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Re: [bug, bisected] pfifo_fast causes packet reordering

On Tue, Mar 13, 2018 at 11:24 AM, Jakob Unterwurzacher
<jakob.unterwurzacher@xxxxxxxxxxxxxxxxxxxxx> wrote:
> During stress-testing our "ucan" USB/CAN adapter SocketCAN driver on Linux
> v4.16-rc4-383-ged58d66f60b3 we observed that a small fraction of packets are
> delivered out-of-order.
> We have tracked the problem down to the driver interface level, and it seems
> that the driver's net_device_ops.ndo_start_xmit() function gets the packets
> handed over in the wrong order.
> This behavior was not observed on Linux v4.15 and I have bisected the
> problem down to this patch:
>> commit c5ad119fb6c09b0297446be05bd66602fa564758
>> Author: John Fastabend <john.fastabend@xxxxxxxxx>
>> Date:   Thu Dec 7 09:58:19 2017 -0800
>>    net: sched: pfifo_fast use skb_array
>>    This converts the pfifo_fast qdisc to use the skb_array data structure
>>    and set the lockless qdisc bit. pfifo_fast is the first qdisc to
>> support
>>    the lockless bit that can be a child of a qdisc requiring locking. So
>>    we add logic to clear the lock bit on initialization in these cases
>> when
>>    the qdisc graft operation occurs.
>>    This also removes the logic used to pick the next band to dequeue from
>>    and instead just checks a per priority array for packets from top
>> priority
>>    to lowest. This might need to be a bit more clever but seems to work
>>    for now.
>>    Signed-off-by: John Fastabend <john.fastabend@xxxxxxxxx>
>>    Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> The patch does not revert cleanly, but moving to one commit earlier makes
> the problem go away.
> Selecting the "fq" scheduler instead of "pfifo_fast" makes the problem go
> away as well.

I am of course, a fan of obsoleting pfifo_fast. There's no good reason
for it anymore.

> Is this an unintended side-effect of the patch or is there something the
> driver has to do to request in-order delivery?
> Thanks,
> Jakob


Dave Täht
CEO, TekLibre, LLC
Tel: 1-669-226-2619