From ed242de3f5b709e9e0925513ab976886a04358c0 Mon Sep 17 00:00:00 2001
From: Jeff Brown <jeffbrown@google.com>
Date: Fri, 17 Sep 2010 17:01:23 -0700
Subject: [PATCH] Reduce lock thrashing in native Looper.

In the common case, there is nothing interesting happening on the native
Looper besides occasional wake ups.  There is no point grabbing the
semaphore then.

Change-Id: Ib5c426d0e158dfa37891b7ff5537b6f833592fad
---
 libs/utils/Looper.cpp | 70 ++++++++++++++++++++++++++++-----------------------
 1 file changed, 38 insertions(+), 32 deletions(-)

diff --git a/libs/utils/Looper.cpp b/libs/utils/Looper.cpp
index 4aa50d675f..b46279efc1 100644
--- a/libs/utils/Looper.cpp
+++ b/libs/utils/Looper.cpp
@@ -184,44 +184,50 @@ int Looper::pollInner(int timeoutMillis) {
 #if DEBUG_POLL_AND_WAKE
     LOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount);
 #endif
-    { // acquire lock
-        AutoMutex _l(mLock);
-        for (int i = 0; i < eventCount; i++) {
-            int fd = eventItems[i].data.fd;
-            uint32_t epollEvents = eventItems[i].events;
-            if (fd == mWakeReadPipeFd) {
-                if (epollEvents & EPOLLIN) {
+    bool acquiredLock = false;
+    for (int i = 0; i < eventCount; i++) {
+        int fd = eventItems[i].data.fd;
+        uint32_t epollEvents = eventItems[i].events;
+        if (fd == mWakeReadPipeFd) {
+            if (epollEvents & EPOLLIN) {
 #if DEBUG_POLL_AND_WAKE
-                    LOGD("%p ~ pollOnce - awoken", this);
+                LOGD("%p ~ pollOnce - awoken", this);
 #endif
-                    char buffer[16];
-                    ssize_t nRead;
-                    do {
-                        nRead = read(mWakeReadPipeFd, buffer, sizeof(buffer));
-                    } while ((nRead == -1 && errno == EINTR) || nRead == sizeof(buffer));
-                } else {
-                    LOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents);
-                }
+                char buffer[16];
+                ssize_t nRead;
+                do {
+                    nRead = read(mWakeReadPipeFd, buffer, sizeof(buffer));
+                } while ((nRead == -1 && errno == EINTR) || nRead == sizeof(buffer));
+            } else {
+                LOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents);
+            }
+        } else {
+            if (! acquiredLock) {
+                mLock.lock();
+                acquiredLock = true;
+            }
+
+            ssize_t requestIndex = mRequests.indexOfKey(fd);
+            if (requestIndex >= 0) {
+                int events = 0;
+                if (epollEvents & EPOLLIN) events |= ALOOPER_EVENT_INPUT;
+                if (epollEvents & EPOLLOUT) events |= ALOOPER_EVENT_OUTPUT;
+                if (epollEvents & EPOLLERR) events |= ALOOPER_EVENT_ERROR;
+                if (epollEvents & EPOLLHUP) events |= ALOOPER_EVENT_HANGUP;
+
+                Response response;
+                response.events = events;
+                response.request = mRequests.valueAt(requestIndex);
+                mResponses.push(response);
             } else {
-                ssize_t requestIndex = mRequests.indexOfKey(fd);
-                if (requestIndex >= 0) {
-                    int events = 0;
-                    if (epollEvents & EPOLLIN) events |= ALOOPER_EVENT_INPUT;
-                    if (epollEvents & EPOLLOUT) events |= ALOOPER_EVENT_OUTPUT;
-                    if (epollEvents & EPOLLERR) events |= ALOOPER_EVENT_ERROR;
-                    if (epollEvents & EPOLLHUP) events |= ALOOPER_EVENT_HANGUP;
-
-                    Response response;
-                    response.events = events;
-                    response.request = mRequests.valueAt(requestIndex);
-                    mResponses.push(response);
-                } else {
-                    LOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
-                            "no longer registered.", epollEvents, fd);
-                }
+                LOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
+                        "no longer registered.", epollEvents, fd);
             }
         }
     }
+    if (acquiredLock) {
+        mLock.unlock();
+    }
 
     for (size_t i = 0; i < mResponses.size(); i++) {
         const Response& response = mResponses.itemAt(i);
-- 
2.11.0