Add fetch scheduling group

This scheduling group is used for consumer fetch processing. We assign
it the same priority as the default group (where most other kafka
handling takes place), but by putting it into its own group we prevent
non-fetch requests from being significantly delayed when fetch requests
use all the CPU.

Issue redpanda-data/core-internal#435

src/v/resource_mgmt/cpu_scheduling.h

@@ -38,6 +38,7 @@ class scheduling_groups final {
           "archival_upload", 100);
         _node_status = co_await ss::create_scheduling_group("node_status", 50);
         _self_test = co_await ss::create_scheduling_group("self_test", 100);
+        _fetch = co_await ss::create_scheduling_group("fetch", 1000);
     }
 
     ss::future<> destroy_groups() {
@@ -52,6 +53,7 @@ class scheduling_groups final {
         co_await destroy_scheduling_group(_archival_upload);
         co_await destroy_scheduling_group(_node_status);
         co_await destroy_scheduling_group(_self_test);
+        co_await destroy_scheduling_group(_fetch);
         co_return;
     }
 
@@ -70,6 +72,16 @@ class scheduling_groups final {
     ss::scheduling_group archival_upload() { return _archival_upload; }
     ss::scheduling_group node_status() { return _node_status; }
     ss::scheduling_group self_test_sg() { return _self_test; }
+    /**
+     * @brief Scheduling group for fetch requests.
+     *
+     * This scheduling group is used for consumer fetch processing. We assign
+     * it the same priority as the default group (where most other kafka
+     * handling takes place), but by putting it into its own group we prevent
+     * non-fetch requests from being significantly delayed when fetch requests
+     * use all the CPU.
+     */
+    ss::scheduling_group fetch_sg() { return _fetch; }
 
     std::vector<std::reference_wrapper<const ss::scheduling_group>>
     all_scheduling_groups() const {
@@ -85,7 +97,8 @@ class scheduling_groups final {
           std::cref(_raft_learner_recovery),
           std::cref(_archival_upload),
           std::cref(_node_status),
-          std::cref(_self_test)};
+          std::cref(_self_test),
+          std::cref(_fetch)};
     }
 
 private:
@@ -102,4 +115,5 @@ class scheduling_groups final {
     ss::scheduling_group _archival_upload;
     ss::scheduling_group _node_status;
     ss::scheduling_group _self_test;
+    ss::scheduling_group _fetch;
 };