bp/Config: add option io_uring_sqpoll

doc/bp.rst

@@ -419,6 +419,11 @@ The following settings are available:
   which can make debugging with ``strace`` easier, because ``strace``
   cannot see ``io_uring`` operations.
 
+- ``io_uring_sqpoll``: Enables ``io_uring`` submit-queue polling.
+  This reduces the number of ``io_uring_enter()`` system calls at the
+  cost of a kernel thread running at 100% all the time, busy-polling
+  for new entries.
+
 - ``verbose_response``: Set to ``yes`` to reveal internal error
   messages in HTTP responses.
 

src/bp/Config.cxx

@@ -62,6 +62,8 @@ BpConfig::HandleSet(std::string_view name, const char *value)
 		use_xattr = ParseBool(value);
 	} else if (name == "use_io_uring"sv) {
 		use_io_uring = ParseBool(value);
+	} else if (name == "io_uring_sqpoll"sv) {
+		io_uring_sqpoll = ParseBool(value);
 	} else if (name == "verbose_response"sv) {
 		verbose_response = ParseBool(value);
 	} else if (name == "session_cookie"sv) {

src/bp/Config.hxx

@@ -95,6 +95,8 @@ struct BpConfig {
 
 	bool use_io_uring = true;
 
+	bool io_uring_sqpoll = false;
+
 	SpawnConfig spawn;
 
 	SslClientConfig ssl_client;

src/bp/Instance.hxx

@@ -89,7 +89,11 @@ struct BpInstance final : PInstance, BengControl::Handler,
 	HttpStats http_stats;
 
 	[[no_unique_address]]
-	UringGlue uring{event_loop, config.use_io_uring};
+	UringGlue uring{
+		event_loop,
+		config.use_io_uring,
+		config.io_uring_sqpoll,
+	};
 
 	const StateDirectories state_directories;
 

src/bp/UringGlue.cxx

@@ -22,14 +22,19 @@
 #include <sys/stat.h>
 
 UringGlue::UringGlue([[maybe_unused]] EventLoop &event_loop,
-		     [[maybe_unused]] bool enable) noexcept
+		     [[maybe_unused]] bool enable,
+		     [[maybe_unused]] bool sqpoll) noexcept
 {
 #ifdef HAVE_URING
 	if (!enable)
 		return;
 
+	unsigned flags = 0;
+	if (sqpoll)
+		flags |= IORING_SETUP_SQPOLL;
+
 	try {
-		uring.emplace(event_loop, 16384);
+		uring.emplace(event_loop, 16384, flags);
 	} catch (...) {
 		fprintf(stderr, "Failed to initialize io_uring: ");
 		PrintException(std::current_exception());

src/bp/UringGlue.hxx

@@ -33,7 +33,8 @@ class UringGlue {
 #endif
 
 public:
-	explicit UringGlue(EventLoop &event_loop, bool enable) noexcept;
+	explicit UringGlue(EventLoop &event_loop, bool enable,
+			   bool sqpoll) noexcept;
 
 	void SetVolatile() noexcept;