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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 changes attach_one_default_qdisc() to use pfifo_fast for "ARPHRD_CAN" network devices. 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 network device type is "ARPHRD_CAN". [1] https://github.com/systemd/systemd/issues/9194 Signed-off-by: Vincent Prince <vincent.prince.fr@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.