_shard(),
+          _partitions_reserve_shard0());
         return errc::topic_invalid_partitions_core_limit;
     }
 
@@ -243,7 +252,7 @@ partition_allocator::allocate(simple_allocation_request simple_req) {
     const uint64_t create_count
       = static_cast<uint64_t>(simple_req.additional_partitions)
         * static_cast<uint64_t>(simple_req.replication_factor);
-    auto cluster_errc = check_cluster_limits(create_count);
+    auto cluster_errc = check_cluster_limits(create_count, simple_req.tp_ns);
     if (cluster_errc) {
         co_return cluster_errc;
     }
@@ -276,7 +285,7 @@ partition_allocator::allocate(allocation_request request) {
         }
     }
 
-    auto cluster_errc = check_cluster_limits(create_count);
+    auto cluster_errc = check_cluster_limits(create_count, request._nt);
     if (cluster_errc) {
         co_return cluster_errc;
     }

src/v/cluster/scheduling/partition_allocator.h

@@ -18,6 +18,7 @@
 #include "cluster/scheduling/types.h"
 #include "config/property.h"
 #include "features/fwd.h"
+#include "model/metadata.h"
 
 namespace cluster {
 
@@ -152,7 +153,8 @@ class partition_allocator {
     // new_partitions_replicas_requested represents the total number of
     // partitions requested by a request. i.e. partitions * replicas requested.
     std::error_code check_cluster_limits(
-      const uint64_t new_partitions_replicas_requested) const;
+      const uint64_t new_partitions_replicas_requested,
+      const model::topic_namespace& topic) const;
 
     ss::future<result<allocation_units::pointer>>
       do_allocate(allocation_request);

src/v/cluster/tests/partition_allocator_fixture.h

@@ -32,7 +32,6 @@
 
 struct partition_allocator_fixture {
     static constexpr uint32_t partitions_per_shard = 1000;
-    static constexpr uint32_t partitions_reserve_shard0 = 2;
 
     partition_allocator_fixture()
       : partition_allocator_fixture(std::nullopt, std::nullopt) {}
@@ -68,7 +67,7 @@ struct partition_allocator_fixture {
           broker.id(),
           broker.properties().cores,
           config::mock_binding<uint32_t>(uint32_t{partitions_per_shard}),
-          config::mock_binding<uint32_t>(uint32_t{partitions_reserve_shard0}),
+          partitions_reserve_shard0.bind(),
           kafka_internal_topics.bind()));
     }
 
@@ -138,6 +137,7 @@ struct partition_allocator_fixture {
     cluster::partition_allocator& allocator() { return _allocator.local(); }
 
     config::mock_property<std::vector<ss::sstring>> kafka_internal_topics{{}};
+    config::mock_property<uint32_t> partitions_reserve_shard0{2};
     model::topic_namespace tn{model::kafka_namespace, model::topic{"test"}};
     ss::sharded<cluster::members_table> members;
     ss::sharded<features::feature_table> features;
@@ -158,7 +158,7 @@ struct partition_allocator_fixture {
             config::mock_binding<std::optional<size_t>>(memory_per_partition),
             config::mock_binding<std::optional<int32_t>>(fds_per_partition),
             config::mock_binding<uint32_t>(uint32_t{partitions_per_shard}),
-            config::mock_binding<uint32_t>(uint32_t{partitions_reserve_shard0}),
+            partitions_reserve_shard0.bind(),
             kafka_internal_topics.bind(),
             config::mock_binding<bool>(true))
           .get();

src/v/cluster/tests/partition_allocator_tests.cc

@@ -194,6 +194,28 @@ FIXTURE_TEST(allocation_over_capacity, partition_allocator_fixture) {
       allocator().allocate(make_allocation_request(int_2, 1, 1)).get());
 }
 
+FIXTURE_TEST(
+  allocation_over_capacity_without_shard0, partition_allocator_fixture) {
+    // Disable shard0 reservations
+    partitions_reserve_shard0.update(0);
+
+    register_node(0, 6);
+    register_node(1, 6);
+    register_node(2, 6);
+
+    saturate_all_machines();
+    auto gr = allocator().state().last_group_id();
+    BOOST_REQUIRE(
+      allocator().allocate(make_allocation_request(1, 1)).get().has_error());
+    // group id hasn't changed
+    BOOST_REQUIRE_EQUAL(allocator().state().last_group_id(), gr);
+
+    // Make the topic internal and retry, should work.
+    kafka_internal_topics.update({tn.tp()});
+    BOOST_REQUIRE(allocator().allocate(make_allocation_request(1, 1)).get());
+    BOOST_REQUIRE_GT(allocator().state().last_group_id(), gr);
+}
+
 FIXTURE_TEST(max_allocation, partition_allocator_fixture) {
     register_node(0, 2);
     register_node(1, 2);
@@ -530,7 +552,7 @@ FIXTURE_TEST(updating_nodes_properties, partition_allocator_fixture) {
     BOOST_REQUIRE_EQUAL(
       it->second->max_capacity(),
       10 * partition_allocator_fixture::partitions_per_shard
-        - partition_allocator_fixture::partitions_reserve_shard0);
+        - partitions_reserve_shard0());
 }
 
 FIXTURE_TEST(change_replication_factor, partition_allocator_fixture) {