Workaround for stupid callback scheduler.

There is no reak callback scheduler and previous behavior causes
bad things during hard congestion (like BGP hold timeouts).

Smart callback scheduler is still missing, but main loop was
changed such that it first processes all tx callbacks (which
are fast enough) (but max 4* per socket) + rx callbacks for CLI,
and in the second phase it processes one rx callback per
socket up to four sockets (as rx callback can be slow when
there are too many protocols, because route redistribution
is done synchronously inside rx callback). If there is event
callback ready, second phase is skipped in 90% of iterations
(to speed up CLI during congestion).

lib/socket.h

@@ -51,6 +51,7 @@ void sk_reallocate(sock *);		/* Free and allocate tbuf & rbuf */
 void sk_dump_all(void);
 int sk_set_ttl(sock *s, int ttl);	/* Set TTL for given socket */
 int sk_set_md5_auth(sock *s, ip_addr a, char *passwd);	/* Add or remove security associations for given passive socket */
+int sk_rx_ready(sock *s);
 
 static inline int
 sk_send_buffer_empty(sock *sk)

proto/bgp/bgp.c

@@ -431,8 +431,15 @@ bgp_hold_timeout(timer *t)
 {
   struct bgp_conn *conn = t->data;
 
-  DBG("BGP: Hold timeout, closing connection\n");
-  bgp_error(conn, 4, 0, NULL, 0);
+  DBG("BGP: Hold timeout\n");
+
+  /* If there is something in input queue, we are probably congested
+     and perhaps just not processed BGP packets in time. */
+
+  if (sk_rx_ready(conn->sk) > 0)
+    bgp_start_timer(conn->hold_timer, 10);
+  else
+    bgp_error(conn, 4, 0, NULL, 0);
 }
 
 static void

sysdep/unix/io.c

@@ -30,12 +30,17 @@
 #include "lib/unix.h"
 #include "lib/sysio.h"
 
-/* Maximum number of calls of rx/tx handler for one socket in one
+/* Maximum number of calls of tx handler for one socket in one
  * select iteration. Should be small enough to not monopolize CPU by
  * one protocol instance.
  */
 #define MAX_STEPS 4
 
+/* Maximum number of calls of rx handler for all sockets in one select
+   iteration. RX callbacks are often much more costly so we limit
+   this to gen small latencies */
+#define MAX_RX_STEPS 4
+
 /*
  *	Tracked Files
  */
@@ -493,6 +498,7 @@ tm_format_reltime(char *x, bird_clock_t t)
 
 static list sock_list;
 static struct birdsock *current_sock;
+static struct birdsock *stored_sock;
 static int sock_recalc_fdsets_p;
 
 static inline sock *
@@ -541,6 +547,8 @@ sk_free(resource *r)
       close(s->fd);
       if (s == current_sock)
 	current_sock = sk_next(s);
+      if (s == stored_sock)
+	stored_sock = sk_next(s);
       rem_node(&s->n);
       sock_recalc_fdsets_p = 1;
     }
@@ -1071,6 +1079,29 @@ sk_maybe_write(sock *s)
     }
 }
 
+int
+sk_rx_ready(sock *s)
+{
+  fd_set rd, wr;
+  struct timeval timo;
+  int rv;
+
+  FD_ZERO(&rd);
+  FD_ZERO(&wr);
+  FD_SET(s->fd, &rd);
+
+  timo.tv_sec = 0;
+  timo.tv_usec = 0;
+
+ redo:
+  rv = select(s->fd+1, &rd, &wr, NULL, &timo);
+  
+  if ((rv < 0) && (errno == EINTR || errno == EAGAIN))
+    goto redo;
+
+  return rv;
+}
+
 /**
  * sk_send - send data to a socket
  * @s: socket
@@ -1239,6 +1270,9 @@ io_init(void)
   srandom((int) now_real);
 }
 
+static int short_loops = 0;
+#define SHORT_LOOP_MAX 10
+
 void
 io_loop(void)
 {
@@ -1317,8 +1351,8 @@ io_loop(void)
 	}
 
       /* And finally enter select() to find active sockets */
-
       hi = select(hi+1, &rd, &wr, NULL, &timo);
+
       if (hi < 0)
 	{
 	  if (errno == EINTR || errno == EAGAIN)
@@ -1327,13 +1361,17 @@ io_loop(void)
 	}
       if (hi)
 	{
-	  current_sock = SKIP_BACK(sock, n, HEAD(sock_list));	/* guaranteed to be non-empty */
+	  /* guaranteed to be non-empty */
+	  current_sock = SKIP_BACK(sock, n, HEAD(sock_list));
+
 	  while (current_sock)
 	    {
 	      sock *s = current_sock;
 	      int e;
-	      int steps = MAX_STEPS;
-	      if (FD_ISSET(s->fd, &rd) && s->rx_hook)
+	      int steps;
+
+	      steps = MAX_STEPS;
+	      if ((s->type >= SK_MAGIC) && FD_ISSET(s->fd, &rd) && s->rx_hook)
 		do
 		  {
 		    steps--;
@@ -1356,6 +1394,35 @@ io_loop(void)
 	      current_sock = sk_next(s);
 	    next: ;
 	    }
+
+	  short_loops++;
+	  if (events && (short_loops < SHORT_LOOP_MAX))
+	    continue;
+	  short_loops = 0;
+
+	  int count = 0;
+	  current_sock = stored_sock;
+	  if (current_sock == NULL)
+	    current_sock = SKIP_BACK(sock, n, HEAD(sock_list));
+
+	  while (current_sock && count < MAX_RX_STEPS)
+	    {
+	      sock *s = current_sock;
+	      int e;
+	      int steps;
+
+	      if ((s->type < SK_MAGIC) && FD_ISSET(s->fd, &rd) && s->rx_hook)
+		{
+		  count++;
+		  e = sk_read(s);
+		  if (s != current_sock)
+		      goto next2;
+		}
+	      current_sock = sk_next(s);
+	    next2: ;
+	    }
+
+	  stored_sock = current_sock;
 	}
     }
 }