netdev - Re: [PATCH] af_packet: Don't use skb after dev_queue

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Date:	Mon, 18 Jan 2010 22:50:48 +0100
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Michael Breuer <mbreuer@...jas.com>
Cc:	Stephen Hemminger <shemminger@...ux-foundation.org>,
	David Miller <davem@...emloft.net>, akpm@...ux-foundation.org,
	flyboy@...il.com, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH] af_packet: Don't use skb after dev_queue_xmit()

On Mon, Jan 18, 2010 at 04:24:52PM -0500, Michael Breuer wrote:
> On 1/18/2010 4:00 PM, Stephen Hemminger wrote:
> >On Mon, 18 Jan 2010 15:56:45 -0500
> >Michael Breuer<mbreuer@...jas.com>  wrote:
> >
> >>>>2. The dropped tx packet (DHCP) is a bit harder to recreate, but it
> >>>>still happens.
> >>>>
> >You might want to use tc filter rule to set priority of DHCP packets
> >higher. This would cause them to be in a separate queue and eliminate
> >the problem.
> >
> Ok - for fun, tried that - no change. Not sure I see why this might
> be a factor. The packet loss happens when TX load is low and RX
> high.
> Also,  packets only being dropped if traversing a router vs.to  the
> router itself.  Keep in mind that pings to the router did not lose
> packets, pings through the router lost packets. The router was not
> under load (traffic is being generated from a device connected via
> the 1Gb switch, not the wifi router), and tcpdump on the router
> input port shows the pings to the router, but not the ones through
> the router.
> 
> One added note, when I just tried this, the test data ended while
> the packet loss was occurring. The DHCPOFFER packet loss did not
> clear until about a minute after the throughput abated. I really
> think something is getting hosed, and I'd but some weird interaction
> with the arp logic high on the list of suspects. Not sure what else
> would be a factor when looking at the extra hop on the same subnet.

Good point! Actually, IIRC, your setup might be a problem: you seem
to have two switches on the path (I guess the router is a bridge for
these wireless), so I wonder if it's not something between them.

Jarek P.
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