netdev - Re: [PATCH RFC] net: tcp: GRO should be ECN friendly

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Date:	Mon, 06 Aug 2012 13:41:18 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH RFC] net: tcp: GRO should be ECN friendly

From: Herbert Xu <herbert@...dor.apana.org.au>
Date: Mon, 6 Aug 2012 17:01:50 +0800

> On Mon, Aug 06, 2012 at 10:34:50AM +0200, Eric Dumazet wrote:
>> From: Eric Dumazet <edumazet@...gle.com>
>> 
>> While doing TCP ECN tests, I discovered GRO was reordering packets if it
>> receives one packet with CE set, while previous packets in same NAPI run
>> have ECT(0) for the same flow :
>> 
>> 09:25:25.857620 IP (tos 0x2,ECT(0), ttl 64, id 27893, offset 0, flags
>> [DF], proto TCP (6), length 4396)
>>     172.30.42.19.54550 > 172.30.42.13.44139: Flags [.], seq
>> 233801:238145, ack 1, win 115, options [nop,nop,TS val 3397779 ecr
>> 1990627], length 4344
>> 
>> 09:25:25.857626 IP (tos 0x3,CE, ttl 64, id 27892, offset 0, flags [DF],
>> proto TCP (6), length 1500)
>>     172.30.42.19.54550 > 172.30.42.13.44139: Flags [.], seq
>> 232353:233801, ack 1, win 115, options [nop,nop,TS val 3397779 ecr
>> 1990627], length 1448
>> 
>> 09:25:25.857638 IP (tos 0x0, ttl 64, id 34581, offset 0, flags [DF],
>> proto TCP (6), length 64)
>>     172.30.42.13.44139 > 172.30.42.19.54550: Flags [.], cksum 0xac8f
>> (incorrect -> 0xca69), ack 232353, win 1271, options [nop,nop,TS val
>> 1990627 ecr 3397779,nop,nop,sack 1 {233801:238145}], length 0
>> 
>> We have two problems here :
>> 
>> 1) GRO reorders packets
>> 
>>   If NIC gave packet1, then packet2, which happen to be from "different
>> flows"  GRO feeds stack with packet2, then packet1. I have yet to
>> understand how to solve this problem.
>> 
>> 2) GRO is not ECN friendly
>> 
>> Delivering packets out of order makes TCP stack not as fast as it could
>> be.
>> 
>> In this patch I suggest we make the tos test not part of the 'same_flow'
>> determination, but part of the 'should flush' logic
>> 
>> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> 
> Acked-by: Herbert Xu <herbert@...dor.apana.org.au>
> 
> Good catch, thanks Eric!

Applied to net-next, thanks Eric.
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