Bug 1685938 - Add RunningTimes::mPostMeasurementTimeStamp and use as sample timestamp - r=canaltinova

mPostMeasurementTimeStamp records the time right after CPU measurements (point-based or interval) ended.
It is then used as the main sample timestamp, to both avoid another TimeStamp::Now() call, and to keep measurements and timestamp as close together as possible (and even closer in the next patch).

Differential Revision: https://phabricator.services.mozilla.com/D101545

tools/profiler/core/platform-linux-android.cpp

@@ -342,6 +342,9 @@ static RunningTimes GetThreadRunningTimesDiff(
     }
   }
 
+  // Reminder: This must stay *after* the CPU measurements.
+  newRunningTimes.SetPostMeasurementTimeStamp(TimeStamp::NowUnfuzzed());
+
   RunningTimes diff =
       newRunningTimes - platformData->PreviousThreadRunningTimesRef();
   platformData->PreviousThreadRunningTimesRef() = newRunningTimes;

tools/profiler/core/platform-macos.cpp

@@ -109,6 +109,9 @@ static RunningTimes GetThreadRunningTimesDiff(
     newRunningTimes.SetThreadCPUDelta(userTimeUs + systemTimeUs);
   }
 
+  // Reminder: This must stay *after* the CPU measurements.
+  newRunningTimes.SetPostMeasurementTimeStamp(TimeStamp::NowUnfuzzed());
+
   RunningTimes diff =
       newRunningTimes - platformData->PreviousThreadRunningTimesRef();
   platformData->PreviousThreadRunningTimesRef() = newRunningTimes;

tools/profiler/core/platform-win32.cpp

@@ -150,6 +150,9 @@ static RunningTimes GetThreadRunningTimesDiff(
     }
   }
 
+  // Reminder: This must stay *after* the CPU measurements.
+  newRunningTimes.SetPostMeasurementTimeStamp(TimeStamp::NowUnfuzzed());
+
   RunningTimes diff =
       newRunningTimes - platformData->PreviousThreadRunningTimesRef();
   platformData->PreviousThreadRunningTimesRef() = newRunningTimes;

tools/profiler/core/platform.cpp

@@ -3369,13 +3369,15 @@ void SamplerThread::Run() {
                 thread.mProfiledThreadData.get();
             RefPtr<ThreadInfo> info = registeredThread->Info();
 
-            RunningTimes runningTimesDiff;
-            if (cpuUtilization) {
-              runningTimesDiff =
-                  GetThreadRunningTimesDiff(lock, *registeredThread);
+            const RunningTimes runningTimesDiff = [&]() {
+              if (!cpuUtilization) {
+                // If we don't need CPU measurements, we only need a timestamp.
+                return RunningTimes(TimeStamp::NowUnfuzzed());
               }
+              return GetThreadRunningTimesDiff(lock, *registeredThread);
+            }();
 
-            TimeStamp now = TimeStamp::NowUnfuzzed();
+            const TimeStamp& now = runningTimesDiff.PostMeasurementTimeStamp();
             double threadSampleDeltaMs =
                 (now - CorePS::ProcessStartTime()).ToMilliseconds();
 

tools/profiler/core/platform.h

@@ -34,6 +34,7 @@
 #include "mozilla/Logging.h"
 #include "mozilla/MathAlgorithms.h"
 #include "mozilla/ProfileBufferEntrySerialization.h"
+#include "mozilla/TimeStamp.h"
 #include "mozilla/UniquePtr.h"
 #include "mozilla/Vector.h"
 #include "nsString.h"
@@ -141,10 +142,24 @@ class RunningTimes {
  public:
   constexpr RunningTimes() = default;
 
+  // Constructor with only a timestamp, useful when no measurements will be
+  // taken.
+  constexpr explicit RunningTimes(const mozilla::TimeStamp& aTimeStamp)
+      : mPostMeasurementTimeStamp(aTimeStamp) {}
+
   constexpr void Clear() { *this = RunningTimes{}; }
 
   constexpr bool IsEmpty() const { return mKnownBits == 0; }
 
+  // This should be called right after CPU measurements have been taken.
+  void SetPostMeasurementTimeStamp(const mozilla::TimeStamp& aTimeStamp) {
+    mPostMeasurementTimeStamp = aTimeStamp;
+  }
+
+  const mozilla::TimeStamp& PostMeasurementTimeStamp() const {
+    return mPostMeasurementTimeStamp;
+  }
+
   // Should be filled for any registered thread.
 
 #define RUNNING_TIME_MEMBER(index, name, unit, jsonProperty)          \
@@ -187,8 +202,12 @@ class RunningTimes {
   }
 
   // Difference from `aBefore` to `this`. Any unknown makes the result unknown.
+  // PostMeasurementTimeStamp set to `this` PostMeasurementTimeStamp, to keep
+  // the most recent timestamp associated with the end of the interval over
+  // which the difference applies.
   RunningTimes operator-(const RunningTimes& aBefore) const {
     RunningTimes diff;
+    diff.mPostMeasurementTimeStamp = mPostMeasurementTimeStamp;
 #define RUNNING_TIME_SUB(index, name, unit, jsonProperty)           \
   if (Is##name##unit##Known() && aBefore.Is##name##unit##Known()) { \
     diff.Set##name##unit(m##name##unit - aBefore.m##name##unit);    \
@@ -204,6 +223,8 @@ class RunningTimes {
   friend mozilla::ProfileBufferEntryWriter::Serializer<RunningTimes>;
   friend mozilla::ProfileBufferEntryReader::Deserializer<RunningTimes>;
 
+  mozilla::TimeStamp mPostMeasurementTimeStamp;
+
   uint32_t mKnownBits = 0u;
 
 #define RUNNING_TIME_MEMBER(index, name, unit, jsonProperty) \