RE: [EXT] Re: [PATCH net-next] net: ethernet: fec: prevent tx starvation under high rx load

["Tobias Waldekranz" <tobias@waldekranz.com>]

On Tue Jun 30, 2020 at 10:26 AM CEST, Andy Duan wrote:
> From: Tobias Waldekranz <tobias@waldekranz.com> Sent: Tuesday, June 30,
> 2020 3:31 PM
> > On Tue Jun 30, 2020 at 8:27 AM CEST, Andy Duan wrote:
> > > From: Tobias Waldekranz <tobias@waldekranz.com> Sent: Tuesday, June
> > > 30,
> > > 2020 12:29 AM
> > > > On Sun Jun 28, 2020 at 8:23 AM CEST, Andy Duan wrote:
> > > > > I never seem bandwidth test cause netdev watchdog trip.
> > > > > Can you describe the reproduce steps on the commit, then we can
> > > > > reproduce it on my local. Thanks.
> > > >
> > > > My setup uses a i.MX8M Nano EVK connected to an ethernet switch, but
> > > > can get the same results with a direct connection to a PC.
> > > >
> > > > On the iMX, configure two VLANs on top of the FEC and enable IPv4
> > > > forwarding.
> > > >
> > > > On the PC, configure two VLANs and put them in different namespaces.
> > > > From one namespace, use trafgen to generate a flow that the iMX will
> > > > route from the first VLAN to the second and then back towards the
> > > > second namespace on the PC.
> > > >
> > > > Something like:
> > > >
> > > >     {
> > > >         eth(sa=PC_MAC, da=IMX_MAC),
> > > >         ipv4(saddr=10.0.2.2, daddr=10.0.3.2, ttl=2)
> > > >         udp(sp=1, dp=2),
> > > >         "Hello world"
> > > >     }
> > > >
> > > > Wait a couple of seconds and then you'll see the output from fec_dump.
> > > >
> > > > In the same setup I also see a weird issue when running a TCP flow
> > > > using iperf3. Most of the time (~70%) when i start the iperf3 client
> > > > I'll see ~450Mbps of throughput. In the other case (~30%) I'll see
> > > > ~790Mbps. The system is "stably bi-modal", i.e. whichever rate is
> > > > reached in the beginning is then sustained for as long as the session is kept
> > alive.
> > > >
> > > > I've inserted some tracepoints in the driver to try to understand
> > > > what's going
> > > > on:
> > > > https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsv
> > > > gsha
> > re.com%2Fi%2FMVp.svg&amp;data=02%7C01%7Cfugang.duan%40nxp.com%
> > > >
> > 7C12854e21ea124b4cc2e008d81c59d618%7C686ea1d3bc2b4c6fa92cd99c5c
> > > >
> > 301635%7C0%7C0%7C637290519453656013&amp;sdata=by4ShOkmTaRkFfE
> > > > 0xJkrTptC%2B2egFf9iM4E5hx4jiSU%3D&amp;reserved=0
> > > >
> > > > What I can't figure out is why the Tx buffers seem to be collected
> > > > at a much slower rate in the slow case (top in the picture). If we
> > > > fall behind in one NAPI poll, we should catch up at the next call (which we
> > can see in the fast case).
> > > > But in the slow case we keep falling further and further behind
> > > > until we freeze the queue. Is this something you've ever observed? Any
> > ideas?
> > >
> > > Before, our cases don't reproduce the issue, cpu resource has better
> > > bandwidth than ethernet uDMA then there have chance to complete
> > > current NAPI. The next, work_tx get the update, never catch the issue.
> > 
> > It appears it has nothing to do with routing back out through the same
> > interface.
> > 
> > I get the same bi-modal behavior if just run the iperf3 server on the iMX and
> > then have it be the transmitting part, i.e. on the PC I run:
> > 
> >     iperf3 -c $IMX_IP -R
> > 
> > I would be very interesting to see what numbers you see in this scenario.
> I just have on imx8mn evk in my hands, and run the case, the numbers is
> ~940Mbps
> as below.
>
> root@imx8mnevk:~# iperf3 -s
> -----------------------------------------------------------
> Server listening on 5201
> -----------------------------------------------------------
> Accepted connection from 10.192.242.132, port 43402
> [ 5] local 10.192.242.96 port 5201 connected to 10.192.242.132 port
> 43404
> [ ID] Interval Transfer Bitrate Retr Cwnd
> [ 5] 0.00-1.00 sec 109 MBytes 913 Mbits/sec 0 428 KBytes
> [ 5] 1.00-2.00 sec 112 MBytes 943 Mbits/sec 0 447 KBytes
> [ 5] 2.00-3.00 sec 112 MBytes 941 Mbits/sec 0 472 KBytes
> [ 5] 3.00-4.00 sec 113 MBytes 944 Mbits/sec 0 472 KBytes
> [ 5] 4.00-5.00 sec 112 MBytes 942 Mbits/sec 0 472 KBytes
> [ 5] 5.00-6.00 sec 112 MBytes 936 Mbits/sec 0 472 KBytes
> [ 5] 6.00-7.00 sec 113 MBytes 945 Mbits/sec 0 472 KBytes
> [ 5] 7.00-8.00 sec 112 MBytes 944 Mbits/sec 0 472 KBytes
> [ 5] 8.00-9.00 sec 112 MBytes 941 Mbits/sec 0 472 KBytes
> [ 5] 9.00-10.00 sec 112 MBytes 940 Mbits/sec 0 472 KBytes
> [ 5] 10.00-10.04 sec 4.16 MBytes 873 Mbits/sec 0 472 KBytes
> - - - - - - - - - - - - - - - - - - - - - - - - -
> [ ID] Interval Transfer Bitrate Retr
> [ 5] 0.00-10.04 sec 1.10 GBytes 939 Mbits/sec 0 sender

Are you running the client with -R so that the iMX is the transmitter?
What if you run the test multiple times, do you get the same result
each time?