在 Android 系统中我们常用 wake lock 锁来保持设备唤醒，使设备不进入休眠状态，本文从源码角度分析一下 wake_lock 的基本流程原理
1.WakeLock 使用
WakeLock 有三种表现形式:
PowerManger.WakeLock：PMS 暴露给应用层和其他组件用来申请 WakeLock 的接口；
PowerManagerService.WakeLock: PowerManager.WakeLock 在 PMS 中的表现形式；
SuspendBlocker: PowerManagerService.WakeLock 在向底层节点操作时的表现形式。
下面是一个申请 WakeLock 锁的示例:
// 获取PowerManager对象
PowerManager pm = (PowerManager) getSystemService(Context.POWER_SERVICE);
// 创建WakeLock锁实例
PowerManager.WakeLock wl = pm.newWakeLock(PowerManager.SCREEN_DIM_WAKE_LOCK, "My Tag");
// 申请WakeLock
wl.acquire();
// 释放WakeLock
wl.release();
2.结构图
3.acquire
/**
* Acquires the wake lock.
* <p>
* Ensures that the device is on at the level requested when
* the wake lock was created.
* </p>
*/
public void acquire() {
synchronized (mToken) {
acquireLocked();
}
}
/**
* Acquires the wake lock with a timeout.
* <p>
* Ensures that the device is on at the level requested when
* the wake lock was created. The lock will be released after the given timeout
* expires.
* </p>
*
* @param timeout The timeout after which to release the wake lock, in milliseconds.
*/
public void acquire(long timeout) {
synchronized (mToken) {
acquireLocked();
//如果是超时锁，延时释放
mHandler.postDelayed(mReleaser, timeout);
}
}
private void acquireLocked() {
mInternalCount++;
mExternalCount++;
if (!mRefCounted || mInternalCount == 1) {
mHandler.removeCallbacks(mReleaser);
Trace.asyncTraceBegin(Trace.TRACE_TAG_POWER, mTraceName, 0);
try {
mService.acquireWakeLock(mToken, mFlags, mTag, mPackageName, mWorkSource,
mHistoryTag);
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
mHeld = true;
}
}
newWakeLoc WakeLock 对象的创建
// frameworks/base/core/java/android/os/PowerManager.java
public WakeLock newWakeLock(int levelAndFlags, String tag) {
validateWakeLockParameters(levelAndFlags, tag); // 校验Flag
// 创建WakeLock对象
return new WakeLock(levelAndFlags, tag, mContext.getOpPackageName());
}
4.PowerManagerService
//// frameworks/base/services/core/java/com/android/server/power/PowerManagerService.java
@Override // Binder call
public void acquireWakeLock(IBinder lock, int flags, String tag, String packageName,
WorkSource ws, String historyTag) {
if (lock == null) {
throw new IllegalArgumentException("lock must not be null");
}
if (packageName == null) {
throw new IllegalArgumentException("packageName must not be null");
}
PowerManager.validateWakeLockParameters(flags, tag);
mContext.enforceCallingOrSelfPermission(android.Manifest.permission.WAKE_LOCK, null);
if ((flags & PowerManager.DOZE_WAKE_LOCK) != 0) {
mContext.enforceCallingOrSelfPermission(
android.Manifest.permission.DEVICE_POWER, null);
}
if (ws != null && !ws.isEmpty()) {
mContext.enforceCallingOrSelfPermission(
android.Manifest.permission.UPDATE_DEVICE_STATS, null);
} else {
ws = null;
}
final int uid = Binder.getCallingUid();
final int pid = Binder.getCallingPid();
final long ident = Binder.clearCallingIdentity();
try {
acquireWakeLockInternal(lock, flags, tag, packageName, ws, historyTag, uid, pid);
} finally {
Binder.restoreCallingIdentity(ident);
}
}
acquireWakeLockInternal
// frameworks/base/services/core/java/com/android/server/power/PowerManagerService.java
private void acquireWakeLockInternal(IBinder lock, int flags, String tag, String packageName,
WorkSource ws, String historyTag, int uid, int pid) {
synchronized (mLock) {
if (DEBUG_SPEW) {
Slog.d(TAG, "acquireWakeLockInternal: lock=" + Objects.hashCode(lock)
+ ", flags=0x" + Integer.toHexString(flags)
+ ", tag=\"" + tag + "\", ws=" + ws + ", uid=" + uid + ", pid=" + pid);
}
WakeLock wakeLock;
//如果之前申请过，从列表里拿出来直接用
int index = findWakeLockIndexLocked(lock);
boolean notifyAcquire;
if (index >= 0) {
wakeLock = mWakeLocks.get(index);
if (!wakeLock.hasSameProperties(flags, tag, ws, uid, pid)) {
/** Update existing wake lock. This shouldn't happen but is harmless.**/
notifyWakeLockChangingLocked(wakeLock, flags, tag, packageName,
uid, pid, ws, historyTag);
wakeLock.updateProperties(flags, tag, packageName, ws, historyTag, uid, pid);
}
notifyAcquire = false;
} else {
UidState state = mUidState.get(uid);
if (state == null) {
state = new UidState(uid);
state.mProcState = ActivityManager.PROCESS_STATE_NONEXISTENT;
mUidState.put(uid, state);
}
state.mNumWakeLocks++;
//新创建一个lock放到列表
wakeLock = new WakeLock(lock, flags, tag, packageName, ws, historyTag, uid, pid,
state);
try {
lock.linkToDeath(wakeLock, 0);
} catch (RemoteException ex) {
throw new IllegalArgumentException("Wake lock is already dead.");
}
mWakeLocks.add(wakeLock);
setWakeLockDisabledStateLocked(wakeLock);
notifyAcquire = true;
}
applyWakeLockFlagsOnAcquireLocked(wakeLock, uid);
mDirty |= DIRTY_WAKE_LOCKS;
updatePowerStateLocked();
if (notifyAcquire) {
// This needs to be done last so we are sure we have acquired the
// kernel wake lock. Otherwise we have a race where the system may
// go to sleep between the time we start the accounting in battery
// stats and when we actually get around to telling the kernel to
// stay awake.
notifyWakeLockAcquiredLocked(wakeLock);
}
}
}
setWakeLockDisabledStateLocked
这里特别注意下，在某些情况下 wake-lock 锁可能被 disable 掉。
private boolean setWakeLockDisabledStateLocked(WakeLock wakeLock) {
if ((wakeLock.mFlags & PowerManager.WAKE_LOCK_LEVEL_MASK)
== PowerManager.PARTIAL_WAKE_LOCK) {
boolean disabled = false;
final int appid = UserHandle.getAppId(wakeLock.mOwnerUid);
if (appid >= Process.FIRST_APPLICATION_UID) {
// Cached inactive processes are never allowed to hold wake locks.
if (mConstants.NO_CACHED_WAKE_LOCKS) {
// 强制进入suspend、对应uid进程没有处于active且进程adj大于PROCESS_STATE_RECEIVER
disabled = mForceSuspendActive
|| (!wakeLock.mUidState.mActive && wakeLock.mUidState.mProcState
!= ActivityManager.PROCESS_STATE_NONEXISTENT &&
wakeLock.mUidState.mProcState > ActivityManager.PROCESS_STATE_RECEIVER);
}
if (mDeviceIdleMode) {
// If we are in idle mode, we will also ignore all partial wake locks that are
// for application uids that are not whitelisted.
final UidState state = wakeLock.mUidState;
if (Arrays.binarySearch(mDeviceIdleWhitelist, appid) < 0 &&
Arrays.binarySearch(mDeviceIdleTempWhitelist, appid) < 0 &&
state.mProcState != ActivityManager.PROCESS_STATE_NONEXISTENT &&
state.mProcState >
ActivityManager.PROCESS_STATE_BOUND_FOREGROUND_SERVICE) {
//处于idle状态时，将非白名单应用wakeLock 禁用
disabled = true;
}
}
}
if (wakeLock.mDisabled != disabled) {
wakeLock.mDisabled = disabled;
return true;
}
}
return false;
}
updatePowerStateLocked
/**
* Updates the global power state based on dirty bits recorded in mDirty.
*
* This is the main function that performs power state transitions.
* We centralize them here so that we can recompute the power state completely
* each time something important changes, and ensure that we do it the same
* way each time. The point is to gather all of the transition logic here.
*/
private void updatePowerStateLocked() {
if (!mSystemReady || mDirty == 0) {
return;
}
if (!Thread.holdsLock(mLock)) {
Slog.wtf(TAG, "Power manager lock was not held when calling updatePowerStateLocked");
}
Trace.traceBegin(Trace.TRACE_TAG_POWER, "updatePowerState");
try {
// Phase 0: Basic state updates.
updateIsPoweredLocked(mDirty);
updateStayOnLocked(mDirty);
updateScreenBrightnessBoostLocked(mDirty);
// Phase 1: Update wakefulness.
// Loop because the wake lock and user activity computations are influenced
// by changes in wakefulness.
final long now = SystemClock.uptimeMillis();
int dirtyPhase2 = 0;
for (;;) {
int dirtyPhase1 = mDirty;
dirtyPhase2 |= dirtyPhase1;
mDirty = 0;
updateWakeLockSummaryLocked(dirtyPhase1);
updateUserActivitySummaryLocked(now, dirtyPhase1);
if (!updateWakefulnessLocked(dirtyPhase1)) {
break;
}
}
// Phase 2: Lock profiles that became inactive/not kept awake.
updateProfilesLocked(now);
// Phase 3: Update display power state.
final boolean displayBecameReady = updateDisplayPowerStateLocked(dirtyPhase2);
// Phase 4: Update dream state (depends on display ready signal).
updateDreamLocked(dirtyPhase2, displayBecameReady);
// Phase 5: Send notifications, if needed.
finishWakefulnessChangeIfNeededLocked();
// Phase 6: Update suspend blocker.
// Because we might release the last suspend blocker here, we need to make sure
// we finished everything else first!
updateSuspendBlockerLocked();
} finally {
Trace.traceEnd(Trace.TRACE_TAG_POWER);
}
}
主要有三种情况下会禁用 Partical WakeLock：
强制进入 suspend；
WakeLock 所属进程不处于 active 状态，且进程 adj 大于 PROCESS_STATE_RECEIVER；
DeviceIdle 处于 IDLE 状态，且所属进程不在 doze 白名单中；
updateSuspendBlockerLocked
// frameworks/base/services/core/java/com/android/server/power/PowerManagerService.java
private void updateSuspendBlockerLocked() {
// 是否因持有WakeLock锁而需要CPU保持唤醒
final boolean needWakeLockSuspendBlocker = ((mWakeLockSummary & WAKE_LOCK_CPU) != 0);
// 是否因Display状态而需要CPU保持唤醒
final boolean needDisplaySuspendBlocker = needDisplaySuspendBlockerLocked();
// 是否开启auto_suspend模式
final boolean autoSuspend = !needDisplaySuspendBlocker;
// 是否处于交互状态
final boolean interactive = mDisplayPowerRequest.isBrightOrDim();
// 如果持有Display SuspendBlocker，则关闭auto-suspend模式
if (!autoSuspend && mDecoupleHalAutoSuspendModeFromDisplayConfig) {
setHalAutoSuspendModeLocked(false);
}
// 申请mWakeLockSuspendBlocker锁
if (needWakeLockSuspendBlocker && !mHoldingWakeLockSuspendBlocker) {
mWakeLockSuspendBlocker.acquire();
mHoldingWakeLockSuspendBlocker = true;
}
// 申请mDisplaySuspendBlocker锁
if (needDisplaySuspendBlocker && !mHoldingDisplaySuspendBlocker) {
mDisplaySuspendBlocker.acquire();
mHoldingDisplaySuspendBlocker = true;
}
// 设置交互状态
if (mDecoupleHalInteractiveModeFromDisplayConfig) {
if (interactive || mDisplayReady) {
setHalInteractiveModeLocked(interactive);
}
}
// 释放mWakeLockSuspendBlocker
if (!needWakeLockSuspendBlocker && mHoldingWakeLockSuspendBlocker) {
mWakeLockSuspendBlocker.release();
mHoldingWakeLockSuspendBlocker = false;
}
// 释放mDisplaySuspendBlocker
if (!needDisplaySuspendBlocker && mHoldingDisplaySuspendBlocker) {
mDisplaySuspendBlocker.release();
mHoldingDisplaySuspendBlocker = false;
}
// 开启auto_suspend模式
if (autoSuspend && mDecoupleHalAutoSuspendModeFromDisplayConfig) {
setHalAutoSuspendModeLocked(true);
}
}
mWakeLockSuspendBlocker 会调用 JNI 的方法 nativeAcquireSuspendBlocker
nativeAcquireSuspendBlocker
// frameworks/base/services/core/jni/com_android_server_power_PowerManagerService.cpp
static void nativeAcquireSuspendBlocker(JNIEnv *env, jclass /* clazz */, jstring nameStr) {
ScopedUtfChars name(env, nameStr);
acquire_wake_lock(PARTIAL_WAKE_LOCK, name.c_str());
}
5.power.c
acquire_wake_lock 的实现在 power.c
acquire_wake_lock
///hardware/libhardware_legacy/power.c
int acquire_wake_lock(int lock, const char* id)
{
initialize_fds();
// ALOGI("acquire_wake_lock lock=%d id='%s'\n", lock, id);
if (g_error) return g_error;
int fd;
ssize_t ret;
if (lock != PARTIAL_WAKE_LOCK) {
return -EINVAL;
}
fd = g_fds[ACQUIRE_PARTIAL_WAKE_LOCK];
ret = write(fd, id, strlen(id));
if (ret < 0) {
return -errno;
}
return ret;
}
const char * const OLD_PATHS[] = {
"/sys/android_power/acquire_partial_wake_lock",
"/sys/android_power/release_wake_lock",
};
const char * const NEW_PATHS[] = {
"/sys/power/wake_lock",
"/sys/power/wake_unlock",
};
最终是往指定的 path 的文件节点下写入一份 string 数据，新版本是“/sys/power/wake_lock”旧版本“/sys/android_power/acquire_partial_wake_lock”。
至此 wake_lock 锁的大概流程讲述完，release 过程与 acquire 类似。最后是一个时序图。
参考文章：https://juejin.cn/post/6947092185930350622
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