audioflinger: reduce sleep time to avoid underrun

Progressively reduce the sleep time applied in MixerThread::threadLoop()
in case of consecutive application underruns to avoid starving the audio HAL.
As the default sleep time is longer than the duration of an audio buffer
we ended up writing less data than needed by the audio HAL if
the condition persisted.

Issue 5553055.

Change-Id: I2b23ee79c032efa945025db228beaecd1e07a2e5

Author: Eric Laurent

services/audioflinger/AudioFlinger.cpp

@@ -89,6 +89,12 @@ static const int kRecordThreadSleepUs = 5000;
 
 static const nsecs_t kSetParametersTimeout = seconds(2);
 
+// minimum sleep time for the mixer thread loop when tracks are active but in underrun
+static const uint32_t kMinThreadSleepTimeUs = 5000;
+// maximum divider applied to the active sleep time in the mixer thread loop
+static const uint32_t kMaxThreadSleepTimeShift = 2;
+
+
 // ----------------------------------------------------------------------------
 
 static bool recordingAllowed() {
@@ -1846,6 +1852,7 @@ bool AudioFlinger::MixerThread::threadLoop()
     uint32_t activeSleepTime = activeSleepTimeUs();
     uint32_t idleSleepTime = idleSleepTimeUs();
     uint32_t sleepTime = idleSleepTime;
+    uint32_t sleepTimeShift = 0;
     Vector< sp<EffectChain> > effectChains;
 #ifdef DEBUG_CPU_USAGE
     ThreadCpuUsage cpu;
@@ -1937,6 +1944,7 @@ bool AudioFlinger::MixerThread::threadLoop()
 
                     standbyTime = systemTime() + kStandbyTimeInNsecs;
                     sleepTime = idleSleepTime;
+                    sleepTimeShift = 0;
                     continue;
                 }
             }
@@ -1953,14 +1961,28 @@ bool AudioFlinger::MixerThread::threadLoop()
             // mix buffers...
             mAudioMixer->process();
             sleepTime = 0;
+            // increase sleep time progressively when application underrun condition clears
+            if (sleepTimeShift > 0) {
+                sleepTimeShift--;
+            }
             standbyTime = systemTime() + kStandbyTimeInNsecs;
             //TODO: delay standby when effects have a tail
         } else {
             // If no tracks are ready, sleep once for the duration of an output
             // buffer size, then write 0s to the output
             if (sleepTime == 0) {
                 if (mixerStatus == MIXER_TRACKS_ENABLED) {
-                    sleepTime = activeSleepTime;
+                    sleepTime = activeSleepTime >> sleepTimeShift;
+                    if (sleepTime < kMinThreadSleepTimeUs) {
+                        sleepTime = kMinThreadSleepTimeUs;
+                    }
+                    // reduce sleep time in case of consecutive application underruns to avoid
+                    // starving the audio HAL. As activeSleepTimeUs() is larger than a buffer
+                    // duration we would end up writing less data than needed by the audio HAL if
+                    // the condition persists.
+                    if (sleepTimeShift < kMaxThreadSleepTimeShift) {
+                        sleepTimeShift++;
+                    }
                 } else {
                     sleepTime = idleSleepTime;
                 }