netdev - Re: [PATCH v5] net: sch_generic: Use pfifo_fast as fallback scheduler for CAN hardware

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Message-Id: <20191025.192045.1462505833001638832.davem@davemloft.net>
Date:   Fri, 25 Oct 2019 19:20:45 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     vincent.prince.fr@...il.com
Cc:     mkl@...gutronix.de, dave.taht@...il.com, jhs@...atatu.com,
        jiri@...nulli.us, kernel@...gutronix.de, linux-can@...r.kernel.org,
        netdev@...r.kernel.org, xiyou.wangcong@...il.com
Subject: Re: [PATCH v5] net: sch_generic: Use pfifo_fast as fallback
 scheduler for CAN hardware

From: Vincent Prince <vincent.prince.fr@...il.com>
Date: Wed, 23 Oct 2019 15:44:20 +0200

> 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
> 
> Suggested-by: Marc Kleine-Budde <mkl@...gutronix.de>
> Signed-off-by: Marc Kleine-Budde <mkl@...gutronix.de>
> Signed-off-by: Vincent Prince <vincent.prince.fr@...il.com>
> Acked-by: Dave Taht <dave.taht@...il.com>

Applied to net-next.