netdev - RE: [PATCH net-next] loopback: sctp: add NETIF_F_SCTP

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Date:	Mon, 24 Feb 2014 13:24:58 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Daniel Borkmann' <dborkman@...hat.com>
CC:	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>
Subject: RE: [PATCH net-next] loopback: sctp: add NETIF_F_SCTP_CSUM to
 device features

From: Daniel Borkmann
> On 02/24/2014 11:42 AM, David Laight wrote:
> ...
> > I'm sure it shouldn't be that expensive, you are implying that it spent
> > about 70% of the time doing crc32.
> 
> In this scenario, the following perf log I get that shows where cycles
> are being spent on my machine:
> 
>   65.95%       netperf  [kernel.kallsyms]  [k] __crc32c_le

WTF is that function doing!
Even doing the crc naively bit by bit shouldn't manage to use 65%.

Maybe the _le has something to do with it.
Could it be bit-reversing the crc and data bytes all the time?
The packet will (one would hope) want the crc in the same bit-order
as the data, so no bit reversal is needed - just the correct logic
and lookup table.

Which architecture and which version of crc32_le() does your kernel use?
(I'd guess there are several lurking).
Whichever function you are using wants killing.

	David

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