Do not disable tracing in zygote.

We now use the sequence number of the atrace tag rather than a Binder
transaction, so zygote can observe changes to the tags.

Test: m
Bug: 137366208
Bug: 147667830

This is a cherry-pick of e38797be84.

Change-Id: I50e32a19c538dd255097c035836504b4746b172b
Merged-In: I50e32a19c538dd255097c035836504b4746b172b

core/java/android/os/Trace.java

@@ -175,11 +175,7 @@ public final class Trace {
     }
 
     /**
-     * Set whether tracing is enabled in this process.  Tracing is disabled shortly after Zygote
-     * initializes and re-enabled after processes fork from Zygote.  This is done because Zygote
-     * has no way to be notified about changes to the tracing tags, and if Zygote ever reads and
-     * caches the tracing tags, forked processes will inherit those stale tags.
-     *
+     * Set whether tracing is enabled in this process.
      * @hide
      */
     public static void setTracingEnabled(boolean enabled, int debugFlags) {

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

@@ -261,10 +261,8 @@ public final class Zygote {
         int pid = nativeForkAndSpecialize(
                 uid, gid, gids, runtimeFlags, rlimits, mountExternal, seInfo, niceName, fdsToClose,
                 fdsToIgnore, startChildZygote, instructionSet, appDataDir);
-        // Enable tracing as soon as possible for the child process.
         if (pid == 0) {
             Zygote.disableExecuteOnly(targetSdkVersion);
-            Trace.setTracingEnabled(true, runtimeFlags);
 
             // Note that this event ends at the end of handleChildProc,
             Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "PostFork");
@@ -309,9 +307,6 @@ public final class Zygote {
         nativeSpecializeAppProcess(uid, gid, gids, runtimeFlags, rlimits, mountExternal, seInfo,
                 niceName, startChildZygote, instructionSet, appDataDir);
 
-        // Enable tracing as soon as possible for the child process.
-        Trace.setTracingEnabled(true, runtimeFlags);
-
         // Note that this event ends at the end of handleChildProc.
         Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "PostFork");
 
@@ -367,11 +362,6 @@ public final class Zygote {
                 uid, gid, gids, runtimeFlags, rlimits,
                 permittedCapabilities, effectiveCapabilities);
 
-        // Enable tracing as soon as we enter the system_server.
-        if (pid == 0) {
-            Trace.setTracingEnabled(true, runtimeFlags);
-        }
-
         // Set the Java Language thread priority to the default value for new apps.
         Thread.currentThread().setPriority(Thread.NORM_PRIORITY);
 

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

@@ -908,10 +908,6 @@ public class ZygoteInit {
             bootTimingsTraceLog.traceEnd(); // PostZygoteInitGC
 
             bootTimingsTraceLog.traceEnd(); // ZygoteInit
-            // Disable tracing so that forked processes do not inherit stale tracing tags from
-            // Zygote.
-            Trace.setTracingEnabled(false, 0);
-
 
             Zygote.initNativeState(isPrimaryZygote);