Subject: Re: [PATCH v2] net: sch_generic: Use pfifo_fast as fallback scheduler for CAN hardware

From: Stephen Hemminger <[email-masked]>
Date: Tue, 22 Oct 2019 09:42:54 -0700

On Tue, 22 Oct 2019 16:53:44 +0200
Marc Kleine-Budde <[email-masked]> wrote:

> On 10/22/19 3:23 PM, Vincent Prince wrote:
> > Signed-off-by: Vincent Prince <[email-masked]>  
> 
> please include a patch description. I.e. this one:
> 
> -------->8-------->8-------->8-------->8-------->8-------->8-------->8--------  
> 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".
> -------->8-------->8-------->8-------->8-------->8-------->8-------->8--------  
> 
> Marc
> 

Why not fix fq_codel to return the same errors as other qdisc?