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Date: Tue, 21 May 2019 12:28:48 +0200
From: Rafał Miłecki <zajec5@...il.com>
To: Network Development <netdev@...r.kernel.org>,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc: linux-block@...r.kernel.org, Felix Fietkau <nbd@....name>,
Jonas Gorski <jonas.gorski@...il.com>,
Jo-Philipp Wich <jo@...n.io>, John Crispin <john@...ozen.org>
Subject: ARM router NAT performance affected by random/unrelated commits
Hi,
I work on home routers based on Broadcom's Northstar SoCs. Those devices
have ARM Cortex-A9 and most of them are dual-core.
As for home routers, my main concern is network performance. That CPU
isn't powerful enough to handle gigabit traffic so all kind of
optimizations do matter. I noticed some unexpected changes in NAT
performance when switching between kernels.
My hardware is BCM47094 SoC (dual core ARM) with integrated network
controller and external BCM53012 switch.
Relevant setup:
* SoC network controller is wired to the hardware switch
* Switch passes 802.1q frames with VID 1 to four LAN ports
* Switch passes 802.1q frames with VID 2 to WAN port
* Linux does NAT for LAN (eth0.1) to WAN (eth0.2)
* Linux uses pfifo and "echo 2 > rps_cpus"
* Ryzen 5 PRO 2500U (x86_64) laptop connected to a LAN port
* Intel i7-2670QM laptop connected to a WAN port
*****
I found a very nice example of commit that does /nothing/ yet it affects
NAT performance: 9316a9ed6895 ("blk-mq: provide helper for setting up an
SQ queue and tag set")
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9316a9ed6895c4ad2f0cde171d486f80c55d8283
All it does is exporting an unused symbol (function).
Let me share some numbers (I use iperf for testing):
git reset --hard v4.19
git am OpenWrt-mtd-chages.patch
[ 3] 0.0-30.0 sec 2.60 GBytes 745 Mbits/sec
[ 3] 0.0-30.0 sec 2.60 GBytes 745 Mbits/sec
[ 3] 0.0-30.0 sec 2.60 GBytes 744 Mbits/sec
[ 3] 0.0-30.0 sec 2.59 GBytes 742 Mbits/sec
[ 3] 0.0-30.0 sec 2.59 GBytes 740 Mbits/sec
[ 3] 0.0-30.0 sec 2.59 GBytes 740 Mbits/sec
[ 3] 0.0-30.0 sec 2.58 GBytes 738 Mbits/sec
[ 3] 0.0-30.0 sec 2.58 GBytes 738 Mbits/sec
[ 3] 0.0-30.0 sec 2.58 GBytes 738 Mbits/sec
[ 3] 0.0-30.0 sec 2.57 GBytes 735 Mbits/sec
Average: 741 Mb/s
git reset --hard v4.19
git am OpenWrt-mtd-chages.patch
git cherry-pick -x 9316a9ed6895
[ 3] 0.0-30.0 sec 2.73 GBytes 780 Mbits/sec
[ 3] 0.0-30.0 sec 2.72 GBytes 777 Mbits/sec
[ 3] 0.0-30.0 sec 2.71 GBytes 775 Mbits/sec
[ 3] 0.0-30.0 sec 2.70 GBytes 773 Mbits/sec
[ 3] 0.0-30.0 sec 2.70 GBytes 771 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 771 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 771 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 770 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 769 Mbits/sec
[ 3] 0.0-30.0 sec 2.68 GBytes 768 Mbits/sec
Average: 773 Mb/s
As you can see cherry-picking (on top of Linux 4.19) a single commit
that does /nothing/ can improve NAT performance by 4,5%.
*****
I was hoping to learn something from profiling kernel with the "perf"
tool. Eanbling CONFIG_PERF_EVENTS resulted in smaller NAT performance
gain: 741 Mb/s → 750 Mb/s. I tried it anyway.
Without cherry-picking I got:
+ 9,04% swapper [kernel.kallsyms] [k] v7_dma_inv_range
+ 5,54% swapper [kernel.kallsyms] [k] __irqentry_text_end
+ 5,12% swapper [kernel.kallsyms] [k] l2c210_inv_range
+ 4,30% ksoftirqd/1 [kernel.kallsyms] [k] v7_dma_clean_range
+ 4,02% swapper [kernel.kallsyms] [k] bcma_host_soc_read32
+ 3,13% swapper [kernel.kallsyms] [k] arch_cpu_idle
+ 2,88% ksoftirqd/1 [kernel.kallsyms] [k] __netif_receive_skb_core
+ 2,51% ksoftirqd/1 [kernel.kallsyms] [k] l2c210_clean_range
+ 1,88% ksoftirqd/1 [kernel.kallsyms] [k] fib_table_lookup
(741 Mb/s while *not* running perf)
With cherry-picked 9316a9ed6895 I got:
+ 9,16% swapper [kernel.kallsyms] [k] v7_dma_inv_range
+ 5,64% swapper [kernel.kallsyms] [k] __irqentry_text_end
+ 5,05% swapper [kernel.kallsyms] [k] l2c210_inv_range
+ 4,25% ksoftirqd/1 [kernel.kallsyms] [k] v7_dma_clean_range
+ 4,10% swapper [kernel.kallsyms] [k] bcma_host_soc_read32
+ 3,35% ksoftirqd/1 [kernel.kallsyms] [k] __netif_receive_skb_core
+ 3,17% swapper [kernel.kallsyms] [k] arch_cpu_idle
+ 2,49% ksoftirqd/1 [kernel.kallsyms] [k] l2c210_clean_range
+ 2,03% ksoftirqd/1 [kernel.kallsyms] [k] fib_table_lookup
(750 Mb/s while *not* running perf)
Changes seem quite minimal and I'm not sure if they tell what is causing
that NAT performance change at all.
*****
I also tried running cachestat but didn't get anything interesting:
Counting cache functions... Output every 1 seconds.
TIME HITS MISSES DIRTIES RATIO BUFFERS_MB CACHE_MB
10:06:59 1020 5 0 99.5% 0 2
10:07:00 1029 0 0 100.0% 0 2
10:07:01 1013 0 0 100.0% 0 2
10:07:02 1029 0 0 100.0% 0 2
10:07:03 1029 0 0 100.0% 0 2
10:07:04 997 0 0 100.0% 0 2
10:07:05 1013 0 0 100.0% 0 2
(I started iperf at 10:07:00).
*****
There were more situations with such unexpected performance changes.
Another example: cherry-picking 5b0890a97204 ("flow_dissector: Parse
batman-adv unicast headers")
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5b0890a97204627d75a333fc30f29f737e2bfad6
to some Linux 4.14.x release was lowering NAT performance by 55 Mb/s.
The tricky part is there aren't any ETH_P_BATMAN packets in my traffic.
Extra tests revealed that any __skb_flow_dissect() modification was
lowering my NAT performance (e.g. commenting out ETH_P_TIPC or
ETH_P_FCOE switch cases).
*****
I would like every kernel to provide a maximum NAT performance, no
matter what random commits it contains.
Suffering from such a random changes makes it also really hard to notice
a real performance regression.
Do you have any idea what is causing those performance changes? Can I
provide any extra info to help debugging this?
View attachment "openwrt-mtd-patches.txt" of type "text/plain" (1435 bytes)
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