ircd::fs::aio: Simplify various conditions; add various comments; cleanup.

ircd/aio.cc

@@ -71,7 +71,11 @@ ircd::fs::aio::init::init()
 	if(!bool(aio::enable))
 		return;
 
-	system = new struct system;
+	system = new struct aio::system
+	(
+		size_t(max_events),
+		size_t(max_submit)
+	);
 }
 
 ircd::fs::aio::init::~init()
@@ -308,8 +312,7 @@ ircd::fs::aio::request::operator()()
 	// Wait here until there's room to submit a request
 	system->dock.wait([]
 	{
-		const size_t count(system->qcount + system->in_flight);
-		return count < size_t(max_events);
+		return system->request_avail() > 0;
 	});
 
 	// Submit to system
@@ -368,30 +371,31 @@ ircd::fs::aio::system::eventfd_flags
 // system::system
 //
 
-ircd::fs::aio::system::system()
+ircd::fs::aio::system::system(const size_t &max_events,
+                              const size_t &max_submit)
 try
 :event
 {
-	size_t(max_events)
+	max_events
 }
 ,queue
 {
-	size_t(max_submit)
+	max_submit
 }
 ,resfd
 {
 	ios::get(), int(syscall(::eventfd, ecount, eventfd_flags))
 }
 {
-	syscall<SYS_io_setup>(size_t(max_events), &idp);
+	syscall<SYS_io_setup>(this->max_events(), &idp);
 	set_handle();
 	log::debug
 	{
 		"Established AIO(%p) context (fd:%d max_events:%zu max_submit:%zu)",
 		this,
 		int(resfd.native_handle()),
-		size_t(max_events),
-		size_t(max_submit)
+		this->max_events(),
+		this->max_submit(),
 	};
 }
 catch(const std::exception &e)
@@ -539,24 +543,22 @@ ircd::fs::aio::system::submit(request &request)
 {
 	assert(request.opts);
 	assert(qcount < queue.size());
-	assert(qcount + in_flight < max_events);
+	assert(qcount + in_flight < max_events());
 	assert(request.aio_data == uintptr_t(&request));
 
 	const ctx::critical_assertion ca;
 	queue.at(qcount++) = static_cast<iocb *>(&request);
 	stats.cur_queued++;
 
-	// Determine whether the user wants (or needs) to submit without delay.
+	// Determine whether this request will trigger a flush of the queue
+	// and be submitted itself as well.
 	const bool submit_now
 	{
-		// The nodelay flag is set
+		// The nodelay flag is set by the user.
 		request.opts->nodelay
 
-		// The queue has reached the configured size
-		|| qcount >= size_t(max_submit)
-
-		// The queue has reached its maximum size
-		|| qcount >= queue.size()
+		// The queue has reached its limits.
+		|| qcount >= max_submit()
 	};
 
 	const size_t submitted
@@ -564,13 +566,16 @@ ircd::fs::aio::system::submit(request &request)
 		submit_now? submit() : 0
 	};
 
+	// Only post the chaser when the queue has one item. If it has more
+	// items the chaser was already posted after the first item and will
+	// flush the whole queue down to 0.
 	if(qcount == 1)
 		ircd::post(std::bind(&system::chase, this));
 }
 
-/// The chaser is posted to the IRCd event loop after the first
-/// request is queued. Ideally more requests will queue up before
-/// the chaser is executed.
+/// The chaser is posted to the IRCd event loop after the first request is
+/// Ideally more requests will queue up before the chaser reaches the front
+/// of the IRCd event queue and executes.
 void
 ircd::fs::aio::system::chase()
 noexcept try
@@ -600,7 +605,7 @@ try
 {
 	assert(qcount > 0);
 	assert(in_flight + qcount < MAX_EVENTS);
-	assert(in_flight + qcount <= size_t(max_events));
+	assert(in_flight + qcount <= max_events());
 
 	const auto submitted
 	{
@@ -738,15 +743,24 @@ void
 ircd::fs::aio::system::handle_event(const io_event &event)
 noexcept try
 {
-	// Our extended control block is passed in event.data
+	// The kernel always references the iocb in `event.obj`
+	auto *const iocb
+	{
+		reinterpret_cast<struct ::iocb *>(event.obj)
+	};
+
+	// We referenced our request (which extends the same iocb anyway)
+	// for the kernel to carry through as an opaque in `event.data`.
 	auto &request
 	{
 		*reinterpret_cast<aio::request *>(event.data)
 	};
 
-	auto *const iocb(reinterpret_cast<struct ::iocb *>(event.obj));
+	// Check that everything lines up.
 	assert(iocb == static_cast<struct ::iocb *>(&request));
 	assert(reinterpret_cast<aio::request *>(iocb->aio_data) == &request);
+
+	// Assert that we understand the return-value semantics of this interface.
 	assert(event.res2 >= 0);
 	assert(event.res == -1 || event.res2 == 0);
 
@@ -755,12 +769,10 @@ noexcept try
 	request.errcode = event.res >= -1? event.res2 : std::abs(event.res);
 
 	// Notify the waiting context. Note that we are on the main async stack
-	// but it is safe to notify from here. The waiter may be null if it left.
-	assert(!request.waiter || request.waiter != ctx::current);
+	// but it is safe to notify from here.
+	assert(request.waiter);
 	assert(ctx::current == nullptr);
-	if(likely(request.waiter))
 	ctx::notify(*request.waiter);
-
 	stats.events++;
 }
 catch(const std::exception &e)
@@ -774,6 +786,35 @@ catch(const std::exception &e)
 	};
 }
 
+size_t
+ircd::fs::aio::system::request_avail()
+const
+{
+	assert(request_count() <= max_events());
+	return max_events() - request_count();
+}
+
+size_t
+ircd::fs::aio::system::request_count()
+const
+{
+	return qcount + in_flight;
+}
+
+size_t
+ircd::fs::aio::system::max_submit()
+const
+{
+	return queue.size();
+}
+
+size_t
+ircd::fs::aio::system::max_events()
+const
+{
+	return event.size();
+}
+
 //
 // internal util
 //

ircd/aio.h

@@ -51,6 +51,11 @@ struct ircd::fs::aio::system
 	/// Handler to the io context we submit requests to the system with
 	aio_context_t idp {0};
 
+	size_t max_events() const;
+	size_t max_submit() const;
+	size_t request_count() const; // qcount + in_flight
+	size_t request_avail() const; // max_events - request_count()
+
 	// Callback stack invoked when the sigfd is notified of completed events.
 	void handle_event(const io_event &) noexcept;
 	void handle_events() noexcept;
@@ -68,7 +73,9 @@ struct ircd::fs::aio::system
 	bool wait();
 	bool interrupt();
 
-	system();
+	system(const size_t &max_events,
+	       const size_t &max_submit);
+
 	~system() noexcept;
 };