CpuBoundWork#CpuBoundWork(): don't spin on atomic int to acquire slot

This is inefficient and involves possible bad surprises regarding waiting durations on busy nodes. Instead, use AsioConditionVariable#Wait() if there are no free slots. It's notified by others' CpuBoundWork#~CpuBoundWork() once finished.

lib/base/io-engine.cpp

@@ -16,30 +16,121 @@
 
 using namespace icinga;
 
-CpuBoundWork::CpuBoundWork(boost::asio::yield_context yc, boost::asio::io_context::strand&)
+/**
+ * Acquires a slot for CPU-bound work.
+ *
+ * If and as long as the lock-free TryAcquireSlot() doesn't succeed,
+ * subscribes to the slow path by waiting on a condition variable.
+ * It is woken up by Done() which is called by the destructor.
+ *
+ * @param yc Needed to asynchronously wait for the condition variable.
+ * @param strand Where to post the wake-up of the condition variable.
+ */
+CpuBoundWork::CpuBoundWork(boost::asio::yield_context yc, boost::asio::io_context::strand& strand)
 	: m_Done(false)
 {
-	auto& ioEngine (IoEngine::Get());
+	VERIFY(strand.running_in_this_thread());
 
-	for (;;) {
-		auto availableSlots (ioEngine.m_CpuBoundSemaphore.fetch_sub(1));
+	auto& ie (IoEngine::Get());
+	Shared<AsioConditionVariable>::Ptr cv;
 
-		if (availableSlots < 1) {
-			ioEngine.m_CpuBoundSemaphore.fetch_add(1);
-			IoEngine::YieldCurrentCoroutine(yc);
-			continue;
-		}
+	while (!TryAcquireSlot()) {
+		if (!cv) {
+			cv = Shared<AsioConditionVariable>::Make(ie.GetIoContext());
 
+			// The above line may take a little bit, so let's optimistically re-check
+			if (TryAcquireSlot()) {
 				break;
 			}
 		}
 
+		{
+			std::unique_lock lock (ie.m_CpuBoundWaitingMutex);
+
+			// The above line may take even longer, so let's check again.
+			// Also mitigate lost wake-ups by re-checking during the lock:
+			//
+			// During our lock, Done() can't retrieve the subscribers to wake up,
+			// so any ongoing wake-up is either done at this point or has not started yet.
+			// If such a wake-up is done, it's a lost wake-up to us unless we re-check here
+			// whether the slot being freed (just before the wake-up) is still available.
+			if (TryAcquireSlot()) {
+				break;
+			}
+
+			// If the (hypothetical) slot mentioned above was taken by another coroutine,
+			// there are no free slots again, just as if no wake-ups happened just now.
+			ie.m_CpuBoundWaiting.emplace_back(strand, cv);
+		}
+
+		cv->Wait(yc);
+	}
+}
+
+/**
+ * Tries to acquire a slot for CPU-bound work.
+ *
+ * Specifically, decrements the number of free slots (semaphore) by one,
+ * but only if it's currently greater than zero.
+ * Not falling below zero requires an atomic#compare_exchange_weak() loop
+ * instead of a simple atomic#fetch_sub() call, but it's also atomic.
+ *
+ * @return Whether a slot was acquired.
+ */
+bool CpuBoundWork::TryAcquireSlot()
+{
+	auto& ie (IoEngine::Get());
+	auto freeSlots (ie.m_CpuBoundSemaphore.load());
+
+	while (freeSlots > 0) {
+		// If ie.m_CpuBoundSemaphore was changed after the last load,
+		// compare_exchange_weak() will load its latest value into freeSlots for us to retry until...
+		if (ie.m_CpuBoundSemaphore.compare_exchange_weak(freeSlots, freeSlots - 1)) {
+			// ... either we successfully decrement ie.m_CpuBoundSemaphore by one, ...
+			return true;
+		}
+	}
+
+	// ... or it becomes zero due to another coroutine.
+	return false;
+}
+
+/**
+ * Releases the own slot acquired by the constructor (TryAcquireSlot()) if not already done.
+ *
+ * Precisely, increments the number of free slots (semaphore) by one.
+ * Also wakes up all waiting constructors (slow path) if necessary.
+ */
 void CpuBoundWork::Done()
 {
 	if (!m_Done) {
-		IoEngine::Get().m_CpuBoundSemaphore.fetch_add(1);
-
 		m_Done = true;
+
+		auto& ie (IoEngine::Get());
+
+		// The constructor takes the slow path only if the semaphore is full,
+		// so we only have to wake up constructors if the semaphore was full.
+		// This works because after fetch_add(), TryAcquireSlot() (fast path) will succeed.
+		if (ie.m_CpuBoundSemaphore.fetch_add(1) < 1) {
+			// So now there are only slow path subscribers from just before the fetch_add() to be woken up.
+			// Precisely, only subscribers from just before the fetch_add() which turned 0 to 1.
+
+			decltype(ie.m_CpuBoundWaiting) subscribers;
+
+			{
+				// Locking after fetch_add() is safe because a delayed wake-up is fine.
+				// Wake-up of constructors which subscribed after the fetch_add() is also not a problem.
+				// In worst case, they will just re-subscribe to the slow path.
+				// Lost wake-ups are mitigated by the constructor, see its implementation comments.
+				std::unique_lock lock (ie.m_CpuBoundWaitingMutex);
+				std::swap(subscribers, ie.m_CpuBoundWaiting);
+			}
+
+			// Again, a delayed wake-up is fine, hence unlocked.
+			for (auto& [strand, cv] : subscribers) {
+				boost::asio::post(strand, [cv = std::move(cv)] { cv->NotifyOne(); });
+			}
+		}
 	}
 }
 

lib/base/io-engine.hpp

@@ -12,6 +12,7 @@
 #include <atomic>
 #include <exception>
 #include <memory>
+#include <mutex>
 #include <thread>
 #include <utility>
 #include <vector>
@@ -52,6 +53,8 @@ public:
 	void Done();
 
 private:
+	static bool TryAcquireSlot();
+
 	bool m_Done;
 };
 
@@ -153,7 +156,10 @@ private:
 	boost::asio::executor_work_guard<boost::asio::io_context::executor_type> m_KeepAlive;
 	std::vector<std::thread> m_Threads;
 	boost::asio::deadline_timer m_AlreadyExpiredTimer;
+
 	std::atomic_int_fast32_t m_CpuBoundSemaphore;
+	std::mutex m_CpuBoundWaitingMutex;
+	std::vector<std::pair<boost::asio::io_context::strand, Shared<AsioConditionVariable>::Ptr>> m_CpuBoundWaiting;
 };
 
 class TerminateIoThread : public std::exception