Messages in this thread | | | From | Jakob Unterwurzacher <> | Subject | [bug, bisected] pfifo_fast causes packet reordering | Date | Tue, 13 Mar 2018 19:24:44 +0100 |
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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@gmail.com> > 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@gmail.com> > Signed-off-by: David S. Miller <davem@davemloft.net>
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.
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
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