netdev - TCP performance regression

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Date:	Mon, 11 Nov 2013 11:00:21 +0530
From:	Sujith Manoharan <sujith@...jith.org>
To:	Eric Dumazet <edumazet@...gle.com>
CC:	netdev@...r.kernel.org
Subject: TCP performance regression

Hi,

The commit, "tcp: TSQ can use a dynamic limit" causes a large
performance drop in TCP transmission with the wireless driver ath9k.

With a 2-stream card (AR9462), the usual throughput is around 195 Mbps.
But, with this commit, it drops to ~125 Mbps, occasionally reaching 130.

If the commit is reverted, performance is normal again and I can get
190+ Mbps. Apparently, ath10k is also affected and a 250 Mbps drop
is seen (from an original 740 Mbps).

I am using Linville's wireless-testing tree.

>From the test machine:

root@...ux-test ~# uname -a
Linux linux-test 3.12.0-wl-nodebug #104 SMP PREEMPT Mon Nov 11 10:27:56 IST 2013 x86_64 GNU/Linux

root@...ux-test ~# tc -d -s qdisc show dev wlan0 
qdisc mq 0: root 
 Sent 342682272 bytes 226366 pkt (dropped 0, overlimits 0 requeues 0) 
 backlog 0b 0p requeues 0 

root@...ux-test ~# zgrep -i net_sch /proc/config.gz
CONFIG_NET_SCHED=y
CONFIG_NET_SCH_CBQ=m
CONFIG_NET_SCH_HTB=m
CONFIG_NET_SCH_HFSC=m
CONFIG_NET_SCH_ATM=m
CONFIG_NET_SCH_PRIO=m
CONFIG_NET_SCH_MULTIQ=m
CONFIG_NET_SCH_RED=m
CONFIG_NET_SCH_SFB=m
CONFIG_NET_SCH_SFQ=m
CONFIG_NET_SCH_TEQL=m
CONFIG_NET_SCH_TBF=m
CONFIG_NET_SCH_GRED=m
CONFIG_NET_SCH_DSMARK=m
CONFIG_NET_SCH_NETEM=m
CONFIG_NET_SCH_DRR=m
CONFIG_NET_SCH_MQPRIO=m
CONFIG_NET_SCH_CHOKE=m
CONFIG_NET_SCH_QFQ=m
CONFIG_NET_SCH_CODEL=m
CONFIG_NET_SCH_FQ_CODEL=m
CONFIG_NET_SCH_FQ=m
CONFIG_NET_SCH_INGRESS=m
# CONFIG_NET_SCH_PLUG is not set
CONFIG_NET_SCH_FIFO=y


If more information is required, please let me know.

Sujith
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