netdev - Re: [PATCH v2] net-tcp: TCP/IP stack bypass for loopback connections

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Message-ID: <1345722015.5904.675.camel@edumazet-glaptop>
Date:	Thu, 23 Aug 2012 13:40:15 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Pádraig Brady <P@...igBrady.com>
Cc:	"Bruce \"Brutus\" Curtis" <brutus@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v2] net-tcp: TCP/IP stack bypass for loopback
 connections

On Thu, 2012-08-23 at 11:57 +0100, Pádraig Brady wrote:

> Just to quantify the loopback testing compat issue.
> I often do stuff like the following to test latency.
> Will that be impacted?
> 
>   tc qdisc add dev lo root handle 1:0 netem delay 20msec
> 

Yes this will. At least for tcp traffic this wont "work".

TCP friends bypass layers, by directly queuing skbs to sockets.

-> no iptables, 
   no qdisc (by default there is no qdisc on lo),
   no loopback stats (ifconfig lo).
   some SNMP stats missing as well (netstat -s)

> As for the stated tcpdump change,
> I don't suppose it would be possible to dynamically
> disable this (for new connections at least)
> while lo is being dumped?

This could be done, but it might be better to let the admin globally
enable/disable TCP friends...



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