Improve `reactor_tick_time` metric (#11724)

The existing implementation of the `python_twisted_reactor_tick_time` metric is pretty useless, because it *only* 
measures the time taken to execute timed calls and callbacks from threads. That neglects everything that 
happens off the back of I/O, which is obviously quite a lot for us.

To improve this, I've hooked into a different place in the reactor - in particular, where it calls `epoll`. That call is 
the only place it should wait for something to happen - the rest of the loop *should* be quick.

I've also removed `python_twisted_reactor_pending_calls`, because I don't believe anyone ever looks at it, and
it's a nuisance to populate.

5 files changed, 86 insertions, 109 deletions

diff --git a/changelog.d/11724.misc b/changelog.d/11724.misc
new file mode 100644
index 0000000000..e9d5dae857
--- /dev/null
+++ b/changelog.d/11724.misc
@@ -0,0 +1 @@
+Improve accuracy of `python_twisted_reactor_tick_time` prometheus metric.
diff --git a/changelog.d/11724.removal b/changelog.d/11724.removal
new file mode 100644
index 0000000000..088c3ff31f
--- /dev/null
+++ b/changelog.d/11724.removal
@@ -0,0 +1 @@
+Remove `python_twisted_reactor_pending_calls` prometheus metric.
diff --git a/contrib/prometheus/consoles/synapse.html b/contrib/prometheus/consoles/synapse.html
index cd9ad15231..d17c8a08d9 100644
--- a/contrib/prometheus/consoles/synapse.html
+++ b/contrib/prometheus/consoles/synapse.html
@@ -92,22 +92,6 @@ new PromConsole.Graph({
 })
 </script>
 
-<h3>Pending calls per tick</h3>
-<div id="reactor_pending_calls"></div>
-<script>
-new PromConsole.Graph({
-  node: document.querySelector("#reactor_pending_calls"),
-  expr: "rate(python_twisted_reactor_pending_calls_sum[30s]) / rate(python_twisted_reactor_pending_calls_count[30s])",
-  name: "[[job]]-[[index]]",
-  min: 0,
-  renderer: "line",
-  height: 150,
-  yAxisFormatter: PromConsole.NumberFormatter.humanize,
-  yHoverFormatter: PromConsole.NumberFormatter.humanize,
-  yTitle: "Pending Calls"
-})
-</script>
-
 <h1>Storage</h1>
 
 <h3>Queries</h3>
diff --git a/synapse/metrics/__init__.py b/synapse/metrics/__init__.py
index ba7ca0f2d4..9e6c1b2f3b 100644
--- a/synapse/metrics/__init__.py
+++ b/synapse/metrics/__init__.py
@@ -12,15 +12,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import functools
 import itertools
 import logging
 import os
 import platform
 import threading
-import time
 from typing import (
-    Any,
     Callable,
     Dict,
     Generic,
@@ -33,7 +30,6 @@ from typing import (
     Type,
     TypeVar,
     Union,
-    cast,
 )
 
 import attr
@@ -44,11 +40,9 @@ from prometheus_client.core import (
     GaugeMetricFamily,
 )
 
-from twisted.internet import reactor
-from twisted.internet.base import ReactorBase
 from twisted.python.threadpool import ThreadPool
 
-import synapse
+import synapse.metrics._reactor_metrics
 from synapse.metrics._exposition import (
     MetricsResource,
     generate_latest,
@@ -369,21 +363,6 @@ REGISTRY.register(CPUMetrics())
 
 
 #
-# Twisted reactor metrics
-#
-
-tick_time = Histogram(
-    "python_twisted_reactor_tick_time",
-    "Tick time of the Twisted reactor (sec)",
-    buckets=[0.001, 0.002, 0.005, 0.01, 0.025, 0.05, 0.1, 0.2, 0.5, 1, 2, 5],
-)
-pending_calls_metric = Histogram(
-    "python_twisted_reactor_pending_calls",
-    "Pending calls",
-    buckets=[1, 2, 5, 10, 25, 50, 100, 250, 500, 1000],
-)
-
-#
 # Federation Metrics
 #
 
@@ -434,8 +413,6 @@ build_info.labels(
     " ".join([platform.system(), platform.release()]),
 ).set(1)
 
-last_ticked = time.time()
-
 # 3PID send info
 threepid_send_requests = Histogram(
     "synapse_threepid_send_requests_with_tries",
@@ -483,75 +460,6 @@ def register_threadpool(name: str, threadpool: ThreadPool) -> None:
     )
 
 
-class ReactorLastSeenMetric:
-    def collect(self) -> Iterable[Metric]:
-        cm = GaugeMetricFamily(
-            "python_twisted_reactor_last_seen",
-            "Seconds since the Twisted reactor was last seen",
-        )
-        cm.add_metric([], time.time() - last_ticked)
-        yield cm
-
-
-REGISTRY.register(ReactorLastSeenMetric())
-
-F = TypeVar("F", bound=Callable[..., Any])
-
-
-def runUntilCurrentTimer(reactor: ReactorBase, func: F) -> F:
-    @functools.wraps(func)
-    def f(*args: Any, **kwargs: Any) -> Any:
-        now = reactor.seconds()
-        num_pending = 0
-
-        # _newTimedCalls is one long list of *all* pending calls. Below loop
-        # is based off of impl of reactor.runUntilCurrent
-        for delayed_call in reactor._newTimedCalls:
-            if delayed_call.time > now:
-                break
-
-            if delayed_call.delayed_time > 0:
-                continue
-
-            num_pending += 1
-
-        num_pending += len(reactor.threadCallQueue)
-        start = time.time()
-        ret = func(*args, **kwargs)
-        end = time.time()
-
-        # record the amount of wallclock time spent running pending calls.
-        # This is a proxy for the actual amount of time between reactor polls,
-        # since about 25% of time is actually spent running things triggered by
-        # I/O events, but that is harder to capture without rewriting half the
-        # reactor.
-        tick_time.observe(end - start)
-        pending_calls_metric.observe(num_pending)
-
-        # Update the time we last ticked, for the metric to test whether
-        # Synapse's reactor has frozen
-        global last_ticked
-        last_ticked = end
-
-        return ret
-
-    return cast(F, f)
-
-
-try:
-    # Ensure the reactor has all the attributes we expect
-    reactor.seconds  # type: ignore
-    reactor.runUntilCurrent  # type: ignore
-    reactor._newTimedCalls  # type: ignore
-    reactor.threadCallQueue  # type: ignore
-
-    # runUntilCurrent is called when we have pending calls. It is called once
-    # per iteratation after fd polling.
-    reactor.runUntilCurrent = runUntilCurrentTimer(reactor, reactor.runUntilCurrent)  # type: ignore
-except AttributeError:
-    pass
-
-
 __all__ = [
     "MetricsResource",
     "generate_latest",
diff --git a/synapse/metrics/_reactor_metrics.py b/synapse/metrics/_reactor_metrics.py
new file mode 100644
index 0000000000..ce0688621c
--- /dev/null
+++ b/synapse/metrics/_reactor_metrics.py
@@ -0,0 +1,83 @@
+# 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 select
+import time
+from typing import Any, Iterable, List, Tuple
+
+from prometheus_client import Histogram, Metric
+from prometheus_client.core import REGISTRY, GaugeMetricFamily
+
+from twisted.internet import reactor
+
+#
+# Twisted reactor metrics
+#
+
+tick_time = Histogram(
+    "python_twisted_reactor_tick_time",
+    "Tick time of the Twisted reactor (sec)",
+    buckets=[0.001, 0.002, 0.005, 0.01, 0.025, 0.05, 0.1, 0.2, 0.5, 1, 2, 5],
+)
+
+
+class EpollWrapper:
+    """a wrapper for an epoll object which records the time between polls"""
+
+    def __init__(self, poller: "select.epoll"):
+        self.last_polled = time.time()
+        self._poller = poller
+
+    def poll(self, *args, **kwargs) -> List[Tuple[int, int]]:  # type: ignore[no-untyped-def]
+        # record the time since poll() was last called. This gives a good proxy for
+        # how long it takes to run everything in the reactor - ie, how long anything
+        # waiting for the next tick will have to wait.
+        tick_time.observe(time.time() - self.last_polled)
+
+        ret = self._poller.poll(*args, **kwargs)
+
+        self.last_polled = time.time()
+        return ret
+
+    def __getattr__(self, item: str) -> Any:
+        return getattr(self._poller, item)
+
+
+class ReactorLastSeenMetric:
+    def __init__(self, epoll_wrapper: EpollWrapper):
+        self._epoll_wrapper = epoll_wrapper
+
+    def collect(self) -> Iterable[Metric]:
+        cm = GaugeMetricFamily(
+            "python_twisted_reactor_last_seen",
+            "Seconds since the Twisted reactor was last seen",
+        )
+        cm.add_metric([], time.time() - self._epoll_wrapper.last_polled)
+        yield cm
+
+
+try:
+    # if the reactor has a `_poller` attribute, which is an `epoll` object
+    # (ie, it's an EPollReactor), we wrap the `epoll` with a thing that will
+    # measure the time between ticks
+    from select import epoll
+
+    poller = reactor._poller  # type: ignore[attr-defined]
+except (AttributeError, ImportError):
+    pass
+else:
+    if isinstance(poller, epoll):
+        poller = EpollWrapper(poller)
+        reactor._poller = poller  # type: ignore[attr-defined]
+        REGISTRY.register(ReactorLastSeenMetric(poller))