Zygote: Reset niceness if lazy preloading is enabled.

By definition, the zygote in question is lower priority if lazy
preloading is enabled. We therefore reset the zygote process'
niceness to 0 as soon as it comes up, rather than having it stick
around as a higher priority process until we eventually ask it
to load the default set of resources.

Test: manual.

Change-Id: I3ccfc0e7c3fdebf5f798d90b53140a554bf64c20

core/java/com/android/internal/os/Zygote.java

@@ -173,6 +173,10 @@ public final class Zygote {
         VM_HOOKS.postForkChild(debugFlags, isSystemServer, instructionSet);
     }
 
+    /**
+     * Resets this process' priority to the default value (0).
+     */
+    native static void nativeResetNicePriority();
 
     /**
      * Executes "/system/bin/sh -c <command>" using the exec() system call.

core/java/com/android/internal/os/ZygoteInit.java

@@ -712,6 +712,8 @@ public class ZygoteInit {
                 EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
                     SystemClock.uptimeMillis());
                 bootTimingsTraceLog.traceEnd(); // ZygotePreload
+            } else {
+                Zygote.nativeResetNicePriority();
             }
 
             // Finish profiling the zygote initialization.

core/jni/com_android_internal_os_Zygote.cpp

@@ -806,6 +806,10 @@ static void com_android_internal_os_Zygote_nativeUnmountStorageOnInit(JNIEnv* en
     UnmountTree("/storage");
 }
 
+static void com_android_internal_os_Zygote_nativeResetNicePriority(JNIEnv* env, jclass) {
+    ResetNicePriority(env);
+}
+
 static const JNINativeMethod gMethods[] = {
     { "nativeForkAndSpecialize",
       "(II[II[[IILjava/lang/String;Ljava/lang/String;[I[ILjava/lang/String;Ljava/lang/String;)I",
@@ -815,7 +819,9 @@ static const JNINativeMethod gMethods[] = {
     { "nativeAllowFileAcrossFork", "(Ljava/lang/String;)V",
       (void *) com_android_internal_os_Zygote_nativeAllowFileAcrossFork },
     { "nativeUnmountStorageOnInit", "()V",
-      (void *) com_android_internal_os_Zygote_nativeUnmountStorageOnInit }
+      (void *) com_android_internal_os_Zygote_nativeUnmountStorageOnInit },
+    { "nativeResetNicePriority", "()V",
+      (void *) com_android_internal_os_Zygote_nativeResetNicePriority }
 };
 
 int register_com_android_internal_os_Zygote(JNIEnv* env) {