test.py: reduce cgroup overhead in resource metrics gathering

Only enable the memory controller in cgroup subtree_control instead of
all available controllers. cpu.stat is available in cgroup v2 without
enabling the cpu controller (base accounting), and enabling io/pids/cpu
controllers adds unnecessary per-operation kernel overhead to Scylla
processes - particularly the memory controller's per-page-cache-operation
accounting combined with io controller overhead during heavy I/O.

Additionally, restrict SystemResourceMonitor to the master process only.
System-wide metrics (CPU%, memory) are identical from any process, so
running a monitoring thread in each xdist worker was redundant and added
unnecessary SQLite write contention and thread scheduling noise.

Co-Authored-By: Claude Opus 4.6 (200K context) <noreply@anthropic.com>

test/pylib/resource_gather.py

@@ -279,13 +279,16 @@ def _is_cgroup_rw() -> bool:
     return False
 
 def propagate_subtree_controls(group: Path):
-    with open(group / 'cgroup.controllers', 'r') as f:
-        controllers = f.readline().strip()
-    if not controllers:
-        return
-    controllers = " ".join(map(lambda x: f"+{x}", controllers.split(" ")))
-    with open(group / 'cgroup.subtree_control', 'w') as f:
-        f.write(controllers)
+    # Only enable the memory controller. cpu.stat is available without
+    # enabling the cpu controller (it's base cgroup v2 accounting).
+    # Enabling all controllers (cpu, io, pids, etc.) adds unnecessary
+    # per-operation kernel overhead to child processes - in particular,
+    # the io controller adds accounting to every I/O operation.
+    with open(group / "cgroup.controllers", "r") as f:
+        if "memory" not in f.readline().split():
+            return
+    with open(group / "cgroup.subtree_control", "w") as f:
+        f.write("+memory")
 
 
 def setup_cgroup(is_required: bool) -> None:

test/pylib/runner.py

@@ -304,8 +304,11 @@ def pytest_sessionstart(session: pytest.Session) -> None:
         if not is_xdist_worker and SCYLLA_TEST_CGROUP_BASE_ENV not in os.environ:
             setup_cgroup(is_required=True)
         setup_worker_cgroup()
-        _system_resource_monitor = SystemResourceMonitor(temp_dir)
-        _system_resource_monitor.start()
+        # System-wide resource metrics (CPU%, memory) are identical from any process.
+        # Only the master needs to record them.
+        if not is_xdist_worker:
+            _system_resource_monitor = SystemResourceMonitor(temp_dir)
+            _system_resource_monitor.start()