Don't process UEvents in Dalvik unless they match a pattern.

On some devices, vsync is delivered from the kernel to userspace
over a netlink socket via UEvent.  The result is that a thread wakes
up, reads a message, creates a new String then searches it for matches
against a pattern on every single frame.

Reduce the overhead by performing the initial pattern matching in
native code.

Bug: 7326329
Change-Id: Icb22db1c38330694207bec1153840e0c06f502d6

Author: Jeff Brown

core/java/android/os/UEventObserver.java

@@ -16,6 +16,8 @@
 
 package android.os;
 
+import android.util.Log;
+
 import java.util.ArrayList;
 import java.util.HashMap;
 
@@ -37,14 +39,20 @@
  * @hide
 */
 public abstract class UEventObserver {
+    private static final String TAG = "UEventObserver";
+    private static final boolean DEBUG = false;
+
     private static UEventThread sThread;
 
-    private static native void native_setup();
-    private static native int next_event(byte[] buffer);
+    private static native void nativeSetup();
+    private static native String nativeWaitForNextEvent();
+    private static native void nativeAddMatch(String match);
+    private static native void nativeRemoveMatch(String match);
 
     public UEventObserver() {
     }
 
+    @Override
     protected void finalize() throws Throwable {
         try {
             stopObserving();
@@ -78,10 +86,18 @@ private static UEventThread peekThread() {
      * This method can be called multiple times to register multiple matches.
      * Only one call to stopObserving is required even with multiple registered
      * matches.
-     * @param match A substring of the UEvent to match. Use "" to match all
-     *              UEvent's
+     *
+     * @param match A substring of the UEvent to match.  Try to be as specific
+     * as possible to avoid incurring unintended additional cost from processing
+     * irrelevant messages.  Netlink messages can be moderately high bandwidth and
+     * are expensive to parse.  For example, some devices may send one netlink message
+     * for each vsync period.
      */
     public final void startObserving(String match) {
+        if (match == null || match.isEmpty()) {
+            throw new IllegalArgumentException("match substring must be non-empty");
+        }
+
         final UEventThread t = getThread();
         t.addObserver(match, this);
     }
@@ -117,7 +133,7 @@ public UEvent(String message) {
 
             while (offset < length) {
                 int equals = message.indexOf('=', offset);
-                int at = message.indexOf(0, offset);
+                int at = message.indexOf('\0', offset);
                 if (at < 0) break;
 
                 if (equals > offset && equals < at) {
@@ -158,15 +174,17 @@ public UEventThread() {
             super("UEventObserver");
         }
 
+        @Override
         public void run() {
-            native_setup();
+            nativeSetup();
 
-            byte[] buffer = new byte[1024];
-            int len;
             while (true) {
-                len = next_event(buffer);
-                if (len > 0) {
-                    sendEvent(new String(buffer, 0, len));
+                String message = nativeWaitForNextEvent();
+                if (message != null) {
+                    if (DEBUG) {
+                        Log.d(TAG, message);
+                    }
+                    sendEvent(message);
                 }
             }
         }
@@ -176,7 +194,7 @@ private void sendEvent(String message) {
                 final int N = mKeysAndObservers.size();
                 for (int i = 0; i < N; i += 2) {
                     final String key = (String)mKeysAndObservers.get(i);
-                    if (message.indexOf(key) != -1) {
+                    if (message.contains(key)) {
                         final UEventObserver observer =
                                 (UEventObserver)mKeysAndObservers.get(i + 1);
                         mTempObserversToSignal.add(observer);
@@ -199,6 +217,7 @@ public void addObserver(String match, UEventObserver observer) {
             synchronized (mKeysAndObservers) {
                 mKeysAndObservers.add(match);
                 mKeysAndObservers.add(observer);
+                nativeAddMatch(match);
             }
         }
 
@@ -208,7 +227,8 @@ public void removeObserver(UEventObserver observer) {
                 for (int i = 0; i < mKeysAndObservers.size(); ) {
                     if (mKeysAndObservers.get(i + 1) == observer) {
                         mKeysAndObservers.remove(i + 1);
-                        mKeysAndObservers.remove(i);
+                        final String match = (String)mKeysAndObservers.remove(i);
+                        nativeRemoveMatch(match);
                     } else {
                         i += 2;
                     }

core/jni/android_os_UEventObserver.cpp

@@ -15,41 +15,103 @@
  */
 
 #define LOG_TAG "UEventObserver"
+//#define LOG_NDEBUG 0
+
 #include "utils/Log.h"
 
 #include "hardware_legacy/uevent.h"
 #include "jni.h"
 #include "JNIHelp.h"
 #include "android_runtime/AndroidRuntime.h"
 
-namespace android
-{
+#include <utils/Mutex.h>
+#include <utils/Vector.h>
+#include <utils/String8.h>
+#include <ScopedUtfChars.h>
 
-static void
-android_os_UEventObserver_native_setup(JNIEnv *env, jclass clazz)
-{
+namespace android {
+
+static Mutex gMatchesMutex;
+static Vector<String8> gMatches;
+
+static void nativeSetup(JNIEnv *env, jclass clazz) {
     if (!uevent_init()) {
         jniThrowException(env, "java/lang/RuntimeException",
-                          "Unable to open socket for UEventObserver");
+                "Unable to open socket for UEventObserver");
     }
 }
 
-static int
-android_os_UEventObserver_next_event(JNIEnv *env, jclass clazz, jbyteArray jbuffer)
-{
-    int buf_sz = env->GetArrayLength(jbuffer);
-    char *buffer = (char*)env->GetByteArrayElements(jbuffer, NULL);
+static bool isMatch(const char* buffer, size_t length) {
+    AutoMutex _l(gMatchesMutex);
+
+    for (size_t i = 0; i < gMatches.size(); i++) {
+        const String8& match = gMatches.itemAt(i);
+
+        // Consider all zero-delimited fields of the buffer.
+        const char* field = buffer;
+        const char* end = buffer + length;
+        do {
+            if (strstr(field, match.string())) {
+                ALOGV("Matched uevent message with pattern: %s", match.string());
+                return true;
+            }
+            field += strlen(field) + 1;
+        } while (field != end);
+    }
+    return false;
+}
 
-    int length = uevent_next_event(buffer, buf_sz - 1);
+static jstring nativeWaitForNextEvent(JNIEnv *env, jclass clazz) {
+    char buffer[1024];
 
-    env->ReleaseByteArrayElements(jbuffer, (jbyte*)buffer, 0);
+    for (;;) {
+        int length = uevent_next_event(buffer, sizeof(buffer) - 1);
+        if (length <= 0) {
+            return NULL;
+        }
+        buffer[length] = '\0';
 
-    return length;
+        ALOGV("Received uevent message: %s", buffer);
+
+        if (isMatch(buffer, length)) {
+            // Assume the message is ASCII.
+            jchar message[length];
+            for (int i = 0; i < length; i++) {
+                message[i] = buffer[i];
+            }
+            return env->NewString(message, length);
+        }
+    }
+}
+
+static void nativeAddMatch(JNIEnv* env, jclass clazz, jstring matchStr) {
+    ScopedUtfChars match(env, matchStr);
+
+    AutoMutex _l(gMatchesMutex);
+    gMatches.add(String8(match.c_str()));
+}
+
+static void nativeRemoveMatch(JNIEnv* env, jclass clazz, jstring matchStr) {
+    ScopedUtfChars match(env, matchStr);
+
+    AutoMutex _l(gMatchesMutex);
+    for (size_t i = 0; i < gMatches.size(); i++) {
+        if (gMatches.itemAt(i) == match.c_str()) {
+            gMatches.removeAt(i);
+            break; // only remove first occurrence
+        }
+    }
 }
 
 static JNINativeMethod gMethods[] = {
-    {"native_setup", "()V",   (void *)android_os_UEventObserver_native_setup},
-    {"next_event",   "([B)I", (void *)android_os_UEventObserver_next_event},
+    { "nativeSetup", "()V",
+            (void *)nativeSetup },
+    { "nativeWaitForNextEvent", "()Ljava/lang/String;",
+            (void *)nativeWaitForNextEvent },
+    { "nativeAddMatch", "(Ljava/lang/String;)V",
+            (void *)nativeAddMatch },
+    { "nativeRemoveMatch", "(Ljava/lang/String;)V",
+            (void *)nativeRemoveMatch },
 };
 
 
@@ -64,7 +126,7 @@ int register_android_os_UEventObserver(JNIEnv *env)
     }
 
     return AndroidRuntime::registerNativeMethods(env,
-                "android/os/UEventObserver", gMethods, NELEM(gMethods));
+            "android/os/UEventObserver", gMethods, NELEM(gMethods));
 }
 
 }   // namespace android