netdev - Re: [PATCH net-next 1/1] net: usb: qmi_wwan: add default rx_urb

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Date:   Wed, 4 Nov 2020 18:01:34 +0100
From:   Kristian Evensen <kristian.evensen@...il.com>
To:     Daniele Palmas <dnlplm@...il.com>
Cc:     Bjørn Mork <bjorn@...k.no>,
        Paul Gildea <paul.gildea@...il.com>,
        Carl Yin <carl.yin@...ctel.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        linux-usb@...r.kernel.org
Subject: Re: [PATCH net-next 1/1] net: usb: qmi_wwan: add default rx_urb_size

Hi,

On Wed, Sep 9, 2020 at 11:14 AM Daniele Palmas <dnlplm@...il.com> wrote:
>
> Add default rx_urb_size to support QMAP download data aggregation
> without needing additional setup steps in userspace.
>
> The value chosen is the current highest one seen in available modems.
>
> The patch has the side-effect of fixing a babble issue in raw-ip mode
> reported by multiple users.
>
> Signed-off-by: Daniele Palmas <dnlplm@...il.com>
> ---
> Resending with mailing lists added: sorry for the noise.
>
> Hi Bjørn and all,
>
> this patch tries to address the issue reported in the following threads
>
> https://www.spinics.net/lists/netdev/msg635944.html
> https://www.spinics.net/lists/linux-usb/msg198846.html
> https://www.spinics.net/lists/linux-usb/msg198025.html
>
> so I'm adding the people involved, maybe you can give it a try to
> double check if this is good for you.
>
> On my side, I performed tests with different QC chipsets without
> experiencing problems.
>
> Thanks,
> Daniele

First of all, I am very sorry for not providing any feedback earlier.
I applied your patch and have been running it on my devices more or
less since it was submitted. My devices are equipped with different
generations of modems (cat. 4, cat. 6, cat. 12, 5G NSA), and I haven't
noticed any problems and the babble-issue is gone. Over the last
couple of days I also finally had a chance to experiment with QMAP,
using an SDX55-based modem (i..e,32KB datagram support). Increasing
the datagram size to 32KB gives a nice performance boost over for
example 16KB. When measuring using iperf3 (on the same device), the
throughput goes from around 210 Mbit/s and to 230 Mbit/s. The CPU was
more or less saturated during all of my experiments, so the main
performance gain was from the increased aggregated datagram size.

As a side question, and perhaps this should be a separate thread, does
anyone have any suggestion on how to improve QMI performance further?
The device that I used for my iperf3-tests is mt7621-based, and using
for example an Ethernet dongle I am able to reach somere between 400
and 500 Mbit/s over USB. The Ethernet dongle is able to make use of
for example scatter-gather, but I would still expect at least a bit
more using QMI. I tried to replace the alloc()/put() in the
qmimux_rx_fixup() function with clone() and then doing push()/pull(),
but this resulted in a decrease in performance. I have probably
overlooked something, but I think at least my use of the functions was
correct. The packets looked correct when adding some debug output,
error counters did not increase, etc., etc. The mobile network is not
the bottleneck, on my phone I reliably get around 400 Mbit/s.

BR,
Kristian