AudioFlinger: refine mixer sleep time logic

When an AudioTrack is in underrun state, the AudioFlinger mixer will
sleep for a short period of time to give the app a chance to fill the
AudioTrack buffer. If the AudioTrack is still not ready during next mixing round,
the mixer will proceed with other tracks.

If an application keeps a steady underrun condition, the AudioFlinger mixer will
alternate between ready and not ready states. In the longer term this will cause the
audio HAL to underrun.
There is a mechanism to reduce the sleep period if the mixer is not ready several times in a
row but this mechanism is defeated by the alternating ready/not ready conditions.

The fix consists in only increasing sleep time if the mixer is ready for at least two
consecutive times.

Issue 5904527.

Change-Id: Id0139bca9be8c4e425ec6d428515c4d8f718e8c9

Author: Eric Laurent

services/audioflinger/AudioFlinger.cpp

@@ -1973,11 +1973,14 @@ bool AudioFlinger::MixerThread::threadLoop()
         if (LIKELY(mixerStatus == MIXER_TRACKS_READY)) {
             // mix buffers...
             mAudioMixer->process();
-            sleepTime = 0;
-            // increase sleep time progressively when application underrun condition clears
-            if (sleepTimeShift > 0) {
+            // increase sleep time progressively when application underrun condition clears.
+            // Only increase sleep time if the mixer is ready for two consecutive times to avoid
+            // that a steady state of alternating ready/not ready conditions keeps the sleep time
+            // such that we would underrun the audio HAL.
+            if ((sleepTime == 0) && (sleepTimeShift > 0)) {
                 sleepTimeShift--;
             }
+            sleepTime = 0;
             standbyTime = systemTime() + kStandbyTimeInNsecs;
             //TODO: delay standby when effects have a tail
         } else {