Merge branch 'improve-reap-completions' into 'master'

[IoContext] wrap CQ locking in if constexpr

See merge request i4/manycore/emper!218

emper/io/IoContext.cpp

@@ -211,24 +211,26 @@ reap_cqes:
 	LOGD("Reaping completions for worker " << std::to_string(worker->getWorkerId()));
 	std::array<struct io_uring_cqe *, CQE_BATCH_COUNT> cqes;
 
-	// Someone else is currently reaping completions
-	if constexpr (callerEnvironment == CallerEnvironment::EMPER) {
-		if (unlikely(!cq_lock.try_lock())) {
-			LOGD("worker unsuccessful try_lock");
-			return 0;
-		}
-	} else {
-		// We can not reap completions of this IoContext to not race
-		// with the sleeping worker.
-		if (waitInflight.load(std::memory_order_acquire)) {
-			LOGD("Not reaping worker " << std::to_string(worker->getWorkerId())
-																 << " since this worker is already waiting for its CQEs");
-			return 0;
-		}
+	if constexpr (needsCqLock) {
+		// Someone else is currently reaping completions
+		if constexpr (callerEnvironment == CallerEnvironment::EMPER) {
+			if (unlikely(!cq_lock.try_lock())) {
+				LOGD("worker unsuccessful try_lock");
+				return 0;
+			}
+		} else {
+			// We can not reap completions of this IoContext to not race
+			// with the sleeping worker.
+			if (waitInflight.load(std::memory_order_acquire)) {
+				LOGD("Not reaping worker " << std::to_string(worker->getWorkerId())
+																	 << " since this worker is already waiting for its CQEs");
+				return 0;
+			}
 
-		if (!cq_lock.try_lock_or_increment()) {
-			LOGD("Global completer unsuccessful try_lock_or_increment");
-			return 0;
+			if (!cq_lock.try_lock_or_increment()) {
+				LOGD("Global completer unsuccessful try_lock_or_increment");
+				return 0;
+			}
 		}
 	}
 
@@ -245,7 +247,10 @@ reap_cqes:
 
 	io_uring_cq_advance(&ring, count);
 
-	uint32_t globalCompleterAttempts = cq_lock.unlock();
+	uint32_t globalCompleterAttempts;
+	if constexpr (needsCqLock) {
+		globalCompleterAttempts = cq_lock.unlock();
+	}
 
 	LOGD("got " << count << " cqes from worker " << worker->getWorkerId() << "'s io_uring");
 
@@ -336,7 +341,7 @@ reap_cqes:
 	}
 
 	// check if lost wakeup was possible
-	if constexpr (callerEnvironment == CallerEnvironment::EMPER) {
+	if constexpr (needsCqLock && callerEnvironment == CallerEnvironment::EMPER) {
 		bool reReap = false;
 		// TODO: How sure are we that this is unlikely?
 		if (unlikely(globalCompleterAttempts > maxRaceFreeCompleterAttempts)) {

emper/io/IoContext.hpp

@@ -68,6 +68,8 @@ class IoContext : public Logger<LogSubsystem::IO> {
 	Runtime &runtime;
 
 	static thread_local IoContext *workerIo;
+	static constexpr bool needsCqLock =
+			emper::IO_COMPLETER_BEHAVIOR != emper::IoCompleterBehavior::none;
 	// TryLock protecting the completion queue of ring.
 	ALIGN_TO_CACHE_LINE CqLock cq_lock;
 	struct io_uring ring;