netdev - Re: UDP multicast packet loss not reported if TX ring overrun?

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Date:	Tue, 25 Aug 2009 15:35:34 -0700
From:	Sridhar Samudrala <sri@...ibm.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	David Stevens <dlstevens@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	netdev-owner@...r.kernel.org, niv@...ux.vnet.ibm.com,
	sri@...ux.vnet.ibm.com
Subject: Re: UDP multicast packet loss not reported if TX ring overrun?

On Tue, 2009-08-25 at 15:15 -0400, Christoph Lameter wrote:
> On Tue, 25 Aug 2009, David Stevens wrote:
> 
> > Christoph Lameter <cl@...ux-foundation.org> wrote on 08/25/2009 06:48:24
> > AM:
> >
> > > On Mon, 24 Aug 2009, Sridhar Samudrala wrote:
> >
> > > > If we count these drops as qdisc drops, should we also count them as
> > IP OUTDISCARDS?
> > >
> > > Yes.
> >
> > Actually, no. (!)
> >
> > IP_OUTDISCARDS should count the packets IP dropped, not
> > anything dropped at a lower layer (which, in general, it
> > is not aware of). If you count these in multiple layers,
> > then you don't really know who dropped it.
> 
> You are right. I skipped that IP OUTDICARDS reference. They need to be
> accounted at the qdisc level though.

Yes. Now that we agree that drops at dev_queue_xmit level should be counted
under qdisc stats, the following patch should address 1 of the 3 places where
NET_XMIT_DROP is returned, but qdisc drop stats is not incremented.
The other 2 places are in ipsec output functions esp_output and esp6_output.
I am not sure where these drops should be accounted.

Could you check if the UDP packet losses you are seeing are accounted for in
qdisc drops with this patch. But i am not completely positive on this as this
case happens only if qdisc is deactivated.

diff --git a/net/core/dev.c b/net/core/dev.c
index 6a94475..8b6a075 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1864,8 +1864,7 @@ gso:
 		spin_lock(root_lock);
 
 		if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
-			kfree_skb(skb);
-			rc = NET_XMIT_DROP;
+			rc = qdisc_drop(skb, q);
 		} else {
 			rc = qdisc_enqueue_root(skb, q);
 			qdisc_run(q);

Thanks
Sridhar

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