netdev - Fighting out-of-order reception with RPS?

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Date:	Wed, 08 Jul 2015 19:49:00 +0200
From:	Oliver Hartkopp <socketcan@...tkopp.net>
To:	netdev@...r.kernel.org, therbert.google.com@...r.kernel.org
CC:	"linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
	sunil.kovvuri@...il.com, jonathon.reinhart@...il.com
Subject: Fighting out-of-order reception with RPS?

I'm picking up the request 'Setting RPS affinities from network driver' from
Sunil Kovvuri http://marc.info/?t=142424023500001&r=1&w=2 as I assume to have
the same issue here.

When receiving CAN frames from a specific CAN network interface (e.g. can0)
the frames are sporadically out-of-order on SMP systems like my Core i7 laptop
with 4 CPUs. This out-of-order reception kills reliable communication e.g. for
CAN transport protocols.

First approach was to set the smp_affinity for the USB adapter on irq 28 with:

	echo 1 > /proc/irq/28/smp_affinity

This worked in my case but it looks wrong to pin the USB host adapter to a
single CPU and has to be done by hand depending on where the CAN interfaces
are attached to the system.

Next idea was to use RPS after reading Documentation/networking/scaling.txt

As the only relevant flow identifiction is the number of the incoming CAN
interface I added

	skb_set_hash(skb, dev->ifindex, PKT_HASH_TYPE_L2);

when creating CAN skbs with alloc_can_skb() in drivers/net/can/dev.c

After enabling RPS for my four CPUs with

	echo f > /sys/class/net/can0/queues/rx-0/rps_cpus

I had no more out-of-order frames in my system :-)

My two questions:

1. Is there any better solution to meet the described requirements?
2. If not: How can enable this RPS solution by default for CAN interfaces?

Best regards,
Oliver

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