netdev - Re: alignment faults in 3.6

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Date:	Thu, 11 Oct 2012 14:51:21 +0200
From:	Maxime Bizon <mbizon@...ebox.fr>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Måns Rullgård <mans@...sr.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	netdev@...r.kernel.org, David Laight <David.Laight@...LAB.COM>,
	Jon Masters <jonathan@...masters.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: alignment faults in 3.6

On Thu, 2012-10-11 at 13:54 +0200, Eric Dumazet wrote:

> Of course, if you use a forwarding setup, and the tx driver is not SG
> capable, performance will be bad (You have to copy the data into a
> single skb (linearize the skb)) 
> 
> But in 2012, having to use hardware without SG for a router sounds a bad
> joke (if cpu speed is _also_ too low)

Hey I cannot go back in time, when that hardware was built in 2004 (mips
@250Mhz), it was considered good, and we did manufacture a lot of it, so
it's still maintained.

People run recent kernels on older hardware because they are *encouraged
to do so*.

I fought inside my company to be good kernel citizen, not using
proprietary BSP, rewrite & mainline the drivers, because that was the
community promise: mainline it, we will support it for you, you will get
the latest kernel features for free.

That worked, but with some drawbacks:

 - kernel footprint grew that much (we started from 2.4) that it does
not fit in device flash anymore

 - performance took a hit each time we upgrade, mostly because of cache
footprint growth.

 - as kernel footprint grew, available RAM for conntrack & route cache
entries was smaller each time

But I had to stop upgrading after 2.6.20. Everything below is not
anybody's fault. Bloat is unavoidable for software project that big.

I'm perfectly ok with that, but I don't want to be ridiculed for running
mainline kernel on old hardware.

> Adding get_unaligned() everywhere in linux network stacks is not an
> option.
> 
> We actually want to be able to read the code and fix the bugs, not only
> run it on a cheap low end router.

That was not a request, I just needed a clarification. 

Documentation/unaligned-memory-access.txt does not say it's a big no-no,
it says you can give unaligned pointers to the networking stack if you
arch can do unaligned access (with an "efficiency" notion).

MIPS and ARM have a software handler for this, and performance wise in
my case it's better to take the faults, a driver writer may think a
benchmark will dictate what to do.

-- 
Maxime

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