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

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Message-ID: <d0d865bd-7bc7-abb5-2b54-2c100c4b9a09@pengutronix.de>
Date:   Tue, 22 Oct 2019 18:48:56 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Vincent Prince <vincent.prince.fr@...il.com>, jiri@...nulli.us,
        jhs@...atatu.com, netdev@...r.kernel.org, dave.taht@...il.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 6:42 PM, Stephen Hemminger wrote:
> On Tue, 22 Oct 2019 16:53:44 +0200
> Marc Kleine-Budde <mkl@...gutronix.de> 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.

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

The head drop is the problem. After a send() system call returned to
user space, one would not expect that a later send() will knock an
earlier from the queue.

It's too late to throttle the package generation, as one frame is lost
already.

Marc

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