On Tue, Mar 13, 2018 at 11:24 AM, Jakob Unterwurzacher <jakob.unterwurzac...@theobroma-systems.com> 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.fastab...@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.fastab...@gmail.com> >> Signed-off-by: David S. Miller <da...@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.
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 http://www.teklibre.com Tel: 1-669-226-2619
