diff --git a/synapse/hacks/__init__.py b/synapse/hacks/__init__.py
new file mode 100644
index 0000000000..3a5f22c022
--- /dev/null
+++ b/synapse/hacks/__init__.py
@@ -0,0 +1,13 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
diff --git a/synapse/hacks/selective_scalene_profiling.py b/synapse/hacks/selective_scalene_profiling.py
new file mode 100644
index 0000000000..41829c3094
--- /dev/null
+++ b/synapse/hacks/selective_scalene_profiling.py
@@ -0,0 +1,95 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import os
+import time
+from typing import Any, Callable, Dict, Optional
+
+from scalene import scalene_profiler
+
+
+class ProfilingDecider:
+ INSTANCES: Dict[str, "ProfilingDecider"] = {}
+
+ def __init__(self, name: str, cond: Callable[[], bool]) -> None:
+ ProfilingDecider.INSTANCES[name] = self
+
+ # Default to being armed if SCALENE is available as an env var.
+ self.armed = b"SCALENE" in os.environb
+
+ self._cond = cond
+
+ def decide(self) -> bool:
+ if not self.armed:
+ return False
+
+ if not self._cond():
+ return False
+
+ self.armed = False
+
+ return True
+
+
+class CpuUtimeTracker:
+ def __init__(self) -> None:
+ self._update_times(time.time())
+
+ def _update_times(self, now_wall: float) -> None:
+ utime, _, _, _, elapsed = os.times()
+ self._last_utime = utime
+ self._last_elapsed = elapsed
+ self._last_wall = now_wall
+
+ self.min_elapse = 0.5
+ self.max_elapse = 120.0
+
+ def update_return_utime(self) -> Optional[float]:
+ """
+ Returns CPU usage over this period, provided at least `min_elapse` have
+ elapsed.
+ """
+ wall = time.time()
+ elapsed = wall - self._last_wall
+ if elapsed < self.min_elapse:
+ return None
+
+ last_utime = self._last_utime
+ last_elapsed = self._last_elapsed
+
+ self._update_times(wall)
+
+ if elapsed > self.max_elapse:
+ # the average will be a bit skewy if so much time has elapsed. Ignore.
+ return None
+
+ usage = (self._last_utime - last_utime) / (self._last_elapsed - last_elapsed)
+ return usage
+
+
+class SelectiveProfiling:
+ def __init__(self, decider: ProfilingDecider, enable: bool = False):
+ self._decider = decider
+ self._enable = enable
+
+ def __enter__(self) -> None:
+ if not self._enable:
+ return
+ if not self._decider.decide():
+ self._enable = False
+ return
+ scalene_profiler.start()
+
+ def __exit__(self, exc_type: Any, exc_val: Any, exc_tb: Any) -> None:
+ if not self._enable:
+ scalene_profiler.stop()
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