Event dispatcher slow down using delays rather than thread priority

The Symbian event dispatcher has a mechanism to slow down the Qt app
to prevent viewsrv crashes and keep the device responsive. This was
implemented using a thread priority drop. But that has some bad side
effects, such as app and system performance instability.

This new implementation of a slow down mechanism uses a separate low
priority thread to test when the system is getting too busy. Adaptive
millisecond waits are used to slow the app down just enough to let the
low prioirity thread to run.

In practice this avoids the performance instability of the previous
method, and results in much better system stability where the system
stays more responsive with fewer viewsrv panics when heavy Qt apps are
running.

The slow down code kicks in after 2 seconds of busy time. The delays
grow 1ms at a time to a maximum of 1/4 of average event run time.

Task-number: QTBUG-9489
Reviewed-by: Shane Kearns

2 files changed, 95 insertions, 37 deletions

diff --git a/src/corelib/kernel/qeventdispatcher_symbian.cpp b/src/corelib/kernel/qeventdispatcher_symbian.cpp
index 8c96057..fa337ca 100644
--- a/src/corelib/kernel/qeventdispatcher_symbian.cpp
+++ b/src/corelib/kernel/qeventdispatcher_symbian.cpp
@@ -43,7 +43,6 @@
 #include <private/qthread_p.h>
 #include <qcoreapplication.h>
 #include <private/qcoreapplication_p.h>
-#include <qdatetime.h>
 
 #include <unistd.h>
 #include <errno.h>
@@ -636,6 +635,74 @@ void QSocketActiveObject::deleteLater()
     }
 }
 
+#ifdef QT_SYMBIAN_PRIORITY_DROP
+class QIdleDetectorThread
+{
+public:
+    QIdleDetectorThread()
+    : m_state(STATE_RUN), m_stop(false)
+    {
+        qt_symbian_throwIfError(m_lock.CreateLocal());
+        TInt err = m_idleDetectorThread.Create(KNullDesC(), &idleDetectorThreadFunc, 1024, &User::Allocator(), this);
+        if (err != KErrNone)
+            m_lock.Close();
+        qt_symbian_throwIfError(err);
+        m_idleDetectorThread.SetPriority(EPriorityAbsoluteBackgroundNormal);
+        m_idleDetectorThread.Resume();
+    }
+
+    ~QIdleDetectorThread()
+    {
+        // close down the idle thread because if corelib is loaded temporarily, this would leak threads into the host process
+        m_stop = true;
+        m_lock.Signal();
+        m_idleDetectorThread.SetPriority(EPriorityNormal);
+        TRequestStatus s;
+        m_idleDetectorThread.Logon(s);
+        User::WaitForRequest(s);
+        m_idleDetectorThread.Close();
+        m_lock.Close();
+    }
+
+    void kick()
+    {
+        m_state = STATE_KICKED;
+        m_lock.Signal();
+    }
+
+    bool hasRun()
+    {
+        return m_state == STATE_RUN;
+    }
+
+private:
+    static TInt idleDetectorThreadFunc(TAny* self)
+    {
+        static_cast<QIdleDetectorThread*>(self)->IdleLoop();
+        return KErrNone;
+    }
+
+    void IdleLoop()
+    {
+        while (!m_stop) {
+            m_lock.Wait();
+            m_state = STATE_RUN;
+        }
+    }
+
+private:
+    enum IdleStates {STATE_KICKED, STATE_RUN} m_state;
+    bool m_stop;
+    RThread m_idleDetectorThread;
+    RFastLock m_lock;
+};
+
+Q_GLOBAL_STATIC(QIdleDetectorThread, idleDetectorThread);
+
+const int maxBusyTime = 2000; // maximum time we allow idle detector to be blocked before worrying, in milliseconds
+const int baseDelay = 1000; // minimum delay time used when backing off to allow idling, in microseconds
+#endif
+
 QEventDispatcherSymbian::QEventDispatcherSymbian(QObject *parent)
     : QAbstractEventDispatcher(parent),
       m_activeScheduler(0),
@@ -646,11 +713,15 @@ QEventDispatcherSymbian::QEventDispatcherSymbian(QObject *parent)
       m_iterationCount(0),
       m_noSocketEvents(false)
 {
+#ifdef QT_SYMBIAN_PRIORITY_DROP
+    m_delay = baseDelay;
+    m_avgEventTime = 0;
+    idleDetectorThread();
+#endif
 }
 
 QEventDispatcherSymbian::~QEventDispatcherSymbian()
 {
-    m_processHandle.Close();
 }
 
 void QEventDispatcherSymbian::startingUp()
@@ -711,23 +782,7 @@ bool QEventDispatcherSymbian::processEvents ( QEventLoop::ProcessEventsFlags fla
         m_interrupt = false;
 
 #ifdef QT_SYMBIAN_PRIORITY_DROP
-        /*
-         * This QTime variable is used to measure the time it takes to finish
-         * the event loop. If we take too long in the loop, other processes
-         * may be starved and killed. After the first event has completed, we
-         * take the current time, and if the remaining events take longer than
-         * a preset time, we temporarily lower the priority to force a context
-         * switch. For applications that do not take unecessarily long in the
-         * event loop, the priority will not be altered.
-         */
-        QTime time;
-        enum {
-            FirstRun,
-            SubsequentRun,
-            TimeStarted
-        } timeState = FirstRun;
-
-        TProcessPriority priority;
+        QTime eventTimer;
 #endif
 
         while (1) {
@@ -743,10 +798,18 @@ bool QEventDispatcherSymbian::processEvents ( QEventLoop::ProcessEventsFlags fla
             }
 
 #ifdef QT_SYMBIAN_PRIORITY_DROP
-            if (timeState == SubsequentRun) {
-                time.start();
-                timeState = TimeStarted;
+            if (idleDetectorThread()->hasRun()) {
+                if (m_delay > baseDelay)
+                    m_delay -= baseDelay;
+                m_lastIdleRequestTimer.start();
+                idleDetectorThread()->kick();
+            } else if (m_lastIdleRequestTimer.elapsed() > maxBusyTime) {
+                User::AfterHighRes(m_delay);
+                // allow delay to be up to 1/4 of execution time
+                if (!idleDetectorThread()->hasRun() && m_delay*3 < m_avgEventTime)
+                    m_delay += baseDelay;
             }
+            eventTimer.start();
 #endif
 
             TInt error;
@@ -756,6 +819,12 @@ bool QEventDispatcherSymbian::processEvents ( QEventLoop::ProcessEventsFlags fla
                 CActiveScheduler::Current()->Error(error);
             }
 
+#ifdef QT_SYMBIAN_PRIORITY_DROP
+            int eventDur = eventTimer.elapsed()*1000;
+            // average is calcualted as a 5% decaying exponential average
+            m_avgEventTime = (m_avgEventTime * 95 + eventDur * 5) / 100;
+#endif
+
             if (!handledSymbianEvent) {
                 qFatal("QEventDispatcherSymbian::processEvents(): Caught Symbian stray signal");
             }
@@ -764,20 +833,6 @@ bool QEventDispatcherSymbian::processEvents ( QEventLoop::ProcessEventsFlags fla
                 break;
             }
             block = false;
-#ifdef QT_SYMBIAN_PRIORITY_DROP
-            if (timeState == TimeStarted && time.elapsed() > 100) {
-                priority = m_processHandle.Priority();
-                m_processHandle.SetPriority(EPriorityBackground);
-                time.start();
-                // Slight chance of race condition in the next lines, but nothing fatal
-                // will happen, just wrong priority.
-                if (m_processHandle.Priority() == EPriorityBackground) {
-                    m_processHandle.SetPriority(priority);
-                }
-            }
-            if (timeState == FirstRun)
-                timeState = SubsequentRun;
-#endif
         };
 
         emit awake();
diff --git a/src/corelib/kernel/qeventdispatcher_symbian_p.h b/src/corelib/kernel/qeventdispatcher_symbian_p.h
index 1ab31cc..8a9c9a0 100644
--- a/src/corelib/kernel/qeventdispatcher_symbian_p.h
+++ b/src/corelib/kernel/qeventdispatcher_symbian_p.h
@@ -62,6 +62,7 @@
 #include <qmutex.h>
 #include <qwaitcondition.h>
 #include <qsocketnotifier.h>
+#include <qdatetime.h>
 
 #include <e32base.h>
 
@@ -279,7 +280,9 @@ private:
 
     QList<QActiveObject *> m_deferredActiveObjects;
 
-    RProcess m_processHandle;
+    int m_delay;
+    int m_avgEventTime;
+    QTime m_lastIdleRequestTimer;
 };
 
 #ifdef QT_DEBUG