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QML界面卡死分析 --01

现象 — Canvas与QRasterPaintEngine相关

1. 前置条件：渲染循环使用多线程方式；

2. 发现界面假死，无响应;拷贝dump 分析，主线程堆栈如下：

   #0  SyncSemWait () 
   #1  0x000000007b4f8784 in QMutexPrivate::wait(int) () from  qt_engine/qt-engine/lib/libQt6Core.so.6.5.0
   #2  0x000000007b4f8940 in QBasicMutex::lockInternal(int) () from qt_engine/qt-engine/lib/libQt6Core.so.6.5.0
   #3  0x000000007b4fecb4 in QThreadPool::start(QRunnable*, int) () from qt_engine/qt-engine/lib/libQt6Core.so.6.5.0
   #4  0x000000007b4fed70 in QThreadPool::start(std::__1::function<void ()>, int) () qt_engine/qt-engine/lib/libQt6Core.so.6.5.0
   #5  0x000000007af73d4c in blend_untransformed_argb(int, QT_FT_Span_ const*, void*) ()from qt_engine/qt-engine/lib/libQt6Gui.so.6.5.0
   #6  0x000000007aba991c in fillRect_normalized(QRect const&, QSpanData*, QRasterPaintEnginePrivate*) ()from /qt_engine/qt-engine/lib/libQt6Gui.so.6.5.0
   #7  0x000000007abadafc in QRasterPaintEngine::drawImage(QPointF const&, QImage const&) ()from qt_engine/qt-engine/lib/libQt6Gui.so.6.5.0
   #8  0x000000007abd125c in QPainter::drawImage(QPointF const&, QImage const&) ()from qt_engine/qt-engine/lib/libQt6Gui.so.6.5.0
   #9  0x000000007957db04 in qt_drawImage(QPainter*, QQuickContext2D::State&, QImage, QRectF const&, QRectF const&, bool) () from qt_engine/qt-engine/lib/libQt6Quick.so.6.5.0
   #10 0x000000007957f5cc in QQuickContext2DCommandBuffer::replay(QPainter*, QQuickContext2D::State&, QVector2D const&) ()from qt_engine/qt-engine/lib/libQt6Quick.so.6.5.0
   #11 0x0000000079580df4 in QQuickContext2DTexture::paintWithoutTiles(QQuickContext2DCommandBuffer*) ()
   --Type <RET> for more, q to quit, c to continue without paging--c from qt_engine/qt-engine/lib/libQt6Quick.so.6.5.0
   #12 0x00000000795829b0 in QQuickContext2DTexture::paint(QQuickContext2DCommandBuffer*) () from qt_engine/qt-engine/lib/libQt6Quick.so.6.5.0
   #13 0x000000007955efb8 in QQuickContext2D::flush() () from qt_engine/qt-engine/lib/libQt6Quick.so.6.5.0
   #14 0x000000007955af14 in QQuickCanvasItem::updatePolish() () from qt_engine/qt-engine/lib/libQt6Quick.so.6.5.0
   #15 0x00000000792ba70c in QQuickWindowPrivate::polishItems() () from qt_engine/qt-engine/lib/libQt6Quick.so.6.5.0
   #16 0x00000000794b78fc in QSGThreadedRenderLoop::polishAndSync(QSGThreadedRenderLoop::Window*, bool) () from qt_engine/qt-engine/lib/libQt6Quick.so.6.5.0
   #17 0x00000000794b8874 in QSGThreadedRenderLoop::handleUpdateRequest(QQuickWindow*) () from qt_engine/qt-engine/lib/libQt6Quick.so.6.5.0
   #18 0x00000000792bd61c in QQuickWindow::event(QEvent*) () from qt_engine/qt-engine/lib/libQt6Quick.so.6.5.0
   #19 0x000000007b3597ac in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from qt_engine/qt-engine/lib/libQt6Core.so.6.5.0
   #20 0x000000007aae8a20 in QPlatformWindow::windowEvent(QEvent*) () from qt_engine/qt-engine/lib/libQt6Gui.so.6.5.0
   #21 0x000000007aaa20e8 in QGuiApplication::notify(QObject*, QEvent*) () from qt_engine/qt-engine/lib/libQt6Gui.so.6.5.0
   #22 0x000000007b3597ac in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from qt_engine/qt-engine/lib/libQt6Core.so.6.5.0
   #23 0x000000007b4f6350 in QTimerInfoList::activateTimers() () from qt_engine/qt-engine/lib/libQt6Core.so.6.5.0
   #24 0x000000007b4f418c in QEventDispatcherUNIX::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from qt_engine/qt-engine/lib/libQt6Core.so.6.5.0
   #25 0x000000007af1e474 in QUnixEventDispatcherQPA::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from qt_engine/qt-engine/lib/libQt6Gui.so.6.5.0
   #26 0x000000007b365480 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from qt_engine/qt-engine/lib/libQt6Core.so.6.5.0
   #27 0x000000007b36155c in QCoreApplication::exec() () from qt_engine/qt-engine/lib/libQt6Core.so.6.5.0
   #28 0x00000000101d9eec in main ()
   

分析

1. 根据堆栈分析出现 QRasterPaintEngine相关类； 此类和CPU 绘制相关；在跟踪源码到 blend_untransformed_argb() 发现内部有用线程池在做图像相关处理，主线程卡死在信号量的等待上；
2. 根据纯QML程序 光栅化之类的 在GPU 上运行，如果没有配置软光栅化，按照正常配置，不应该会运行到QRasterPaintEngine类中相关代码，除非qml程序中有用Canvas 画布在绘制；搜索qml 程序 发现代码中用用Canvas 中绘制，且是定时器已25帧间隔一直在掉requestPaint()函数；此函数会造成即时Canvas 不可见也会触发绘制；
3. 通过屏蔽Canvas 代码，发现QRasterPaintEngine类中绘制相关代码再也没有进入过；然后优化Canvas 绘制相关代码，避免不可见时也在后台绘制；整个程序的 cpu 使用率 就将下来了；跑机测试，也未发现主线程卡死现象了。

结论

QRasterPaintEngine 是软光栅化，是CPU绘制相关，会耗费CPU资源；其内部对对象处理主要通过线程池来做资源加载；而Canvas中不管是否可见调用requestPaint()函数；都会触发软光栅化运行，降低性能，在配合上压力测试或者不合理刷新间隔容易造成界面卡顿。