Add config option to determine whether eventfd is used.

Add a new configuration option "reactor" / "use_eventfd" that is used by
the epoll_reactor. When true (the default), the reactor uses an eventfd
as its select_interrupter implementation. When false, a pipe is used
instead.

Author: chriskohlhoff

include/asio/detail/eventfd_select_interrupter.hpp

@@ -29,7 +29,7 @@ class eventfd_select_interrupter
 {
 public:
   // Constructor.
-  ASIO_DECL eventfd_select_interrupter();
+  ASIO_DECL explicit eventfd_select_interrupter(bool use_eventfd = true);
 
   // Destructor.
   ASIO_DECL ~eventfd_select_interrupter();
@@ -51,7 +51,7 @@ class eventfd_select_interrupter
 
 private:
   // Open the descriptors. Throws on error.
-  ASIO_DECL void open_descriptors();
+  ASIO_DECL void open_descriptors(bool use_eventfd);
 
   // Close the descriptors.
   ASIO_DECL void close_descriptors();

include/asio/detail/impl/epoll_reactor.ipp

@@ -41,7 +41,7 @@ epoll_reactor::epoll_reactor(asio::execution_context& ctx)
     scheduler_(use_service<scheduler>(ctx)),
     mutex_(config(ctx).get("reactor", "registration_locking", true),
         config(ctx).get("reactor", "registration_locking_spin_count", 0)),
-    interrupter_(),
+    interrupter_(config(ctx).get("reactor", "use_eventfd", true)),
     epoll_fd_(do_epoll_create()),
     timer_fd_(config(ctx).get("reactor", "use_timerfd", true)
         ? do_timerfd_create() : -1),

include/asio/detail/impl/eventfd_select_interrupter.ipp

@@ -38,38 +38,43 @@
 namespace asio {
 namespace detail {
 
-eventfd_select_interrupter::eventfd_select_interrupter()
+eventfd_select_interrupter::eventfd_select_interrupter(bool use_eventfd)
 {
-  open_descriptors();
+  open_descriptors(use_eventfd);
 }
 
-void eventfd_select_interrupter::open_descriptors()
+void eventfd_select_interrupter::open_descriptors(bool use_eventfd)
 {
-#if __GLIBC__ == 2 && __GLIBC_MINOR__ < 8 && !defined(__UCLIBC__)
-  write_descriptor_ = read_descriptor_ = syscall(__NR_eventfd, 0);
-  if (read_descriptor_ != -1)
+  if (use_eventfd)
   {
-    ::fcntl(read_descriptor_, F_SETFL, O_NONBLOCK);
-    ::fcntl(read_descriptor_, F_SETFD, FD_CLOEXEC);
-  }
+#if __GLIBC__ == 2 && __GLIBC_MINOR__ < 8 && !defined(__UCLIBC__)
+    write_descriptor_ = read_descriptor_ = syscall(__NR_eventfd, 0);
+    if (read_descriptor_ != -1)
+    {
+      ::fcntl(read_descriptor_, F_SETFL, O_NONBLOCK);
+      ::fcntl(read_descriptor_, F_SETFD, FD_CLOEXEC);
+    }
 #else // __GLIBC__ == 2 && __GLIBC_MINOR__ < 8 && !defined(__UCLIBC__)
 # if defined(EFD_CLOEXEC) && defined(EFD_NONBLOCK)
-  write_descriptor_ = read_descriptor_ =
-    ::eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK);
+    write_descriptor_ = read_descriptor_ =
+      ::eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK);
 # else // defined(EFD_CLOEXEC) && defined(EFD_NONBLOCK)
-  errno = EINVAL;
-  write_descriptor_ = read_descriptor_ = -1;
+    errno = EINVAL;
+    write_descriptor_ = read_descriptor_ = -1;
 # endif // defined(EFD_CLOEXEC) && defined(EFD_NONBLOCK)
-  if (read_descriptor_ == -1 && errno == EINVAL)
-  {
-    write_descriptor_ = read_descriptor_ = ::eventfd(0, 0);
-    if (read_descriptor_ != -1)
+    if (read_descriptor_ == -1 && errno == EINVAL)
     {
-      ::fcntl(read_descriptor_, F_SETFL, O_NONBLOCK);
-      ::fcntl(read_descriptor_, F_SETFD, FD_CLOEXEC);
+      write_descriptor_ = read_descriptor_ = ::eventfd(0, 0);
+      if (read_descriptor_ != -1)
+      {
+        ::fcntl(read_descriptor_, F_SETFL, O_NONBLOCK);
+        ::fcntl(read_descriptor_, F_SETFD, FD_CLOEXEC);
+      }
     }
-  }
 #endif // __GLIBC__ == 2 && __GLIBC_MINOR__ < 8 && !defined(__UCLIBC__)
+  }
+  else
+    write_descriptor_ = read_descriptor_ = -1;
 
   if (read_descriptor_ == -1)
   {
@@ -107,12 +112,10 @@ void eventfd_select_interrupter::close_descriptors()
 
 void eventfd_select_interrupter::recreate()
 {
+  bool use_eventfd = (write_descriptor_ == read_descriptor_);
   close_descriptors();
-
-  write_descriptor_ = -1;
-  read_descriptor_ = -1;
-
-  open_descriptors();
+  write_descriptor_ = read_descriptor_ = -1;
+  open_descriptors(use_eventfd);
 }
 
 void eventfd_select_interrupter::interrupt()

include/asio/detail/impl/pipe_select_interrupter.ipp

@@ -37,7 +37,7 @@
 namespace asio {
 namespace detail {
 
-pipe_select_interrupter::pipe_select_interrupter()
+pipe_select_interrupter::pipe_select_interrupter(bool)
 {
   open_descriptors();
 }

include/asio/detail/impl/socket_select_interrupter.ipp

@@ -35,7 +35,7 @@
 namespace asio {
 namespace detail {
 
-socket_select_interrupter::socket_select_interrupter()
+socket_select_interrupter::socket_select_interrupter(bool)
 {
   open_descriptors();
 }

include/asio/detail/pipe_select_interrupter.hpp

@@ -32,7 +32,7 @@ class pipe_select_interrupter
 {
 public:
   // Constructor.
-  ASIO_DECL pipe_select_interrupter();
+  ASIO_DECL explicit pipe_select_interrupter(bool = true);
 
   // Destructor.
   ASIO_DECL ~pipe_select_interrupter();

include/asio/detail/socket_select_interrupter.hpp

@@ -34,7 +34,7 @@ class socket_select_interrupter
 {
 public:
   // Constructor.
-  ASIO_DECL socket_select_interrupter();
+  ASIO_DECL explicit socket_select_interrupter(bool = true);
 
   // Destructor.
   ASIO_DECL ~socket_select_interrupter();

src/doc/overview/configuration.qbk

@@ -177,6 +177,18 @@ below.
       fails with `EAGAIN`.
     ]
   ]
+  [
+    [`reactor`]
+    [`use_eventfd`]
+    [`bool`]
+    [`true`]
+    [
+      Linux [^epoll] backend only.
+
+      When `true`, the reactor uses an [^eventfd] descriptor to wake a blocked
+      [^epoll_wait] call. When `false`, a pipe is used instead.
+    ]
+  ]
   [
     [`reactor`]
     [`use_timerfd`]