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

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Date:   Tue, 22 Oct 2019 16:55:26 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Vincent Prince <vincent.prince.fr@...il.com>
Cc:     jiri@...nulli.us, dave.taht@...il.com, netdev@...r.kernel.org,
        jhs@...atatu.com, linux-can@...r.kernel.org, kernel@...gutronix.de,
        xiyou.wangcong@...il.com, davem@...emloft.net
Subject: Re: [PATCH v2] net: sch_generic: Use pfifo_fast as fallback scheduler
 for CAN hardware

On 10/22/19 4:53 PM, Marc Kleine-Budde wrote:
> On 10/22/19 3:23 PM, Vincent Prince wrote:
>> Signed-off-by: Vincent Prince <vincent.prince.fr@...il.com>
> 
> 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--------

Doh, also include the footnote:

-------->8-------->8-------->8-------->8-------->8-------->8-------->8--------
[1] https://github.com/systemd/systemd/issues/9194
-------->8-------->8-------->8-------->8-------->8-------->8-------->8--------

Marc

-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
Industrial Linux Solutions        | Phone: +49-231-2826-924     |
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