On Wed, Mar 27, 2019 at 9:56 AM Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx> wrote: > > There is networking hardware that isn't based on Ethernet for layers 1 and 2. > > For example CAN. > > CAN is a multi-master serial bus standard for connecting Electronic Control > Units [ECUs] also known as nodes. A frame on the CAN bus carries up to 8 bytes > of payload. Frame corruption is detected by a CRC. However frame loss due to > corruption is possible, but a quite unusual phenomenon. > > While fq_codel works great for TCP/IP, it doesn't for CAN. There are a lot of > legacy protocols on top of CAN, which are not build with flow control or high > CAN frame drop rates in mind. > > When using fq_codel, as soon as the queue reaches a certain delay based length, > skbs from the head of the queue are silently dropped. Silently meaning that the > user space using a send() or similar syscall doesn't get an error. However > TCP's flow control algorithm will detect dropped packages and adjust the > bandwidth accordingly. > > When using fq_codel and sending raw frames over CAN, which is the common use > case, the user space thinks the package has been sent without problems, because > send() returned without an error. pfifo_fast will drop skbs, if the queue > length exceeds the maximum. But with this scheduler the skbs at the tail are > dropped, an error (-ENOBUFS) is propagated to user space. So that the user > space can slow down the package generation. > > On distributions, where fq_codel is made default via CONFIG_DEFAULT_NET_SCH > during compile time, or set default during runtime with sysctl > net.core.default_qdisc (see [1]), we get a bad user experience. In my test case > with pfifo_fast, I can transfer thousands of million CAN frames without a frame > drop. On the other hand with fq_codel there is more then one lost CAN frame per > thousand frames. > > As pointed out fq_codel is not suited for CAN hardware, so this patch > introduces a new netdev_priv_flag called "IFF_FIFO_QUEUE" (in contrast to the > existing "IFF_NO_QUEUE"). > > During transition of a netdev from down to up state the default queuing > discipline is attached by attach_default_qdiscs() with the help of > attach_one_default_qdisc(). This patch modifies attach_one_default_qdisc() to > attach the pfifo_fast (pfifo_fast_ops) if the "IFF_FIFO_QUEUE" flag is set. I wonder if we just need to allow arbitrary default qdisc per netdevice while you are on it. A private flag is simply a boolean, perhaps in the future other type of devices wants other default qdiscs, so that could make it more flexible. Just a thought. Thanks.
