netdev - Re: [QUESTION] poor TX performance on new GbE driver

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Date:   Sun, 22 Oct 2017 20:34:55 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     "<netdev@...r.kernel.org>" <netdev@...r.kernel.org>
Subject: Re: [QUESTION] poor TX performance on new GbE driver

On 22 October 2017 at 20:27, Florian Fainelli <f.fainelli@...il.com> wrote:
> On 10/22/2017 12:14 PM, Ard Biesheuvel wrote:
>> Hello all,
>>
>> I am working on upstreaming a network driver for a Socionext SoC, and
>> I am having some trouble figuring out why my TX performance is
>> horrible when booting a Debian Stretch rootfs, while booting a Ubuntu
>> 17.04 rootfs works absolutely fine. Note that this is using the exact
>> same kernel image, booted off the network.
>>
>> Under Ubuntu, I get the following iperf results from the box to my AMD
>> Seattle based devbox with a 1 Gbit switch in between. (The NIC in
>> question is also 1 Gbit)
>>
>>
>> $ sudo iperf -c dogfood.local -r
>> ------------------------------------------------------------
>> Server listening on TCP port 5001
>> TCP window size: 85.3 KByte (default)
>> ------------------------------------------------------------
>> ------------------------------------------------------------
>> Client connecting to dogfood.local, TCP port 5001
>> TCP window size:  748 KByte (default)
>> ------------------------------------------------------------
>> [  5] local 192.168.1.112 port 51666 connected with 192.168.1.106 port 5001
>> [ ID] Interval       Transfer     Bandwidth
>> [  5]  0.0-10.0 sec  1.07 GBytes   920 Mbits/sec
>> [  4] local 192.168.1.112 port 5001 connected with 192.168.1.106 port 33048
>> [  4]  0.0-10.0 sec  1.10 GBytes   940 Mbits/sec
>>
>> Booting the *exact* same kernel into a Debian based rootfs results in
>> the following numbers
>> $ sudo iperf -c dogfood.local -r
>> ------------------------------------------------------------
>> Server listening on TCP port 5001
>> TCP window size: 85.3 KByte (default)
>> ------------------------------------------------------------
>> ------------------------------------------------------------
>> Client connecting to dogfood.local, TCP port 5001
>> TCP window size: 85.0 KByte (default)
>> ------------------------------------------------------------
>> [  5] local 192.168.1.112 port 40132 connected with 192.168.1.106 port 5001
>> [ ID] Interval       Transfer     Bandwidth
>> [  5]  0.0-10.1 sec  4.12 MBytes  3.43 Mbits/sec
>> [  4] local 192.168.1.112 port 5001 connected with 192.168.1.106 port 33068
>> [  4]  0.0-10.0 sec  1.10 GBytes   939 Mbits/sec
>>
>> The ifconfig stats look perfectly fine to me (TX errors 0  dropped 0
>> overruns 0  carrier 0  collisions 0). During the TX test, the CPUs are
>> almost completely idle. (This system has 24 cores, but not
>> particularly powerful ones.)
>>
>> This test is based on v4.14-rc4, but v4.13 gives the same results.
>>
>> Could anyone please shed a light on this? What tuning parameters
>> and/or stats should I be looking at? I am a seasoned kernel developer
>> but a newbie when it comes to networking, so hopefully I just need a
>> nudge to go looking in the right place.

Hi Florian,

Thanks for your response.

>
> You could look at /proc/net/snmp and see if you get higher level TCP/IP
> drops. The second run appears to be fine, is it possible that somehow
> your TX ring starts in a invalid state of some sort, TX activity cleans
> it up during the first run and the second run operates under normal
> condition?

The 'second' run is the opposite direction, due to the '-r' parameter.
I only included it for reference, since it works fine in both cases.

> At first glance I can't think of any sensible difference
> between the two rootfs that would explain what happens but it might be
> worth comparing /proc/sys/net between the two and spot possible TCP
> parameters differences.
>

Right, I can check that.

> How is UDP doing in your test cases? Once your image is loaded
> everything should be in the page cache already so there should not be
> any heavy NFS activity while you run your tests right?

I don't use NFS, only the kernel image is booted off the network, but
the rootfses are actually on SSDs

> You might also
> want to try to take a perf capture of the first run and see where and
> how packets may be dropped: perf record -g -e skb:kfree_skb iperf -c ..
> may help here.

OK, that looks interesting - let me try that.

Thanks a lot!