从framework角度看app保活问题

问题背景

最近在群里看到群友在讨论app保活的问题，回想之前做应用(运动类)开发时也遇到过类似的需求，于是便又来了兴趣，果断加入其中，和群友展开了激烈的讨论

不少群友的想法和我当初的想法一样，这特么保活不是看系统的心情么，系统想让谁活谁才能活，作为app开发者，根本无能为力，可真的是这样的吗？

保活方案

首先，我整理了从古到今，app开发者所使用过的以及当前还在使用的保活方式，主要思路有两个：保活和复活

保活的方案有：

• 1像素惨案

• 后台无声音乐

• 前台service

• 心跳机制

• socket长连接

• 无障碍服务

• ......

复活的方案有：

• 双进程守护(java层和native层)
• JobScheduler定时任务
• 推送/相互唤醒
• ......

不难看出，app开发者为了能让自己的应用多存活一会儿，可谓是绞尽脑汁，但即使这样，随着Android系统升级，尤其是进入8.0之后，系统对应用的限制越来越高，传统的保活方式已经不生效，这让Android开发者手足无措，于是乎，出现了一种比较和谐的保活方式：

• 引导用户开启手机白名单

这也是目前绝大多数应用所采用的的方式，相对于传统黑科技而言，此方式显得不那么流氓，比较容易被用户所接受。

但跟微信这样的国民级应用比起来，保活效果还是差了一大截，那么微信是怎么实现保活的呢？或者回到我们开头的问题，应用的生死真的只能靠系统调度吗？开发者能否干预控制呢？

进程调度原则

解开这个疑问之前，我们需要了解一下Android系统进程调度原则，主要介绍framework中承载四大组件的进程是如何根据组件状态而动态调节自身状态的。进程有两个比较重要的状态值：

• oom_adj，定义在frameworks/base/services/core/java/com/android/server/am/ProcessList.java当中

• procState，定义在frameworks/base/core/java/android/app/ActivityManager.java当中

OOM_ADJ

以Android10的源码为例，oom_adj划分为20级，取值范围[-10000,1001]，Android6.0以前的取值范围是[-17,16]

• oom_adj值越大，优先级越低

• oom_adj<0的进程都是系统进程。

public final class ProcessList {
    static final String TAG = TAG_WITH_CLASS_NAME ? "ProcessList" : TAG_AM;

    // The minimum time we allow between crashes, for us to consider this
    // application to be bad and stop and its services and reject broadcasts.
    static final int MIN_CRASH_INTERVAL = 60 * 1000;

    // OOM adjustments for processes in various states:

    // Uninitialized value for any major or minor adj fields
    static final int INVALID_ADJ = -10000;

    // Adjustment used in certain places where we don't know it yet.
    // (Generally this is something that is going to be cached, but we
    // don't know the exact value in the cached range to assign yet.)
    static final int UNKNOWN_ADJ = 1001;

    // This is a process only hosting activities that are not visible,
    // so it can be killed without any disruption.
    static final int CACHED_APP_MAX_ADJ = 999;
    static final int CACHED_APP_MIN_ADJ = 900;

    // This is the oom_adj level that we allow to die first. This cannot be equal to
    // CACHED_APP_MAX_ADJ unless processes are actively being assigned an oom_score_adj of
    // CACHED_APP_MAX_ADJ.
    static final int CACHED_APP_LMK_FIRST_ADJ = 950;

    // Number of levels we have available for different service connection group importance
    // levels.
    static final int CACHED_APP_IMPORTANCE_LEVELS = 5;

    // The B list of SERVICE_ADJ -- these are the old and decrepit
    // services that aren't as shiny and interesting as the ones in the A list.
    static final int SERVICE_B_ADJ = 800;

    // This is the process of the previous application that the user was in.
    // This process is kept above other things, because it is very common to
    // switch back to the previous app.  This is important both for recent
    // task switch (toggling between the two top recent apps) as well as normal
    // UI flow such as clicking on a URI in the e-mail app to view in the browser,
    // and then pressing back to return to e-mail.
    static final int PREVIOUS_APP_ADJ = 700;

    // This is a process holding the home application -- we want to try
    // avoiding killing it, even if it would normally be in the background,
    // because the user interacts with it so much.
    static final int HOME_APP_ADJ = 600;

    // This is a process holding an application service -- killing it will not
    // have much of an impact as far as the user is concerned.
    static final int SERVICE_ADJ = 500;

    // This is a process with a heavy-weight application.  It is in the
    // background, but we want to try to avoid killing it.  Value set in
    // system/rootdir/init.rc on startup.
    static final int HEAVY_WEIGHT_APP_ADJ = 400;

    // This is a process currently hosting a backup operation.  Killing it
    // is not entirely fatal but is generally a bad idea.
    static final int BACKUP_APP_ADJ = 300;

    // This is a process bound by the system (or other app) that's more important than services but
    // not so perceptible that it affects the user immediately if killed.
    static final int PERCEPTIBLE_LOW_APP_ADJ = 250;

    // This is a process only hosting components that are perceptible to the
    // user, and we really want to avoid killing them, but they are not
    // immediately visible. An example is background music playback.
    static final int PERCEPTIBLE_APP_ADJ = 200;

    // This is a process only hosting activities that are visible to the
    // user, so we'd prefer they don't disappear.
    static final int VISIBLE_APP_ADJ = 100;
    static final int VISIBLE_APP_LAYER_MAX = PERCEPTIBLE_APP_ADJ - VISIBLE_APP_ADJ - 1;

    // This is a process that was recently TOP and moved to FGS. Continue to treat it almost
    // like a foreground app for a while.
    // @see TOP_TO_FGS_GRACE_PERIOD
    static final int PERCEPTIBLE_RECENT_FOREGROUND_APP_ADJ = 50;

    // This is the process running the current foreground app.  We'd really
    // rather not kill it!
    static final int FOREGROUND_APP_ADJ = 0;

    // This is a process that the system or a persistent process has bound to,
    // and indicated it is important.
    static final int PERSISTENT_SERVICE_ADJ = -700;

    // This is a system persistent process, such as telephony.  Definitely
    // don't want to kill it, but doing so is not completely fatal.
    static final int PERSISTENT_PROC_ADJ = -800;

    // The system process runs at the default adjustment.
    static final int SYSTEM_ADJ = -900;

    // Special code for native processes that are not being managed by the system (so
    // don't have an oom adj assigned by the system).
    static final int NATIVE_ADJ = -1000;

    // Memory pages are 4K.
    static final int PAGE_SIZE = 4 * 1024;
    
    //省略部分代码
}

ADJ级别	取值	说明（可参考源码注释）
INVALID_ADJ	-10000	未初始化adj字段时的默认值
UNKNOWN_ADJ	1001	缓存进程，无法获取具体值
CACHED_APP_MAX_ADJ	999	不可见activity进程的最大值
CACHED_APP_MIN_ADJ	900	不可见activity进程的最小值
CACHED_APP_LMK_FIRST_ADJ	950	lowmemorykiller优先杀死的级别值
SERVICE_B_ADJ	800	旧的service的
PREVIOUS_APP_ADJ	700	上一个应用，常见于应用切换场景
HOME_APP_ADJ	600	home进程
SERVICE_ADJ	500	创建了service的进程
HEAVY_WEIGHT_APP_ADJ	400	后台的重量级进程，system/rootdir/init.rc文件中设置
BACKUP_APP_ADJ	300	备份进程
PERCEPTIBLE_LOW_APP_ADJ	250	受其他进程约束的进程
PERCEPTIBLE_APP_ADJ	200	可感知组件的进程，比如背景音乐播放
VISIBLE_APP_ADJ	100	可见进程
PERCEPTIBLE_RECENT_FOREGROUND_APP_ADJ	50	最近运行的后台进程
FOREGROUND_APP_ADJ	0	前台进程，正在与用户交互
PERSISTENT_SERVICE_ADJ	-700	系统持久化进程已绑定的进程
PERSISTENT_PROC_ADJ	-800	系统持久化进程，比如telephony
SYSTEM_ADJ	-900	系统进程
NATIVE_ADJ	-1000	native进程，不受系统管理

可以通过cat /proc/进程id/oom_score_adj查看目标进程的oom_adj值，例如我们查看电话的adj

值为935，处于不可见进程的范围内，当我启动电话app，再次查看

此时adj值为0，也就是正在与用户交互的进程

ProcessState

process_state划分为23类，取值范围为[-1,21]

@SystemService(Context.ACTIVITY_SERVICE)
public class ActivityManager {
    //省略部分代码
    /** @hide Not a real process state. */
    public static final int PROCESS_STATE_UNKNOWN = -1;

    /** @hide Process is a persistent system process. */
    public static final int PROCESS_STATE_PERSISTENT = 0;

    /** @hide Process is a persistent system process and is doing UI. */
    public static final int PROCESS_STATE_PERSISTENT_UI = 1;

    /** @hide Process is hosting the current top activities.  Note that this covers
     * all activities that are visible to the user. */
    @UnsupportedAppUsage
    public static final int PROCESS_STATE_TOP = 2;

    /** @hide Process is hosting a foreground service with location type. */
    public static final int PROCESS_STATE_FOREGROUND_SERVICE_LOCATION = 3;

    /** @hide Process is bound to a TOP app. This is ranked below SERVICE_LOCATION so that
     * it doesn't get the capability of location access while-in-use. */
    public static final int PROCESS_STATE_BOUND_TOP = 4;

    /** @hide Process is hosting a foreground service. */
    @UnsupportedAppUsage
    public static final int PROCESS_STATE_FOREGROUND_SERVICE = 5;

    /** @hide Process is hosting a foreground service due to a system binding. */
    @UnsupportedAppUsage
    public static final int PROCESS_STATE_BOUND_FOREGROUND_SERVICE = 6;

    /** @hide Process is important to the user, and something they are aware of. */
    public static final int PROCESS_STATE_IMPORTANT_FOREGROUND = 7;

    /** @hide Process is important to the user, but not something they are aware of. */
    @UnsupportedAppUsage
    public static final int PROCESS_STATE_IMPORTANT_BACKGROUND = 8;

    /** @hide Process is in the background transient so we will try to keep running. */
    public static final int PROCESS_STATE_TRANSIENT_BACKGROUND = 9;

    /** @hide Process is in the background running a backup/restore operation. */
    public static final int PROCESS_STATE_BACKUP = 10;

    /** @hide Process is in the background running a service.  Unlike oom_adj, this level
     * is used for both the normal running in background state and the executing
     * operations state. */
    @UnsupportedAppUsage
    public static final int PROCESS_STATE_SERVICE = 11;

    /** @hide Process is in the background running a receiver.   Note that from the
     * perspective of oom_adj, receivers run at a higher foreground level, but for our
     * prioritization here that is not necessary and putting them below services means
     * many fewer changes in some process states as they receive broadcasts. */
    @UnsupportedAppUsage
    public static final int PROCESS_STATE_RECEIVER = 12;

    /** @hide Same as {@link #PROCESS_STATE_TOP} but while device is sleeping. */
    public static final int PROCESS_STATE_TOP_SLEEPING = 13;

    /** @hide Process is in the background, but it can't restore its state so we want
     * to try to avoid killing it. */
    public static final int PROCESS_STATE_HEAVY_WEIGHT = 14;

    /** @hide Process is in the background but hosts the home activity. */
    @UnsupportedAppUsage
    public static final int PROCESS_STATE_HOME = 15;

    /** @hide Process is in the background but hosts the last shown activity. */
    public static final int PROCESS_STATE_LAST_ACTIVITY = 16;

    /** @hide Process is being cached for later use and contains activities. */
    @UnsupportedAppUsage
    public static final int PROCESS_STATE_CACHED_ACTIVITY = 17;

    /** @hide Process is being cached for later use and is a client of another cached
     * process that contains activities. */
    public static final int PROCESS_STATE_CACHED_ACTIVITY_CLIENT = 18;

    /** @hide Process is being cached for later use and has an activity that corresponds
     * to an existing recent task. */
    public static final int PROCESS_STATE_CACHED_RECENT = 19;

    /** @hide Process is being cached for later use and is empty. */
    public static final int PROCESS_STATE_CACHED_EMPTY = 20;

    /** @hide Process does not exist. */
    public static final int PROCESS_STATE_NONEXISTENT = 21;
    //省略部分代码
}

state级别	取值	说明（可参考源码注释）
PROCESS_STATE_UNKNOWN	-1	不是真正的进程状态
PROCESS_STATE_PERSISTENT	0	持久化的系统进程
PROCESS_STATE_PERSISTENT_UI	1	持久化的系统进程，并且正在操作UI
PROCESS_STATE_TOP	2	处于栈顶Activity的进程
PROCESS_STATE_FOREGROUND_SERVICE_LOCATION	3	运行前台位置服务的进程
PROCESS_STATE_BOUND_TOP	4	绑定到top应用的进程
PROCESS_STATE_FOREGROUND_SERVICE	5	运行前台服务的进程
PROCESS_STATE_BOUND_FOREGROUND_SERVICE	6	绑定前台服务的进程
PROCESS_STATE_IMPORTANT_FOREGROUND	7	对用户很重要的前台进程
PROCESS_STATE_IMPORTANT_BACKGROUND	8	对用户很重要的后台进程
PROCESS_STATE_TRANSIENT_BACKGROUND	9	临时处于后台运行的进程
PROCESS_STATE_BACKUP	10	备份进程
PROCESS_STATE_SERVICE	11	运行后台服务的进程
PROCESS_STATE_RECEIVER	12	运动广播的后台进程
PROCESS_STATE_TOP_SLEEPING	13	处于休眠状态的进程
PROCESS_STATE_HEAVY_WEIGHT	14	后台进程，但不能恢复自身状态
PROCESS_STATE_HOME	15	后台进程，在运行home activity
PROCESS_STATE_LAST_ACTIVITY	16	后台进程，在运行最后一次显示的activity
PROCESS_STATE_CACHED_ACTIVITY	17	缓存进程，包含activity
PROCESS_STATE_CACHED_ACTIVITY_CLIENT	18	缓存进程，且该进程是另一个包含activity进程的客户端
PROCESS_STATE_CACHED_RECENT	19	缓存进程，且有一个activity是最近任务里的activity
PROCESS_STATE_CACHED_EMPTY	20	空的缓存进程，备用
PROCESS_STATE_NONEXISTENT	21	不存在的进程

进程调度算法

在frameworks/base/services/core/java/com/android/server/am/OomAdjuster.java中，有三个核心方法用于计算和更新进程的oom_adj值

• updateOomAdjLocked()：更新adj，当目标进程为空，或者被杀则返回false，否则返回true。
• computeOomAdjLocked()：计算adj，计算成功返回true，否则返回false。
• applyOomAdjLocked()：应用adj，当需要杀掉目标进程则返回false，否则返回true。

adj更新时机

也就是updateOomAdjLocked()被调用的时机。通俗的说，只要四大组件被创建或者状态发生变化，或者当前进程绑定了其他进程，都会触发adj更新，具体可在源码中查看此方法被调用的地方，比较多，这里就不列举了

adj的计算过程

computeOomAdjLocked()计算过程相当复杂，将近1000行代码，这里就不贴了，有兴趣可自行查看，总体思路就是根据当前进程的状态，设置对应的adj值，因为状态值很多，所以会有很多个if来判断每个状态是否符合，最终计算出当前进程属于哪种状态。

adj的应用

计算得出的adj值将发送给lowmemorykiller(简称lmk)，由lmk来决定进程的生死，不同的厂商，lmk的算法略有不同，下面是源码中对lmk的介绍

/* drivers/misc/lowmemorykiller.c
 *
 * The lowmemorykiller driver lets user-space specify a set of memory thresholds
 * where processes with a range of oom_score_adj values will get killed. Specify
 * the minimum oom_score_adj values in
 * /sys/module/lowmemorykiller/parameters/adj and the number of free pages in
 * /sys/module/lowmemorykiller/parameters/minfree. Both files take a comma
 * separated list of numbers in ascending order.
 *
 * For example, write "0,8" to /sys/module/lowmemorykiller/parameters/adj and
 * "1024,4096" to /sys/module/lowmemorykiller/parameters/minfree to kill
 * processes with a oom_score_adj value of 8 or higher when the free memory
 * drops below 4096 pages and kill processes with a oom_score_adj value of 0 or
 * higher when the free memory drops below 1024 pages.
 *
 * The driver considers memory used for caches to be free, but if a large
 * percentage of the cached memory is locked this can be very inaccurate
 * and processes may not get killed until the normal oom killer is triggered.
 *
 * Copyright (C) 2007-2008 Google, Inc.
 *
 * This software is licensed under the terms of the GNU General Public
 * License version 2, as published by the Free Software Foundation, and
 * may be copied, distributed, and modified under those terms.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 */

保活核心思路

根据上面的Android进程调度原则得知，我们需要尽可能降低app进程的adj值，从而减少被lmk杀掉的可能性，而我们传统的保活方式最终目的也是降低adj值。而根据adj等级分类可以看出，通过应用层的方式最多能将adj降到100~200之间，我分别测试了微信、支付宝、酷狗音乐，启动后返回桌面并息屏，测试结果如下

微信测试结果：

微信创建了两个进程，查看这两个进程的adj值均为100，对应为adj等级表中的VISIBLE_APP_ADJ，此结果为测试机上微信未登录状态测试结果，当换成我的小米8测试后发现，登录状态下的微信有三个进程在运行

后查阅资料得知，进程名为com.tencent.soter.soterserver的进程是微信指纹支付，此进程的adj值居然为-800，上面我们说过，adj小于0的进程为系统进程，那么微信是如何做到创建一个系统进程的，我和我的小伙伴都惊呆了o.o，为此，我对比了一下支付宝的测试结果

支付宝测试结果：

支付宝创建了六个进程，查看这六个进程的adj值，除了一个为915，其余均为0，怎么肥事，0就意味着正在与用户交互的前台进程啊，我的世界要崩塌了，只有一种可能，支付宝通过未知的黑科技降低了adj值。

酷狗测试结果：

酷狗创建了两个进程，查看这两个进程的adj值分别为700、200，对应为adj等级表中的PREVIOUS_APP_ADJ和PERCEPTIBLE_APP_ADJ，还好，这个在意料之中。

测试思考

通过上面三个app的测试结果可以看出，微信和支付宝一定是使用了某种保活手段，让自身的adj降到最低，尤其是微信，居然可以创建系统进程，简直太逆天了，这是应用层绝对做不到的，一定是在native层完成的，但具体什么黑科技就不得而知了，毕竟反编译技术不是我的强项。

正当我郁郁寡欢之时，我想起了前两天看过的一篇文章《当 App 有了系统权限，真的可以为所欲为？》，文章讲述了第三方App如何利用CVE漏洞获取到系统权限，然后神不知鬼不觉的干一些匪夷所思的事儿，这让我茅塞顿开，或许这些大厂的app就是利用了系统漏洞来保活的，不然真的就说不通了，既然都能获取到系统权限了，那创建个系统进程不是分分钟的事儿吗，还需要啥厂商白名单。

总结

进程保活是一把双刃剑，增加app存活时间的同时牺牲的是用户手机的电量，内存，cpu等资源，甚至还有用户的忍耐度，作为开发者一定要合理取舍，不要为了保活而保活，即使需要保活，也尽量采用白色保活手段，别让用户手机变板砖，然后再来哭爹骂娘。

参考资料：

探讨Android6.0及以上系统APP常驻内存(保活)实现-争宠篇

探讨Android6.0及以上系统APP常驻内存(保活)实现-复活篇

探讨一种新型的双进程守护应用保活

史上最强Android保活思路：深入剖析腾讯TIM的进程永生技术

当 App 有了系统权限，真的可以为所欲为？

「 深蓝洞察 」2022 年度最“不可赦”漏洞