runner: Schedule task types whose queue exists

To avoid the churn of hitting the scheduler too often for noops.

Refs. swh/infra/sysadm-environment#5512

swh/scheduler/celery_backend/runner.py

-# Copyright (C) 2015-2024 The Software Heritage developers
+# Copyright (C) 2015-2025 The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -20,7 +20,7 @@ from kombu.utils.uuid import uuid
 
 from swh.core.statsd import statsd
 from swh.scheduler import get_scheduler
-from swh.scheduler.celery_backend.config import get_available_slots
+from swh.scheduler.celery_backend.config import get_available_slots, get_queue_stats
 from swh.scheduler.interface import SchedulerInterface
 from swh.scheduler.model import TaskRun, TaskType
 from swh.scheduler.utils import utcnow
@@ -73,11 +73,32 @@ def run_ready_tasks(
     """
     all_backend_tasks: List[TaskRun] = []
     while True:
+        # Only the task types whose queue exits in rabbitmq will be scheduled
+        task_types_to_schedule = []
+        # Keep the ignore task types during the loop
+        task_types_to_skip = []
+
+        # If no task type filter provided, we use all scheduler task types registered
         if not task_types:
             task_types = backend.get_task_types()
+
+        # Only schedule task types whose queue exists in rabbitmq
+        for _task_type in task_types:
+            task_type_name = _task_type.type if isinstance(TaskType) else _task_type
+            task_type = backend.get_task_type(task_type_name)
+            queue_stats = get_queue_stats(app, task_type.backend_name)
+            lst = task_types_to_schedule if queue_stats else task_types_to_skip
+            lst.append(task_type)
+
+        if task_types_to_skip:
+            logger.info(
+                "Task types skipped because their queue is non-existent: [%s]",
+                ", ".join(tt.type for tt in task_types_to_skip),
+            )
+
         task_types_d = {}
         pending_tasks = []
-        for task_type in task_types:
+        for task_type in task_types_to_schedule:
             task_type_name = task_type.type
             task_types_d[task_type_name] = task_type
             max_queue_length = task_type.max_queue_length