netdev - Re: [PATCH 1/2] net: sch_generic: add flag IFF_FIFO_QUEUE to use pfifo

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Date:   Wed, 27 Mar 2019 10:14:17 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, linux-can@...r.kernel.org,
        kernel@...gutronix.de, Dave Taht <dave.taht@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>
Subject: Re: [PATCH 1/2] net: sch_generic: add flag IFF_FIFO_QUEUE to use
 pfifo_fast as default scheduler

On Wed, Mar 27, 2019 at 9:56 AM Marc Kleine-Budde <mkl@...gutronix.de> 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.