add a stat for packets dropped from socket receive buffer

include/udx.h

@@ -140,6 +140,7 @@ struct udx_socket_s {
   udx_t *udx;
   udx_cirbuf_t *streams_by_id; // for convenience
 
+  bool cmsg_wanted; // include a control buffer for recvmsg
   int family;
   int status;
   int readers;
@@ -157,6 +158,8 @@ struct udx_socket_s {
 
   uint64_t packets_rx;
   uint64_t packets_tx;
+
+  int64_t packets_dropped_by_kernel;
 };
 
 typedef struct udx_cong_s {

src/io.h

@@ -12,4 +12,7 @@ udx__sendmsg (udx_socket_t *handle, const uv_buf_t bufs[], unsigned int bufs_len
 ssize_t
 udx__recvmsg (udx_socket_t *handle, uv_buf_t *buf, struct sockaddr *addr, int addr_len);
 
+int
+udx__udp_set_rxq_ovfl (int fd);
+
 #endif // UDX_IO_H

src/io_posix.c

@@ -90,9 +90,48 @@ udx__recvmsg (udx_socket_t *handle, uv_buf_t *buf, struct sockaddr *addr, int ad
   h.msg_iov = (struct iovec *) buf;
   h.msg_iovlen = 1;
 
+  union {
+    struct cmsghdr align;
+    uint8_t buf[2048];
+  } u;
+
+  h.msg_control = u.buf;
+  h.msg_controllen = sizeof(u.buf);
+
   do {
     size = recvmsg(handle->io_poll.io_watcher.fd, &h, 0);
   } while (size == -1 && errno == EINTR);
 
+#if defined(__linux__)
+
+  // relies on SO_RXQ_OVFL being set
+  uint32_t packets_dropped_by_kernel = 0;
+
+  for (struct cmsghdr *cmsg = CMSG_FIRSTHDR(&h); cmsg != NULL; cmsg = CMSG_NXTHDR(&h, cmsg)) {
+    if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SO_RXQ_OVFL) {
+      packets_dropped_by_kernel = *(uint32_t *) CMSG_DATA(cmsg);
+    }
+  }
+
+  if (packets_dropped_by_kernel) {
+    handle->packets_dropped_by_kernel = packets_dropped_by_kernel;
+  }
+
+#endif
+
   return size == -1 ? uv_translate_sys_error(errno) : size;
 }
+
+#if defined(__linux__)
+int
+udx__udp_set_rxq_ovfl (int fd) {
+  int on = 1;
+  return setsockopt(fd, SOL_SOCKET, SO_RXQ_OVFL, &on, sizeof(on));
+}
+#else
+int
+udx__udp_set_rxq_ovfl (int fd) {
+  UDX_UNUSED(fd);
+  return -1;
+}
+#endif

src/io_win.c

@@ -81,3 +81,9 @@ udx__recvmsg (udx_socket_t *socket, uv_buf_t *buf, struct sockaddr *addr, int ad
 
   return bytes;
 }
+
+int
+udx__udp_set_rxq_ovfl (int fd) {
+  UDX_UNUSED(fd);
+  return -1;
+}

src/udx.c

@@ -1938,6 +1938,9 @@ udx_socket_init (udx_t *udx, udx_socket_t *socket) {
   socket->packets_rx = 0;
   socket->packets_tx = 0;
 
+  socket->packets_dropped_by_kernel = -1;
+  socket->cmsg_wanted = false;
+
   uv_udp_t *handle = &(socket->handle);
   udx__queue_init(&socket->send_queue);
 
@@ -2027,6 +2030,12 @@ udx_socket_bind (udx_socket_t *socket, const struct sockaddr *addr, unsigned int
   err = uv_poll_init_socket(socket->udx->loop, poll, fd);
   assert(err == 0);
 
+  err = udx__udp_set_rxq_ovfl(fd);
+  if (!err) {
+    socket->cmsg_wanted = true;
+    socket->packets_dropped_by_kernel = 0;
+  }
+
   socket->status |= UDX_SOCKET_BOUND;
   poll->data = socket;
 

test/stream-write-read-perf.c

@@ -149,5 +149,10 @@ main () {
   printf("stats: stream a: bytes_rx=%" PRIu64 " packets_rx=%" PRIu64 " bytes_tx=%" PRIu64 " packets_tx=%" PRIu64 "\n", astream.bytes_rx, astream.packets_rx, astream.bytes_tx, astream.packets_tx);
   printf("stats: stream b: bytes_rx=%" PRIu64 " packets_rx=%" PRIu64 " bytes_tx=%" PRIu64 " packets_tx=%" PRIu64 "\n", bstream.bytes_rx, bstream.packets_rx, bstream.bytes_tx, bstream.packets_tx);
 
+  if (asock.packets_dropped_by_kernel != -1 && bsock.packets_dropped_by_kernel != -1) {
+    printf("stats: socket a: packets_dropped=%" PRIi64 "\n", asock.packets_dropped_by_kernel);
+    printf("stats: socket b: packets_dropped=%" PRIi64 "\n", bsock.packets_dropped_by_kernel);
+  }
+
   return 0;
 }