netdev - Re: [PATCH net] net: Allow flow dissector to handle non 4-byte aligned headers

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Date:	Wed, 3 Feb 2016 12:59:47 -0500
From:	Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:	Tom Herbert <tom@...bertland.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Kernel Team <kernel-team@...com>
Subject: Re: [PATCH net] net: Allow flow dissector to handle non 4-byte
 aligned headers

On (02/03/16 09:51), Tom Herbert wrote:
> > (a) quite noisy
> 
> Try disabling the crash dump. That will improve performance.

huh?? 

there is no crash dump involved. If you meant "disable dump_stack()"
sure, I am aware of that, and that is the default behavior of 
log_unaligned(). I was just trying to be helpful and provide
stack traces (I dropped out quite a few, which come from mld, 
ip_fast_csum() etc, which log_unaligned rate-limits and suppresses
by default, btw)

(Removing the batteries from my fire-alarm doesnt make the fire 
go away :-))

> But as we said it's only for tunnels that specifically encapsulate an
> ethernet header with aligning it. Many other encapsulations (e.g.
> IPIP, GUE, EtherIP,IP/GRE) should be fine. We could take this to IETF
> and point out that alignment is still relevant in protocol
> development. We can't fix this for GRE or VXLAN at this point, but
> maybe there's still hope for VXLAN-GPE or Geneve...

good point about taking to ietf, but the list above is not accurate.
IP/GRE itself generated a few log_unaligned() warnings for me, I'd
have to sift through it carefully - need some time for that.. 

> Right, but there is a big difference between a performance degradation
> and a hard failure. It would at least be nice to know what the
> performance hit actually is, if it's acceptable then this would be a
> far simpler and much less invasive fix than the alternatives.

--Sowmini