netdev - Re: [RFC] net: sch_generic: fq_codel vs pfifo

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Message-ID: <0edf15df-b607-70f7-5fe8-2860eb663a53@pengutronix.de>
Date:   Wed, 27 Mar 2019 20:24:13 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org,
        Dave Taht <dave.taht@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>, kernel@...gutronix.de,
        Cong Wang <xiyou.wangcong@...il.com>,
        linux-can@...r.kernel.org, davem@...emloft.net
Subject: Re: [RFC] net: sch_generic: fq_codel vs pfifo_fast

On 3/27/19 7:30 PM, Stephen Hemminger wrote:
>> on CAN networking hardware we (the CAN community) experience a lot silent,
>> unwanted frame drops inside the kernel. (See first patch for details.) So
>> here's a patch series to keep pfifo_fast as default scheduler for CAN hardware
>> by default.
> 
> Why do you set fq_codel as default qdisc if you know it doesn't work right for your
> environment. Is this a distro's want one value problem?

This is a many fold problem.

- Consider a random Linux developer attaching one of the mainline
supported USB-CAN adapters to his/her development laptop and doing some
random CAN test. As far a I heard all modern Linux distributions (but
not debian) enable fq_codel per kernel .config or via systemd. The user
will experiences dropped CAN frames on the first 1000 packages. This is
not a good user experience.

- From the user space point of view the sysctl net.core.default_qdisc
behaves a bit strange. Why can I enable a queuing discipline by default
that's known not to work on CAN devices. You might think the kernel
(should) know that CAN devices best work with pfifo_fast (or not
NET_SCHED at all).

- Addressing your question directly: Mixed environment. fq_codel works
great on Ethernet, but terrible on CAN. Of course I an use "tc" to
configure the way I want to. But from my point of view it would be nice,
if the kernel uses sane default...and handles the net.core.default_qdisc
default in a sane way.

Marc

-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
Industrial Linux Solutions        | Phone: +49-231-2826-924     |
Vertretung West/Dortmund          | Fax:   +49-5121-206917-5555 |
Amtsgericht Hildesheim, HRA 2686  | http://www.pengutronix.de   |



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