Bug 1708325 - Allow doing an extra refresh driver tick for user input events. r=mstange,bas

Differential Revision: https://phabricator.services.mozilla.com/D113737

dom/events/EventDispatcher.cpp

@@ -1013,6 +1013,17 @@ nsresult EventDispatcher::Dispatch(nsISupports* aTarget,
           chain[i].PreHandleEvent(preVisitor);
         }
 
+        RefPtr<nsRefreshDriver> refreshDriver;
+        if (aPresContext && aPresContext->GetRootPresContext() &&
+            aEvent->IsTrusted() &&
+            (aEvent->mMessage == eKeyPress ||
+             aEvent->mMessage == eMouseClick)) {
+          refreshDriver = aPresContext->GetRootPresContext()->RefreshDriver();
+          if (refreshDriver) {
+            refreshDriver->EnterUserInputProcessing();
+          }
+        }
+
         clearTargets = ShouldClearTargets(aEvent);
 
         // Handle the chain.
@@ -1127,6 +1138,10 @@ nsresult EventDispatcher::Dispatch(nsISupports* aTarget,
                 break;
             }
           }
+
+          if (refreshDriver) {
+            driver->ExitUserInputProcessing();
+          }
         }
 
         preVisitor.mEventStatus = postVisitor.mEventStatus;

layout/base/nsRefreshDriver.cpp

@@ -771,6 +771,12 @@ class VsyncRefreshDriverTimer : public RefreshDriverTimer {
 
   void RunRefreshDrivers(VsyncId aId, TimeStamp aTimeStamp) {
     Tick(aId, aTimeStamp);
+    for (auto& driver : mContentRefreshDrivers) {
+      driver->FinishedVsyncTick();
+    }
+    for (auto& driver : mRootRefreshDrivers) {
+      driver->FinishedVsyncTick();
+    }
   }
 
   // When using local vsync source, we keep a strong ref to it here to ensure
@@ -1124,6 +1130,7 @@ nsRefreshDriver::nsRefreshDriver(nsPresContext* aPresContext)
       mNotifyDOMContentFlushed(false),
       mNeedToUpdateIntersectionObservations(false),
       mInNormalTick(false),
+      mAttemptedExtraTickSinceLastVsync(false),
       mWarningThreshold(REFRESH_WAIT_WARNING) {
   MOZ_ASSERT(NS_IsMainThread());
   MOZ_ASSERT(mPresContext,
@@ -1402,6 +1409,36 @@ bool nsRefreshDriver::CanDoCatchUpTick() {
   return true;
 }
 
+bool nsRefreshDriver::CanDoExtraTick() {
+  // Only allow one extra tick per normal vsync tick.
+  if (mAttemptedExtraTickSinceLastVsync) {
+    return false;
+  }
+
+  // If we don't have a timer, or we didn't tick on the timer's
+  // refresh then we can't do an 'extra' tick (but we may still
+  // do a catch up tick).
+  if (!mActiveTimer ||
+      mActiveTimer->MostRecentRefresh() != mMostRecentRefresh) {
+    return false;
+  }
+
+  // Grab the current timestamp before checking the tick hint to be sure
+  // sure that it's equal or smaller than the value used within checking
+  // the tick hint.
+  TimeStamp now = TimeStamp::Now();
+  Maybe<TimeStamp> nextTick = mActiveTimer->GetNextTickHint();
+  int32_t minimumRequiredTime = StaticPrefs::layout_extra_tick_minimum_ms();
+  // If there's less than 4 milliseconds until the next tick, it's probably
+  // not worth trying to catch up.
+  if (minimumRequiredTime < 0 || !nextTick ||
+      (*nextTick - now) < TimeDuration::FromMilliseconds(minimumRequiredTime)) {
+    return false;
+  }
+
+  return true;
+}
+
 void nsRefreshDriver::EnsureTimerStarted(EnsureTimerStartedFlags aFlags) {
   // FIXME: Bug 1346065: We should also assert the case where we have
   // STYLO_THREADS=1.
@@ -1416,7 +1453,31 @@ void nsRefreshDriver::EnsureTimerStarted(EnsureTimerStartedFlags aFlags) {
   }
 
   // will it already fire, and no other changes needed?
-  if (mActiveTimer && !(aFlags & eForceAdjustTimer)) return;
+  if (mActiveTimer && !(aFlags & eForceAdjustTimer)) {
+    // If we're being called from within a user input handler, and we think
+    // there's time to rush an extra tick immediately, then schedule a runnable
+    // to run the extra tick.
+    if (mUserInputProcessingCount && CanDoExtraTick()) {
+      RefPtr<nsRefreshDriver> self = this;
+      NS_DispatchToCurrentThreadQueue(
+          NS_NewRunnableFunction(
+              "RefreshDriver::EnsureTimerStarted::extra",
+              [self]() -> void {
+                // Re-check if we can still do an extra tick, in case anything
+                // changed while the runnable was pending.
+                if (self->CanDoExtraTick()) {
+                  PROFILER_MARKER_UNTYPED("ExtraRefreshDriverTick", GRAPHICS);
+                  LOG("[%p] Doing extra tick for user input", self.get());
+                  self->mAttemptedExtraTickSinceLastVsync = true;
+                  self->Tick(self->mActiveTimer->MostRecentRefreshVsyncId(),
+                             self->mActiveTimer->MostRecentRefresh(),
+                             IsExtraTick::Yes);
+                }
+              }),
+          EventQueuePriority::Vsync);
+    }
+    return;
+  }
 
   if (IsFrozen() || !mPresContext) {
     // If we don't want to start it now, or we've been disconnected.
@@ -1456,6 +1517,7 @@ void nsRefreshDriver::EnsureTimerStarted(EnsureTimerStartedFlags aFlags) {
                 // Re-check if we can still do a catch-up, in case anything
                 // changed while the runnable was pending.
                 if (self->CanDoCatchUpTick()) {
+                  LOG("[%p] Doing catch up tick", self.get());
                   self->Tick(self->mActiveTimer->MostRecentRefreshVsyncId(),
                              self->mActiveTimer->MostRecentRefresh());
                 }
@@ -1965,7 +2027,8 @@ static CallState ReduceAnimations(Document& aDocument) {
   return CallState::Continue;
 }
 
-void nsRefreshDriver::Tick(VsyncId aId, TimeStamp aNowTime) {
+void nsRefreshDriver::Tick(VsyncId aId, TimeStamp aNowTime,
+                           IsExtraTick aIsExtraTick /* = No */) {
   MOZ_ASSERT(!nsContentUtils::GetCurrentJSContext(),
              "Shouldn't have a JSContext on the stack");
 
@@ -1982,11 +2045,14 @@ void nsRefreshDriver::Tick(VsyncId aId, TimeStamp aNowTime) {
   // go forward in time, not backwards. To prevent the refresh
   // driver from going back in time, just skip this tick and
   // wait until the next tick.
-  if ((aNowTime <= mMostRecentRefresh) && !mTestControllingRefreshes) {
+  // If this is an 'extra' tick, then we expect it to be using the same
+  // vsync id and timestamp as the original tick, so also allow those.
+  if ((aNowTime <= mMostRecentRefresh) && !mTestControllingRefreshes &&
+      aIsExtraTick == IsExtraTick::No) {
     return;
   }
   auto cleanupInExtraTick = MakeScopeExit([&] { mInNormalTick = false; });
-  mInNormalTick = true;
+  mInNormalTick = aIsExtraTick != IsExtraTick::Yes;
 
   bool isPresentingInVR = false;
 #if defined(MOZ_WIDGET_ANDROID)

layout/base/nsRefreshDriver.h

@@ -146,6 +146,16 @@ class nsRefreshDriver final : public mozilla::layers::TransactionIdAllocator,
   bool AddImageRequest(imgIRequest* aRequest);
   void RemoveImageRequest(imgIRequest* aRequest);
 
+  /**
+   * Marks that we're currently in the middle of processing user input.
+   * Called by EventDispatcher when it's handling an input event.
+   */
+  void EnterUserInputProcessing() { mUserInputProcessingCount++; }
+  void ExitUserInputProcessing() {
+    MOZ_ASSERT(mUserInputProcessingCount > 0);
+    mUserInputProcessingCount--;
+  }
+
   /**
    * Add / remove presshells which have pending resize event.
    */
@@ -408,6 +418,10 @@ class nsRefreshDriver final : public mozilla::layers::TransactionIdAllocator,
   void AddForceNotifyContentfulPaintPresContext(nsPresContext* aPresContext);
   void FlushForceNotifyContentfulPaintPresContext();
 
+  // Mark that we've just run a tick from vsync, used to throttle 'extra'
+  // paints to one per vsync (see CanDoExtraTick).
+  void FinishedVsyncTick() { mAttemptedExtraTickSinceLastVsync = false; }
+
  private:
   typedef nsTArray<RefPtr<VVPResizeEvent>> VisualViewportResizeEventArray;
   typedef nsTArray<RefPtr<mozilla::Runnable>> ScrollEventArray;
@@ -443,7 +457,13 @@ class nsRefreshDriver final : public mozilla::layers::TransactionIdAllocator,
   void RunFrameRequestCallbacks(mozilla::TimeStamp aNowTime);
   void UpdateIntersectionObservations(mozilla::TimeStamp aNowTime);
   MOZ_CAN_RUN_SCRIPT_BOUNDARY
-  void Tick(mozilla::VsyncId aId, mozilla::TimeStamp aNowTime);
+
+  enum class IsExtraTick {
+    No,
+    Yes,
+  };
+  void Tick(mozilla::VsyncId aId, mozilla::TimeStamp aNowTime,
+            IsExtraTick aIsExtraTick = IsExtraTick::No);
 
   enum EnsureTimerStartedFlags {
     eNone = 0,
@@ -478,7 +498,18 @@ class nsRefreshDriver final : public mozilla::layers::TransactionIdAllocator,
 
   void FinishedWaitingForTransaction();
 
+  /**
+   * Returns true if we didn't tick on the most recent vsync, but we think
+   * we could run one now instead in order to reduce latency.
+   */
   bool CanDoCatchUpTick();
+  /**
+   * Returns true if we think it's possible to run an repeat tick (between
+   * vsyncs) to hopefully replace the original tick's paint on the compositor.
+   * We allow this sometimes for tick requests coming for user input handling
+   * to reduce latency.
+   */
+  bool CanDoExtraTick();
 
   bool AtPendingTransactionLimit() {
     return mPendingTransactions.Length() == 2;
@@ -502,6 +533,7 @@ class nsRefreshDriver final : public mozilla::layers::TransactionIdAllocator,
   AutoTArray<TransactionId, 3> mPendingTransactions;
 
   uint32_t mFreezeCount;
+  uint32_t mUserInputProcessingCount = 0;
 
   // How long we wait between ticks for throttled (which generally means
   // non-visible) documents registered with a non-throttled refresh driver.
@@ -554,6 +586,10 @@ class nsRefreshDriver final : public mozilla::layers::TransactionIdAllocator,
   // (timer-driven) tick.
   bool mInNormalTick : 1;
 
+  // True if we attempted an extra tick (see CanDoExtraTick) since the last
+  // vsync and thus shouldn't allow another.
+  bool mAttemptedExtraTickSinceLastVsync : 1;
+
   // Number of seconds that the refresh driver is blocked waiting for a
   // compositor transaction to be completed before we append a note to the gfx
   // critical log. The number is doubled every time the threshold is hit.

modules/libpref/init/StaticPrefList.yaml

@@ -7165,6 +7165,18 @@
   value: 1
   mirror: always
 
+# The minimum amount of time (milliseconds) required to be remaining
+# in the current vsync interval for us to attempt an extra tick, or
+# <0 to disable extra ticks entirely.
+- name: layout.extra-tick.minimum-ms
+  type: int32_t
+#if defined(NIGHTLY_BUILD)
+  value: 4
+#else
+  value: -1
+#endif
+  mirror: always
+
 # Enable/disable interruptible reflow, which allows reflows to stop
 # before completion (and display the partial results) when user events
 # are pending.