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

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Date:   Fri, 16 Mar 2018 11:26:47 +0100
From:   Jakob Unterwurzacher <jakob.unterwurzacher@...obroma-systems.com>
To:     John Fastabend <john.fastabend@...il.com>,
        Dave Taht <dave.taht@...il.com>
Cc:     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 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
Thanks,
Jakob