On 03/13/2018 11:35 AM, Dave Taht wrote: > 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. >>
Is the stress-testing tool available somewhere? What type of packets are being sent? >> 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. > Is this a single queue device or a multiqueue device? Running 'tc -s qdisc show dev foo' would help some. > 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? >> If we introduced a OOO edge case somewhere that was not intended so I'll take a look into it. But, if you can provide a bit more details on how stress testing is done to cause the issue that would help. Thanks, John >> Thanks, >> Jakob > > >
