Make sure Zygote is running at process priority 0 after VM has started.

Boosting up zygote priority before VM startup, saves ~450ms of boot time
for N9, 180ms for Nexus 5X.

(cherry picked from commit 5d551a5ac3)

Bug: 28866384
Test: m
Test: Device boots
Change-Id: Ic85892b408e15bbc1de7ce706f113f23974fe478

core/jni/com_android_internal_os_Zygote.cpp

@@ -37,6 +37,7 @@
 #include <sys/prctl.h>
 #include <sys/resource.h>
 #include <sys/stat.h>
+#include <sys/time.h>
 #include <sys/types.h>
 #include <sys/utsname.h>
 #include <sys/wait.h>
@@ -153,6 +154,24 @@ static void SetSigChldHandler() {
   }
 }
 
+// Resets nice priority for zygote process. Zygote priority can be set
+// to high value during boot phase to speed it up. We want to ensure
+// zygote is running at normal priority before childs are forked from it.
+//
+// This ends up being called repeatedly before each fork(), but there's
+// no real harm in that.
+static void ResetNicePriority(JNIEnv* env) {
+  errno = 0;
+  int prio = getpriority(PRIO_PROCESS, 0);
+  if (prio == -1 && errno != 0) {
+    ALOGW("getpriority failed: %s\n", strerror(errno));
+  }
+  if (prio != 0 && setpriority(PRIO_PROCESS, 0, 0) != 0) {
+    ALOGE("setpriority(%d, 0, 0) failed: %s", PRIO_PROCESS, strerror(errno));
+    RuntimeAbort(env, __LINE__, "setpriority failed");
+  }
+}
+
 // Sets the SIGCHLD handler back to default behavior in zygote children.
 static void UnsetSigChldHandler() {
   struct sigaction sa;
@@ -447,6 +466,8 @@ static pid_t ForkAndSpecializeCommon(JNIEnv* env, uid_t uid, gid_t gid, jintArra
     RuntimeAbort(env, __LINE__, "Unable to restat file descriptor table.");
   }
 
+  ResetNicePriority(env);
+
   pid_t pid = fork();
 
   if (pid == 0) {
@@ -719,4 +740,3 @@ int register_com_android_internal_os_Zygote(JNIEnv* env) {
   return RegisterMethodsOrDie(env, "com/android/internal/os/Zygote", gMethods, NELEM(gMethods));
 }
 }  // namespace android
-