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

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Message-ID: <alpine.DEB.1.10.0908271139450.14277@gentwo.org>
Date:	Thu, 27 Aug 2009 11:40:44 -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:

> > ip_local_out is returning ENOBUFS. Something at the qdisc layer is
> > dropping the packet and not incrementing counters.
>
> Is the ENOBUFS return with your/Eric's patch? I thought you were
> were seeing NET_XMIT_DROP without any patches.

Both Erics latest patch and your patch were applied.

> > > 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?
>
> The normal path where the packets are dropped when the tx qlen is exceeded is
>   pfifo_fast_enqueue() -> qdisc_drop()
> In this path, drops are counted.
> The other place is in dev_queue_xmit(), but you are not hitting that case too.
>
> So it looks like there is another place where they are getting dropped.

Hmmm.. I need to find more time to dig into this.

Anyways it seems that Eric's latest patch is doing many good things for
packet loss accounting.

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