core/renderjournal: improve render time heuristics

Instead of giving the rolling average for render times a special case for
when render time suddenly increases, detect how stable render times are.
If they're volatile and increase a lot, increase the predicated render time
beyond the spike, as more render time spikes are likely to follow.

CCBUG: 477959

src/core/renderjournal.cpp

@@ -6,6 +6,8 @@
 
 #include "renderjournal.h"
 
+using namespace std::chrono_literals;
+
 namespace KWin
 {
 
@@ -13,22 +15,29 @@ RenderJournal::RenderJournal()
 {
 }
 
-void RenderJournal::add(std::chrono::nanoseconds renderTime)
+static std::chrono::nanoseconds mix(std::chrono::nanoseconds duration1, std::chrono::nanoseconds duration2, double ratio)
 {
-    if (renderTime > m_result || !m_lastAdd) {
-        m_result = renderTime;
-    } else {
-        static constexpr std::chrono::nanoseconds timeConstant = std::chrono::milliseconds(500);
-        const auto timeDifference = std::chrono::steady_clock::now() - *m_lastAdd;
-        const double ratio = std::min(0.1, double(timeDifference.count()) / double(timeConstant.count()));
-        m_result = std::chrono::nanoseconds(int64_t(renderTime.count() * ratio + m_result.count() * (1 - ratio)));
-    }
-    m_lastAdd = std::chrono::steady_clock::now();
+    return std::chrono::nanoseconds(int64_t(std::round(duration1.count() * ratio + duration2.count() * (1 - ratio))));
+}
+
+void RenderJournal::add(std::chrono::nanoseconds renderTime, std::chrono::nanoseconds presentationTimestamp)
+{
+    const auto timeDifference = m_lastAdd ? presentationTimestamp - *m_lastAdd : 10s;
+    m_lastAdd = presentationTimestamp;
+
+    static constexpr std::chrono::nanoseconds varianceTimeConstant = 3s;
+    const double varianceRatio = std::clamp(timeDifference.count() / double(varianceTimeConstant.count()), 0.1, 1.0);
+    const auto renderTimeDiff = std::max(renderTime - m_result, 0ns);
+    m_variance = std::max(mix(renderTimeDiff, m_variance, varianceRatio), renderTimeDiff);
+
+    static constexpr std::chrono::nanoseconds timeConstant = 500ms;
+    const double ratio = std::clamp(timeDifference.count() / double(timeConstant.count()), 0.1, 1.0);
+    m_result = mix(renderTime, m_result, ratio);
 }
 
 std::chrono::nanoseconds RenderJournal::result() const
 {
-    return m_result;
+    return m_result + m_variance * 2;
 }
 
 } // namespace KWin

src/core/renderjournal.h

@@ -23,13 +23,14 @@ class KWIN_EXPORT RenderJournal
 public:
     RenderJournal();
 
-    void add(std::chrono::nanoseconds renderTime);
+    void add(std::chrono::nanoseconds renderTime, std::chrono::nanoseconds presentationTimestamp);
 
     std::chrono::nanoseconds result() const;
 
 private:
     std::chrono::nanoseconds m_result{0};
-    std::optional<std::chrono::steady_clock::time_point> m_lastAdd;
+    std::chrono::nanoseconds m_variance{0};
+    std::optional<std::chrono::nanoseconds> m_lastAdd;
 };
 
 } // namespace KWin

src/core/renderloop.cpp

@@ -97,7 +97,7 @@ void RenderLoopPrivate::notifyFrameCompleted(std::chrono::nanoseconds timestamp,
 
     notifyVblank(timestamp);
 
-    renderJournal.add(renderTime);
+    renderJournal.add(renderTime, timestamp);
     if (!inhibitCount) {
         maybeScheduleRepaint();
     }