netdev - Re: [PATCH] tcp: autocork should not hold first packet in write queue

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Message-Id: <20131220.175652.1043712694585011764.davem@davemloft.net>
Date:	Fri, 20 Dec 2013 17:56:52 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, willemb@...gle.com, ncardwell@...gle.com
Subject: Re: [PATCH] tcp: autocork should not hold first packet in write
 queue

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 17 Dec 2013 09:58:30 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Willem noticed a TCP_RR regression caused by TCP autocorking
> on a Mellanox test bed. MLX4_EN_TX_COAL_TIME is 16 us, which can be
> right above RTT between hosts.
> 
> We can receive a ACK for a packet still in NIC TX ring buffer or in a
> softnet completion queue.
> 
> Fix this by always pushing the skb if it is at the head of write queue.
> 
> Also, as TX completion is lockless, it's safer to perform sk_wmem_alloc
> test after setting TSQ_THROTTLED.
> 
> erd:~# MIB="MIN_LATENCY,MEAN_LATENCY,MAX_LATENCY,P99_LATENCY,STDDEV_LATENCY"
> erd:~#  ./netperf -H remote -t TCP_RR -- -o $MIB | tail -n 1
> (repeat 3 times)
 ...
> Reported-by: Willem de Bruijn <willemb@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Fixes: f54b311142a9 ("tcp: auto corking")

Applied, but please put "net-next" in the subject line next time.

Thank you.
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