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

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Date:	Wed, 26 Aug 2009 15:09:49 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Sridhar Samudrala <sri@...ibm.com>
cc:	David Stevens <dlstevens@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	niv@...ux.vnet.ibm.com
Subject: Re: UDP multicast packet loss not reported if TX ring overrun?

On Wed, 26 Aug 2009, Sridhar Samudrala wrote:

> >  They are reported for IP and UDP.
> Not clear what you meant by this.

The SNMP and UDP statistics show the loss. qdisc level does not show the
loss.

> > root@...strategy3-deb64:/home/clameter#tc -s qdisc show
> > qdisc pfifo_fast 0: dev eth0 root bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1
> > 1 1 1 1
> >  Sent 6208 bytes 64 pkt (dropped 0, overlimits 0 requeues 0)
> >  rate 0bit 0pps backlog 0b 0p requeues 0
>
> Even the Sent count seems to be too low. Are you looking at the right
> device?

I would think that tc displays all queues? It says eth0 and eth0 is the
device that we sent the data out on.


> So based on the current analysis, the packets are getting dropped after
> the call to ip_local_out() in ip_push_pending_frames(). ip_local_out()
> is failing with NET_XMIT_DROP. But we are not sure where they are
> getting dropped. Is that right?

ip_local_out is returning ENOBUFS. Something at the qdisc layer is
dropping the packet and not incrementing counters.

> I think we need to figure out where they are getting dropped and then
> decide on the appropriate counter to be incremented.

Right. Where in the qdisc layer do drops occur?

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