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

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