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

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Message-ID: <20160203173125.GA14627@oracle.com>
Date:	Wed, 3 Feb 2016 12:31:25 -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 09:07), Tom Herbert wrote:
> > Kernel unaligned access at TPC[9150dc] ipv4_neigh_lookup+0x38/0x170
> 
> Sowmini,
> 
> This doesn't look like a hard crash to me. Instead of trying to fix
> all the alignment issues for Sparc, can we just take the trap, fix up
> the load, and continue without any further fuss? Performance might
> suffer, but it doesn't seem like the bad alignments are happening in
> critical paths.
> 

None of these things is a hard crash, but they are 

(a) quite noisy 
(b) I'm able to generate alignment falls merely by configuring
    tunnels, and it gets worse when I disable RSS and use RFS/RPS
    instead. So "critical path" might need some definition.
(c) a perf risk for other platforms as well, even when they dont
    complain noisily about it.

--Sowmini