Re: [PATCH v4] net: sch_generic: Use pfifo_fast as fallback scheduler for CAN hardware

[Dave Taht <dave.taht@gmail.com>]

On Wed, Oct 23, 2019 at 3:52 AM Vincent Prince
<vincent.prince.fr@gmail.com> 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 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@pengutronix.de>
> Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
> Signed-off-by: Vincent Prince <vincent.prince.fr@gmail.com>
> ---
> Changes in v4:
>  - add Marc credit to commit log
>
> Changes in v3:
>  - add description
>
> Changes in v2:
>  - reformat patch
>
>  net/sched/sch_generic.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index 77b289d..dfb2982 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -1008,6 +1008,8 @@ static void attach_one_default_qdisc(struct net_device *dev,
>
>         if (dev->priv_flags & IFF_NO_QUEUE)
>                 ops = &noqueue_qdisc_ops;
> +       else if(dev->type == ARPHRD_CAN)
> +               ops = &pfifo_fast_ops;
>
>         qdisc = qdisc_create_dflt(dev_queue, ops, TC_H_ROOT, NULL);
>         if (!qdisc) {
> --
> 2.7.4
>

While I'm delighted to see such a simple patch emerge, openwrt long
ago patched out pfifo_fast. pfifo_fast has
additional semantics not needed in the can use case either (I think)
and "pfifo" is fine, but sure, pfifo_fast if you must.

anyway, regardless, that's an easy fix and I hope this fix goes to
stable, as I've had nightmares about cars exploding due to out of
order can bus operations ever since I learned of this bug.

Acked-by: Dave Taht <dave.taht@gmail.com>

-- 

Dave Täht
CTO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-831-205-9740