Add callback for single EVB loop duration violators

Summary:
Context for motivation is in
https://fb.workplace.com/groups/560979627394613/permalink/2194915787334314.
This solution is different than the one we discussed in that post. I considered
implementing a set of post-evb-loop function callbacks that all took in the
ebv loop_time, but that was a bigger change than expected since currently the
callbacks are designed to not take any variables, and modifying/adding classes
like `FunctionLoopCallback` to either take a param or not seemed like overkill.
I instead implemented the callback basically the same as setMaxLatency
exception that the duration is a single-loop duration with no exponential
smoothing or backoffs.

Differential Revision: D30916498

fbshipit-source-id: 27cc7edf919ab11d2ee75a7f722b6a48b193c445

folly/experimental/io/test/IoUringBackendTest.cpp

@@ -1260,6 +1260,8 @@ REGISTER_TYPED_TEST_CASE_P(
     CallbackOrderTest,
     AlwaysEnqueueCallbackOrderTest,
     IdleTime,
+    MaxLatencyUndamped,
+    UnsetMaxLatencyUndamped,
     ThisLoop,
     EventBaseThreadLoop,
     EventBaseThreadName,

folly/io/async/EventBase.cpp

@@ -413,13 +413,18 @@ bool EventBase::loopBody(int flags, bool ignoreKeepAlive) {
                << " notificationQueueSize: " << getNotificationQueueSize()
                << " nothingHandledYet(): " << nothingHandledYet();
 
+      if (maxLatency_ > std::chrono::microseconds::zero()) {
         // see if our average loop time has exceeded our limit
-      if ((maxLatency_ > std::chrono::microseconds::zero()) &&
+        if (dampenMaxLatency_ &&
             (maxLatencyLoopTime_.get() > double(maxLatency_.count()))) {
           maxLatencyCob_();
           // back off temporarily -- don't keep spamming maxLatencyCob_
           // if we're only a bit over the limit
           maxLatencyLoopTime_.dampen(0.9);
+        } else if (!dampenMaxLatency_ && loop_time > maxLatency_) {
+          // If no damping, we compare the raw loop time
+          maxLatencyCob_();
+        }
       }
 
       // Our loop run did real work; reset the idle timer

folly/io/async/EventBase.h

@@ -632,10 +632,14 @@ class EventBase : public TimeoutManager,
    * called when that latency is exceeded.
    * OBS: This functionality depends on time-measurement.
    */
-  void setMaxLatency(std::chrono::microseconds maxLatency, Func maxLatencyCob) {
+  void setMaxLatency(
+      std::chrono::microseconds maxLatency,
+      Func maxLatencyCob,
+      bool dampen = true) {
     assert(enableTimeMeasurement_);
     maxLatency_ = maxLatency;
     maxLatencyCob_ = std::move(maxLatencyCob);
+    dampenMaxLatency_ = dampen;
   }
 
   /**
@@ -917,6 +921,10 @@ class EventBase : public TimeoutManager,
   // to reduce spamminess
   SmoothLoopTime maxLatencyLoopTime_;
 
+  // If true, max latency callbacks will use a dampened SmoothLoopTime, else
+  // they'll use the raw loop time.
+  bool dampenMaxLatency_ = true;
+
   // callback called when latency limit is exceeded
   Func maxLatencyCob_;
 

folly/io/async/test/EventBaseTest.cpp

@@ -60,6 +60,8 @@ REGISTER_TYPED_TEST_CASE_P(
     CallbackOrderTest,
     AlwaysEnqueueCallbackOrderTest,
     IdleTime,
+    MaxLatencyUndamped,
+    UnsetMaxLatencyUndamped,
     ThisLoop,
     EventBaseThreadLoop,
     EventBaseThreadName,

folly/io/async/test/EventBaseTestLib.h

@@ -1979,6 +1979,44 @@ TYPED_TEST_P(EventBaseTest, IdleTime) {
   ASSERT_EQ(21, tos->getTimeouts());
 }
 
+TYPED_TEST_P(EventBaseTest, MaxLatencyUndamped) {
+  auto eventBasePtr = getEventBase<TypeParam>();
+  folly::EventBase& eb = *eventBasePtr;
+  int maxDurationViolations = 0;
+  eb.setMaxLatency(
+      std::chrono::milliseconds{1}, [&]() { maxDurationViolations++; }, false);
+  eb.runInLoop(
+      [&]() {
+        /* sleep override */ std::this_thread::sleep_for(
+            std::chrono::microseconds{1001});
+        eb.terminateLoopSoon();
+      },
+      true);
+  eb.loop();
+  ASSERT_EQ(maxDurationViolations, 1);
+}
+
+TYPED_TEST_P(EventBaseTest, UnsetMaxLatencyUndamped) {
+  auto eventBasePtr = getEventBase<TypeParam>();
+  folly::EventBase& eb = *eventBasePtr;
+  int maxDurationViolations = 0;
+  eb.setMaxLatency(
+      std::chrono::milliseconds{1}, [&]() { maxDurationViolations++; }, false);
+  // Immediately unset it and make sure the counter isn't incremented. If the
+  // function gets called, this will raise an std::bad_function_call.
+  std::function<void()> bad_func = nullptr;
+  eb.setMaxLatency(std::chrono::milliseconds{0}, bad_func, false);
+  eb.runInLoop(
+      [&]() {
+        /* sleep override */ std::this_thread::sleep_for(
+            std::chrono::microseconds{1001});
+        eb.terminateLoopSoon();
+      },
+      true);
+  eb.loop();
+  ASSERT_EQ(maxDurationViolations, 0);
+}
+
 /**
  * Test that thisLoop functionality works with terminateLoopSoon
  */