On Mon, 17 Aug 2009, Sridhar Samudrala wrote:

So it is possible that there is some other place in the stack where the packets
are gettting dropped but not counted.

Such a deed occurs in ip_push_pending_frames():

        /* Netfilter gets whole the not fragmented skb. */
        err = ip_local_out(skb);
        if (err) {
                if (err > 0)
                        err = inet->recverr ? net_xmit_errno(err) : 0;
			^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                if (err)
                        goto error;
        }

out:
        ip_cork_release(inet);
        return err;

error:
        IP_INC_STATS(net, IPSTATS_MIB_OUTDISCARDS);
        goto out;


So if ip_local_out returns NET_XMIT_DROP then its simply going to be
replaced by 0. Then we check err again and there is no error!!!!

The statistics are only generated if IP_RECVERR is set.

Could we move the increment of IPSTATS_MIB_OUTDISCARDS up so that it
is incremented regardless of the setting of IP_RECVERR?

F.e?


Subject: Report TX drops

Incrementing of TX drop counters currently does not work if errors from the
network stack are suppressed (IP_RECVERR off). Increment the statistics
independently of the setting of IP_RECVERR.

Signed-off-by: Christoph Lameter <redacted>

---
 net/ipv4/ip_output.c |   19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

Index: linux-2.6/net/ipv4/ip_output.c
===================================================================

--- linux-2.6.orig/net/ipv4/ip_output.c	2009-08-24 17:04:27.000000000 +0000
+++ linux-2.6/net/ipv4/ip_output.c	2009-08-24 17:32:05.000000000 +0000

@@ -1300,20 +1300,21 @@ int ip_push_pending_frames(struct sock *

 	/* Netfilter gets whole the not fragmented skb. */
 	err = ip_local_out(skb);
-	if (err) {
-		if (err > 0)
-			err = inet->recverr ? net_xmit_errno(err) : 0;
-		if (err)
-			goto error;
+	if (err > 0) {
+		/* The packet was dropped by the network subsystem */
+		IP_INC_STATS(net, IPSTATS_MIB_OUTDISCARDS);
+
+		/*
+		 * Errors are not passed on if the socket
+		 * does not process errors (see IP_RECVERR).
+		 * net_xmit_errno filters NET_XMIT_CN.
+		 */
+		err = inet->recverr ? net_xmit_errno(err) : 0;
 	}

 out:
 	ip_cork_release(inet);
 	return err;
-
-error:
-	IP_INC_STATS(net, IPSTATS_MIB_OUTDISCARDS);
-	goto out;
 }

 /*