Update BackgroundExecutor to use LocklessQueue

This change allows BackgroundExecutor to receive callbacks from multiple
producers.

Bug: 283133164
Test: BackgroundExecutorTest
Change-Id: I63f3266184a2ae90b15d7d1b7d9502847e268763

services/surfaceflinger/BackgroundExecutor.cpp

@@ -28,29 +28,19 @@ namespace android {
 ANDROID_SINGLETON_STATIC_INSTANCE(BackgroundExecutor);
 
 BackgroundExecutor::BackgroundExecutor() : Singleton<BackgroundExecutor>() {
+    // mSemaphore must be initialized before any calls to
+    // BackgroundExecutor::sendCallbacks. For this reason, we initialize it
+    // within the constructor instead of within mThread.
+    LOG_ALWAYS_FATAL_IF(sem_init(&mSemaphore, 0, 0), "sem_init failed");
     mThread = std::thread([&]() {
-        LOG_ALWAYS_FATAL_IF(sem_init(&mSemaphore, 0, 0), "sem_init failed");
         while (!mDone) {
             LOG_ALWAYS_FATAL_IF(sem_wait(&mSemaphore), "sem_wait failed (%d)", errno);
-
-            ftl::SmallVector<Work*, 10> workItems;
-
-            Work* work = mWorks.pop();
-            while (work) {
-                workItems.push_back(work);
-                work = mWorks.pop();
+            auto callbacks = mCallbacksQueue.pop();
+            if (!callbacks) {
+                continue;
             }
-
-            // Sequence numbers are guaranteed to be in intended order, as we assume a single
-            // producer and single consumer.
-            std::stable_sort(workItems.begin(), workItems.end(), [](Work* left, Work* right) {
-                return left->sequence < right->sequence;
-            });
-            for (Work* work : workItems) {
-                for (auto& task : work->tasks) {
-                    task();
-                }
-                delete work;
+            for (auto& callback : *callbacks) {
+                callback();
             }
         }
     });
@@ -66,12 +56,8 @@ BackgroundExecutor::~BackgroundExecutor() {
 }
 
 void BackgroundExecutor::sendCallbacks(Callbacks&& tasks) {
-    Work* work = new Work();
-    work->sequence = mSequence;
-    work->tasks = std::move(tasks);
-    mWorks.push(work);
-    mSequence++;
+    mCallbacksQueue.push(std::move(tasks));
     LOG_ALWAYS_FATAL_IF(sem_post(&mSemaphore), "sem_post failed");
 }
 
-} // namespace android
\ No newline at end of file
+} // namespace android

services/surfaceflinger/BackgroundExecutor.h

@@ -16,15 +16,13 @@
 
 #pragma once
 
-#include <Tracing/LocklessStack.h>
-#include <android-base/thread_annotations.h>
 #include <ftl/small_vector.h>
 #include <semaphore.h>
 #include <utils/Singleton.h>
-#include <mutex>
-#include <queue>
 #include <thread>
 
+#include "LocklessQueue.h"
+
 namespace android {
 
 // Executes tasks off the main thread.
@@ -34,24 +32,14 @@ public:
     ~BackgroundExecutor();
     using Callbacks = ftl::SmallVector<std::function<void()>, 10>;
     // Queues callbacks onto a work queue to be executed by a background thread.
-    // Note that this is not thread-safe - a single producer is assumed.
+    // This is safe to call from multiple threads.
     void sendCallbacks(Callbacks&& tasks);
 
 private:
     sem_t mSemaphore;
     std::atomic_bool mDone = false;
 
-    // Sequence number for work items.
-    // Work items are batched by sequence number. Work items for earlier sequence numbers are
-    // executed first. Work items with the same sequence number are executed in the same order they
-    // were added to the stack (meaning the stack must reverse the order after popping from the
-    // queue)
-    int32_t mSequence = 0;
-    struct Work {
-        int32_t sequence = 0;
-        Callbacks tasks;
-    };
-    LocklessStack<Work> mWorks;
+    LocklessQueue<Callbacks> mCallbacksQueue;
     std::thread mThread;
 };
 

services/surfaceflinger/tests/unittests/Android.bp

@@ -71,6 +71,7 @@ cc_test {
         ":libsurfaceflinger_sources",
         "libsurfaceflinger_unittest_main.cpp",
         "ActiveDisplayRotationFlagsTest.cpp",
+        "BackgroundExecutorTest.cpp",
         "CompositionTest.cpp",
         "DisplayIdGeneratorTest.cpp",
         "DisplayTransactionTest.cpp",

services/surfaceflinger/tests/unittests/BackgroundExecutorTest.cpp

+#include <gtest/gtest.h>
+#include <condition_variable>
+
+#include "BackgroundExecutor.h"
+
+namespace android {
+
+class BackgroundExecutorTest : public testing::Test {};
+
+namespace {
+
+TEST_F(BackgroundExecutorTest, singleProducer) {
+    std::mutex mutex;
+    std::condition_variable condition_variable;
+    bool backgroundTaskComplete = false;
+
+    BackgroundExecutor::getInstance().sendCallbacks(
+            {[&mutex, &condition_variable, &backgroundTaskComplete]() {
+                std::lock_guard<std::mutex> lock{mutex};
+                condition_variable.notify_one();
+                backgroundTaskComplete = true;
+            }});
+
+    std::unique_lock<std::mutex> lock{mutex};
+    condition_variable.wait(lock, [&backgroundTaskComplete]() { return backgroundTaskComplete; });
+    ASSERT_TRUE(backgroundTaskComplete);
+}
+
+TEST_F(BackgroundExecutorTest, multipleProducers) {
+    std::mutex mutex;
+    std::condition_variable condition_variable;
+    const int backgroundTaskCount = 10;
+    int backgroundTaskCompleteCount = 0;
+
+    for (int i = 0; i < backgroundTaskCount; i++) {
+        std::thread([&mutex, &condition_variable, &backgroundTaskCompleteCount]() {
+            BackgroundExecutor::getInstance().sendCallbacks(
+                    {[&mutex, &condition_variable, &backgroundTaskCompleteCount]() {
+                        std::lock_guard<std::mutex> lock{mutex};
+                        backgroundTaskCompleteCount++;
+                        if (backgroundTaskCompleteCount == backgroundTaskCount) {
+                            condition_variable.notify_one();
+                        }
+                    }});
+        }).detach();
+    }
+
+    std::unique_lock<std::mutex> lock{mutex};
+    condition_variable.wait(lock, [&backgroundTaskCompleteCount]() {
+        return backgroundTaskCompleteCount == backgroundTaskCount;
+    });
+    ASSERT_EQ(backgroundTaskCount, backgroundTaskCompleteCount);
+}
+
+} // namespace
+
+} // namespace android