netdev - Re: [bug, bisected] pfifo_fast causes packet reordering

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Date:   Mon, 19 Mar 2018 07:07:05 +0100
From:   Alexander Stein <alexander.stein@...tec-electronic.com>
To:     Jakob Unterwurzacher <jakob.unterwurzacher@...obroma-systems.com>
Cc:     John Fastabend <john.fastabend@...il.com>,
        Dave Taht <dave.taht@...il.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        "linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
        Martin Elshuber <martin.elshuber@...obroma-systems.com>
Subject: Re: [bug, bisected] pfifo_fast causes packet reordering

On Friday, March 16, 2018, 11:26:47 AM CET Jakob Unterwurzacher wrote:
> On 15.03.18 23:30, John Fastabend wrote:
> >> I have reproduced it using two USB network cards connected to each other. The test tool sends UDP packets containing a counter and listens on the other interface, it is available at
> >> https://github.com/jakob-tsd/pfifo_stress/blob/master/pfifo_stress.py
> >>
> > 
> > Great thanks, can you also run this with taskset to bind to
> > a single CPU,
> > 
> >   # taskset 0x1 ./pifof_stress.py
> > 
> > And let me know if you still see the OOO.
> 
> Interesting. Looks like it depends on which core it runs on. CPU0 is 
> clean, CPU1 is not.
> 
> Clean:	taskset --cpu-list 0 ./pfifo_stress.py
> 
> Broken:	taskset --cpu-list 1 ./pfifo_stress.py
> 
> Maybe related: CPU0 is where USB interrupts are handled:
> 
> > root@...399-q7:~# cat /proc/interrupts           
> >            CPU0       CPU1       CPU2       CPU3       CPU4       CPU5
> > 217:    2175353          0          0          0          0          0     GICv3 142 Level     xhci-hcd:usb5

This reminds me somewhat of this thread: https://marc.info/?l=linux-can&m=148007442317274&w=2

Best regards,
Alexander