diff --git a/services/audioflinger/Threads.cpp b/services/audioflinger/Threads.cpp
index 4ae5a02..737d21e 100644
--- a/services/audioflinger/Threads.cpp
+++ b/services/audioflinger/Threads.cpp
@@ -647,8 +647,7 @@
         mAudioSource(AUDIO_SOURCE_DEFAULT), mId(id),
         // mName will be set by concrete (non-virtual) subclass
         mDeathRecipient(new PMDeathRecipient(this)),
-        mSystemReady(systemReady),
-        mNotifiedBatteryStart(false)
+        mSystemReady(systemReady)
 {
     memset(&mPatch, 0, sizeof(struct audio_patch));
 }
@@ -989,10 +988,10 @@
     }
 }
 
-void AudioFlinger::ThreadBase::acquireWakeLock(int uid)
+void AudioFlinger::ThreadBase::acquireWakeLock()
 {
     Mutex::Autolock _l(mLock);
-    acquireWakeLock_l(uid);
+    acquireWakeLock_l();
 }
 
 String16 AudioFlinger::ThreadBase::getWakeLockTag()
@@ -1014,37 +1013,23 @@
     }
 }
 
-void AudioFlinger::ThreadBase::acquireWakeLock_l(int uid)
+void AudioFlinger::ThreadBase::acquireWakeLock_l()
 {
     getPowerManager_l();
     if (mPowerManager != 0) {
         sp<IBinder> binder = new BBinder();
-        status_t status;
-        if (uid >= 0) {
-            status = mPowerManager->acquireWakeLockWithUid(POWERMANAGER_PARTIAL_WAKE_LOCK,
-                    binder,
-                    getWakeLockTag(),
-                    String16("audioserver"),
-                    uid,
-                    true /* FIXME force oneway contrary to .aidl */);
-        } else {
-            status = mPowerManager->acquireWakeLock(POWERMANAGER_PARTIAL_WAKE_LOCK,
+        // Uses AID_AUDIOSERVER for wakelock.  updateWakeLockUids_l() updates with client uids.
+        status_t status = mPowerManager->acquireWakeLock(POWERMANAGER_PARTIAL_WAKE_LOCK,
                     binder,
                     getWakeLockTag(),
                     String16("audioserver"),
                     true /* FIXME force oneway contrary to .aidl */);
-        }
         if (status == NO_ERROR) {
             mWakeLockToken = binder;
         }
         ALOGV("acquireWakeLock_l() %s status %d", mThreadName, status);
     }
 
-    if (!mNotifiedBatteryStart) {
-        // TODO: call this function for each track when it becomes active.
-        BatteryNotifier::getInstance().noteStartAudio(AID_AUDIOSERVER);
-        mNotifiedBatteryStart = true;
-    }
     gBoottime.acquire(mWakeLockToken);
     mTimestamp.mTimebaseOffset[ExtendedTimestamp::TIMEBASE_BOOTTIME] =
             gBoottime.getBoottimeOffset();
@@ -1067,17 +1052,6 @@
         }
         mWakeLockToken.clear();
     }
-
-    if (mNotifiedBatteryStart) {
-        // TODO: call this function for each track when it becomes inactive.
-        BatteryNotifier::getInstance().noteStopAudio(AID_AUDIOSERVER);
-        mNotifiedBatteryStart = false;
-    }
-}
-
-void AudioFlinger::ThreadBase::updateWakeLockUids(const SortedVector<int> &uids) {
-    Mutex::Autolock _l(mLock);
-    updateWakeLockUids_l(uids);
 }
 
 void AudioFlinger::ThreadBase::getPowerManager_l() {
@@ -1096,6 +1070,15 @@
 
 void AudioFlinger::ThreadBase::updateWakeLockUids_l(const SortedVector<int> &uids) {
     getPowerManager_l();
+
+#if !LOG_NDEBUG
+    std::stringstream s;
+    for (int uid : uids) {
+        s << uid << " ";
+    }
+    ALOGD("updateWakeLockUids_l %s uids:%s", mThreadName, s.str().c_str());
+#endif
+
     if (mWakeLockToken == NULL) { // token may be NULL if AudioFlinger::systemReady() not called.
         if (mSystemReady) {
             ALOGE("no wake lock to update, but system ready!");
@@ -1657,6 +1640,41 @@
     mPendingConfigEvents.clear();
 }
 
+template <typename T>
+ssize_t AudioFlinger::ThreadBase::ActiveTracks<T>::add(const sp<T> &track) {
+    ssize_t index = mActiveTracks.indexOf(track);
+    if (index >= 0) {
+        ALOGW("ActiveTracks<T>::add track %p already there", track.get());
+        return index;
+    }
+    mActiveTracksGeneration++;
+    mLatestActiveTrack = track;
+    BatteryNotifier::getInstance().noteStartAudio(track->uid());
+    return mActiveTracks.add(track);
+}
+
+template <typename T>
+ssize_t AudioFlinger::ThreadBase::ActiveTracks<T>::remove(const sp<T> &track) {
+    ssize_t index = mActiveTracks.remove(track);
+    if (index < 0) {
+        ALOGW("ActiveTracks<T>::remove nonexistent track %p", track.get());
+        return index;
+    }
+    mActiveTracksGeneration++;
+    BatteryNotifier::getInstance().noteStopAudio(track->uid());
+    // mLatestActiveTrack is not cleared even if is the same as track.
+    return index;
+}
+
+template <typename T>
+void AudioFlinger::ThreadBase::ActiveTracks<T>::clear() {
+    for (const sp<T> &track : mActiveTracks) {
+        BatteryNotifier::getInstance().noteStopAudio(track->uid());
+    }
+    mLastActiveTracksGeneration = mActiveTracksGeneration;
+    mActiveTracks.clear();
+    mLatestActiveTrack.clear();
+}
 
 // ----------------------------------------------------------------------------
 //      Playback
@@ -1683,7 +1701,6 @@
         mSuspended(0), mBytesWritten(0),
         mFramesWritten(0),
         mSuspendedFrames(0),
-        mActiveTracksGeneration(0),
         // mStreamTypes[] initialized in constructor body
         mOutput(output),
         mLastWriteTime(-1), mNumWrites(0), mNumDelayedWrites(0), mInWrite(false),
@@ -1802,8 +1819,8 @@
         result.append(buffer);
         Track::appendDumpHeader(result);
         for (size_t i = 0; i < numactive; ++i) {
-            sp<Track> track = mActiveTracks[i].promote();
-            if (track != 0 && mTracks.indexOf(track) < 0) {
+            sp<Track> track = mActiveTracks[i];
+            if (mTracks.indexOf(track) < 0) {
                 track->dump(buffer, SIZE, true);
                 result.append(buffer);
             }
@@ -2206,9 +2223,6 @@
         track->mResetDone = false;
         track->mPresentationCompleteFrames = 0;
         mActiveTracks.add(track);
-        mWakeLockUids.add(track->uid());
-        mActiveTracksGeneration++;
-        mLatestActiveTrack = track;
         sp<EffectChain> chain = getEffectChain_l(track->sessionId());
         if (chain != 0) {
             ALOGV("addTrack_l() starting track on chain %p for session %d", chain.get(),
@@ -2843,11 +2857,7 @@
         }
 
         // indicate all active tracks in the chain
-        for (size_t i = 0 ; i < mActiveTracks.size() ; ++i) {
-            sp<Track> track = mActiveTracks[i].promote();
-            if (track == 0) {
-                continue;
-            }
+        for (const sp<Track> &track : mActiveTracks) {
             if (session == track->sessionId()) {
                 ALOGV("addEffectChain_l() activating track %p on session %d", track.get(), session);
                 chain->incActiveTrackCnt();
@@ -2894,11 +2904,7 @@
         if (chain == mEffectChains[i]) {
             mEffectChains.removeAt(i);
             // detach all active tracks from the chain
-            for (size_t i = 0 ; i < mActiveTracks.size() ; ++i) {
-                sp<Track> track = mActiveTracks[i].promote();
-                if (track == 0) {
-                    continue;
-                }
+            for (const sp<Track> &track : mActiveTracks) {
                 if (session == track->sessionId()) {
                     ALOGV("removeEffectChain_l(): stopping track on chain %p for session Id: %d",
                             chain.get(), session);
@@ -2975,8 +2981,6 @@
     // FIXME could this be made local to while loop?
     writeFrames = 0;
 
-    int lastGeneration = 0;
-
     cacheParameters_l();
     mSleepTimeUs = mIdleSleepTimeUs;
 
@@ -3074,10 +3078,9 @@
                     mTimestamp.mTimeNs[ExtendedTimestamp::LOCATION_SERVER] = mLastWriteTime == -1
                             ? systemTime() : mLastWriteTime;
                 }
-                const size_t size = mActiveTracks.size();
-                for (size_t i = 0; i < size; ++i) {
-                    sp<Track> t = mActiveTracks[i].promote();
-                    if (t != 0 && !t->isFastTrack()) {
+
+                for (const sp<Track> &t : mActiveTracks) {
+                    if (!t->isFastTrack()) {
                         t->updateTrackFrameInfo(
                                 t->mAudioTrackServerProxy->framesReleased(),
                                 mFramesWritten,
@@ -3098,8 +3101,6 @@
                 if (!keepWakeLock()) {
                     releaseWakeLock_l();
                     released = true;
-                    mWakeLockUids.clear();
-                    mActiveTracksGeneration++;
                 }
                 ALOGV("wait async completion");
                 mWaitWorkCV.wait(mLock);
@@ -3133,8 +3134,6 @@
                     }
 
                     releaseWakeLock_l();
-                    mWakeLockUids.clear();
-                    mActiveTracksGeneration++;
                     // wait until we have something to do...
                     ALOGV("%s going to sleep", myName.string());
                     mWaitWorkCV.wait(mLock);
@@ -3159,12 +3158,7 @@
             // mMixerStatusIgnoringFastTracks is also updated internally
             mMixerStatus = prepareTracks_l(&tracksToRemove);
 
-            // compare with previously applied list
-            if (lastGeneration != mActiveTracksGeneration) {
-                // update wakelock
-                updateWakeLockUids_l(mWakeLockUids);
-                lastGeneration = mActiveTracksGeneration;
-            }
+            mActiveTracks.updateWakeLockUids(this);
 
             // prevent any changes in effect chain list and in each effect chain
             // during mixing and effect process as the audio buffers could be deleted
@@ -3382,8 +3376,6 @@
     }
 
     releaseWakeLock();
-    mWakeLockUids.clear();
-    mActiveTracksGeneration++;
 
     ALOGV("Thread %p type %d exiting", this, mType);
     return false;
@@ -3397,8 +3389,6 @@
         for (size_t i=0 ; i<count ; i++) {
             const sp<Track>& track = tracksToRemove.itemAt(i);
             mActiveTracks.remove(track);
-            mWakeLockUids.remove(track->uid());
-            mActiveTracksGeneration++;
             ALOGV("removeTracks_l removing track on session %d", track->sessionId());
             sp<EffectChain> chain = getEffectChain_l(track->sessionId());
             if (chain != 0) {
@@ -4017,10 +4007,7 @@
     mEffectBufferValid = false; // mEffectBuffer has no valid data until tracks found.
 
     for (size_t i=0 ; i<count ; i++) {
-        const sp<Track> t = mActiveTracks[i].promote();
-        if (t == 0) {
-            continue;
-        }
+        const sp<Track> t = mActiveTracks[i];
 
         // this const just means the local variable doesn't change
         Track* const track = t.get();
@@ -4516,11 +4503,7 @@
         size_t i = __builtin_ctz(resetMask);
         ALOG_ASSERT(i < count);
         resetMask &= ~(1 << i);
-        sp<Track> t = mActiveTracks[i].promote();
-        if (t == 0) {
-            continue;
-        }
-        Track* track = t.get();
+        sp<Track> track = mActiveTracks[i];
         ALOG_ASSERT(track->isFastTrack() && track->isStopped());
         track->reset();
     }
@@ -4830,7 +4813,7 @@
 void AudioFlinger::DirectOutputThread::onAddNewTrack_l()
 {
     sp<Track> previousTrack = mPreviousTrack.promote();
-    sp<Track> latestTrack = mLatestActiveTrack.promote();
+    sp<Track> latestTrack = mActiveTracks.getLatest();
 
     if (previousTrack != 0 && latestTrack != 0) {
         if (mType == DIRECT) {
@@ -4856,13 +4839,7 @@
     bool doHwResume = false;
 
     // find out which tracks need to be processed
-    for (size_t i = 0; i < count; i++) {
-        sp<Track> t = mActiveTracks[i].promote();
-        // The track died recently
-        if (t == 0) {
-            continue;
-        }
-
+    for (const sp<Track> &t : mActiveTracks) {
         if (t->isInvalid()) {
             ALOGW("An invalidated track shouldn't be in active list");
             tracksToRemove->add(t);
@@ -4877,7 +4854,7 @@
         // In theory an older track could underrun and restart after the new one starts
         // but as we only care about the transition phase between two tracks on a
         // direct output, it is not a problem to ignore the underrun case.
-        sp<Track> l = mLatestActiveTrack.promote();
+        sp<Track> l = mActiveTracks.getLatest();
         bool last = l.get() == track;
 
         if (track->isPausing()) {
@@ -5411,12 +5388,7 @@
     ALOGV("OffloadThread::prepareTracks_l active tracks %zu", count);
 
     // find out which tracks need to be processed
-    for (size_t i = 0; i < count; i++) {
-        sp<Track> t = mActiveTracks[i].promote();
-        // The track died recently
-        if (t == 0) {
-            continue;
-        }
+    for (const sp<Track> &t : mActiveTracks) {
         Track* const track = t.get();
 #ifdef VERY_VERY_VERBOSE_LOGGING
         audio_track_cblk_t* cblk = track->cblk();
@@ -5425,7 +5397,7 @@
         // In theory an older track could underrun and restart after the new one starts
         // but as we only care about the transition phase between two tracks on a
         // direct output, it is not a problem to ignore the underrun case.
-        sp<Track> l = mLatestActiveTrack.promote();
+        sp<Track> l = mActiveTracks.getLatest();
         bool last = l.get() == track;
 
         if (track->isInvalid()) {
@@ -5884,7 +5856,7 @@
 #endif
                                          ) :
     ThreadBase(audioFlinger, id, outDevice, inDevice, RECORD, systemReady),
-    mInput(input), mActiveTracksGen(0), mRsmpInBuffer(NULL),
+    mInput(input), mRsmpInBuffer(NULL),
     // mRsmpInFrames, mRsmpInFramesP2, and mRsmpInFramesOA are set by readInputParameters_l()
     mRsmpInRear(0)
 #ifdef TEE_SINK
@@ -6042,25 +6014,9 @@
 
 reacquire_wakelock:
     sp<RecordTrack> activeTrack;
-    int activeTracksGen;
     {
         Mutex::Autolock _l(mLock);
-        size_t size = mActiveTracks.size();
-        activeTracksGen = mActiveTracksGen;
-        if (size > 0) {
-            // FIXME an arbitrary choice
-            activeTrack = mActiveTracks[0];
-            acquireWakeLock_l(activeTrack->uid());
-            if (size > 1) {
-                SortedVector<int> tmp;
-                for (size_t i = 0; i < size; i++) {
-                    tmp.add(mActiveTracks[i]->uid());
-                }
-                updateWakeLockUids_l(tmp);
-            }
-        } else {
-            acquireWakeLock_l(-1);
-        }
+        acquireWakeLock_l();
     }
 
     // used to request a deferred sleep, to be executed later while mutex is unlocked
@@ -6112,15 +6068,6 @@
                 goto reacquire_wakelock;
             }
 
-            if (mActiveTracksGen != activeTracksGen) {
-                activeTracksGen = mActiveTracksGen;
-                SortedVector<int> tmp;
-                for (size_t i = 0; i < size; i++) {
-                    tmp.add(mActiveTracks[i]->uid());
-                }
-                updateWakeLockUids_l(tmp);
-            }
-
             bool doBroadcast = false;
             bool allStopped = true;
             for (size_t i = 0; i < size; ) {
@@ -6133,7 +6080,6 @@
                     }
                     removeTrack_l(activeTrack);
                     mActiveTracks.remove(activeTrack);
-                    mActiveTracksGen++;
                     size--;
                     continue;
                 }
@@ -6143,7 +6089,6 @@
 
                 case TrackBase::PAUSING:
                     mActiveTracks.remove(activeTrack);
-                    mActiveTracksGen++;
                     doBroadcast = true;
                     size--;
                     continue;
@@ -6183,6 +6128,8 @@
                 }
             }
 
+            mActiveTracks.updateWakeLockUids(this);
+
             if (allStopped) {
                 standbyIfNotAlreadyInStandby();
             }
@@ -6481,7 +6428,6 @@
             track->invalidate();
         }
         mActiveTracks.clear();
-        mActiveTracksGen++;
         mStartStopCond.broadcast();
     }
 
@@ -6735,7 +6681,6 @@
         //      or using a separate command thread
         recordTrack->mState = TrackBase::STARTING_1;
         mActiveTracks.add(recordTrack);
-        mActiveTracksGen++;
         status_t status = NO_ERROR;
         if (recordTrack->isExternalTrack()) {
             mLock.unlock();
@@ -6744,7 +6689,6 @@
             // FIXME should verify that recordTrack is still in mActiveTracks
             if (status != NO_ERROR) {
                 mActiveTracks.remove(recordTrack);
-                mActiveTracksGen++;
                 recordTrack->clearSyncStartEvent();
                 ALOGV("RecordThread::start error %d", status);
                 return status;
diff --git a/services/audioflinger/Threads.h b/services/audioflinger/Threads.h
index da0c705..1fd2adc 100644
--- a/services/audioflinger/Threads.h
+++ b/services/audioflinger/Threads.h
@@ -400,11 +400,10 @@
                     effect_uuid_t mType;    // effect type UUID
                 };
 
-                void        acquireWakeLock(int uid = -1);
-                virtual void acquireWakeLock_l(int uid = -1);
+                void        acquireWakeLock();
+                virtual void acquireWakeLock_l();
                 void        releaseWakeLock();
                 void        releaseWakeLock_l();
-                void        updateWakeLockUids(const SortedVector<int> &uids);
                 void        updateWakeLockUids_l(const SortedVector<int> &uids);
                 void        getPowerManager_l();
                 void setEffectSuspended_l(const effect_uuid_t *type,
@@ -479,8 +478,88 @@
                 static const size_t     kLogSize = 4 * 1024;
                 sp<NBLog::Writer>       mNBLogWriter;
                 bool                    mSystemReady;
-                bool                    mNotifiedBatteryStart;
                 ExtendedTimestamp       mTimestamp;
+
+                // ActiveTracks is a sorted vector of track type T representing the
+                // active tracks of threadLoop() to be considered by the locked prepare portion.
+                // ActiveTracks should be accessed with the ThreadBase lock held.
+                //
+                // During processing and I/O, the threadLoop does not hold the lock;
+                // hence it does not directly use ActiveTracks.  Care should be taken
+                // to hold local strong references or defer removal of tracks
+                // if the threadLoop may still be accessing those tracks due to mix, etc.
+                //
+                // This class updates power information appropriately.
+                //
+
+                template <typename T>
+                class ActiveTracks {
+                public:
+                    ActiveTracks()
+                        : mActiveTracksGeneration(0)
+                        , mLastActiveTracksGeneration(0)
+                    { }
+
+                    ~ActiveTracks() {
+                        clear();
+                    }
+                    // returns the last track added (even though it may have been
+                    // subsequently removed from ActiveTracks).
+                    //
+                    // Used for DirectOutputThread to ensure a flush is called when transitioning
+                    // to a new track (even though it may be on the same session).
+                    // Used for OffloadThread to ensure that volume and mixer state is
+                    // taken from the latest track added.
+                    //
+                    // The latest track is saved with a weak pointer to prevent keeping an
+                    // otherwise useless track alive. Thus the function will return nullptr
+                    // if the latest track has subsequently been removed and destroyed.
+                    sp<T> getLatest() {
+                        return mLatestActiveTrack.promote();
+                    }
+
+                    // Updates ActiveTracks client uids to the thread wakelock.
+                    void updateWakeLockUids(sp<ThreadBase> thread, bool force = false) {
+                        if (mActiveTracksGeneration != mLastActiveTracksGeneration || force) {
+                            thread->updateWakeLockUids_l(getWakeLockUids());
+                            mLastActiveTracksGeneration = mActiveTracksGeneration;
+                        }
+                    }
+
+                    // SortedVector methods
+                    ssize_t         add(const sp<T> &track);
+                    ssize_t         remove(const sp<T> &track);
+                    size_t          size() const {
+                        return mActiveTracks.size();
+                    }
+                    ssize_t         indexOf(const sp<T>& item) {
+                        return mActiveTracks.indexOf(item);
+                    }
+                    sp<T>           operator[](size_t index) const {
+                        return mActiveTracks[index];
+                    }
+                    typename SortedVector<sp<T>>::iterator begin() {
+                        return mActiveTracks.begin();
+                    }
+                    typename SortedVector<sp<T>>::iterator end() {
+                        return mActiveTracks.end();
+                    }
+                    void            clear();
+
+                private:
+                    SortedVector<int> getWakeLockUids() {
+                        SortedVector<int> wakeLockUids;
+                        for (const sp<T> &track : mActiveTracks) {
+                            wakeLockUids.add(track->uid());
+                        }
+                        return wakeLockUids; // moved by underlying SharedBuffer
+                    }
+
+                    SortedVector<sp<T>> mActiveTracks;
+                    int                 mActiveTracksGeneration;
+                    int                 mLastActiveTracksGeneration;
+                    wp<T>               mLatestActiveTrack; // latest track added to ActiveTracks
+                };
 };
 
 // --- PlaybackThread ---
@@ -559,6 +638,10 @@
     virtual     void        preExit();
 
     virtual     bool        keepWakeLock() const { return true; }
+    virtual     void        acquireWakeLock_l() {
+                                ThreadBase::acquireWakeLock_l();
+                                mActiveTracks.updateWakeLockUids(this, true /* force */);
+                            }
 
 public:
 
@@ -731,10 +814,7 @@
     bool                            mMasterMute;
                 void        setMasterMute_l(bool muted) { mMasterMute = muted; }
 protected:
-    SortedVector< wp<Track> >       mActiveTracks;  // FIXME check if this could be sp<>
-    SortedVector<int>               mWakeLockUids;
-    int                             mActiveTracksGeneration;
-    wp<Track>                       mLatestActiveTrack; // latest track added to mActiveTracks
+    ActiveTracks<Track>     mActiveTracks;
 
     // Allocate a track name for a given channel mask.
     //   Returns name >= 0 if successful, -1 on failure.
@@ -900,8 +980,8 @@
     virtual     uint32_t    suspendSleepTimeUs() const;
     virtual     void        cacheParameters_l();
 
-    virtual void acquireWakeLock_l(int uid = -1) {
-        PlaybackThread::acquireWakeLock_l(uid);
+    virtual void acquireWakeLock_l() {
+        PlaybackThread::acquireWakeLock_l();
         if (hasFastMixer()) {
             mFastMixer->setBoottimeOffset(
                     mTimestamp.mTimebaseOffset[ExtendedTimestamp::TIMEBASE_BOOTTIME]);
@@ -1340,6 +1420,11 @@
     virtual status_t    checkEffectCompatibility_l(const effect_descriptor_t *desc,
                                                    audio_session_t sessionId);
 
+    virtual void        acquireWakeLock_l() {
+                            ThreadBase::acquireWakeLock_l();
+                            mActiveTracks.updateWakeLockUids(this, true /* force */);
+                        }
+
 private:
             // Enter standby if not already in standby, and set mStandby flag
             void    standbyIfNotAlreadyInStandby();
@@ -1351,9 +1436,8 @@
             SortedVector < sp<RecordTrack> >    mTracks;
             // mActiveTracks has dual roles:  it indicates the current active track(s), and
             // is used together with mStartStopCond to indicate start()/stop() progress
-            SortedVector< sp<RecordTrack> >     mActiveTracks;
-            // generation counter for mActiveTracks
-            int                                 mActiveTracksGen;
+            ActiveTracks<RecordTrack>           mActiveTracks;
+
             Condition                           mStartStopCond;
 
             // resampler converts input at HAL Hz to output at AudioRecord client Hz