linux-kernel - Re: x86/csum: Remove unnecessary odd handling

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Date: Fri, 5 Jan 2024 16:18:43 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: David Laight <David.Laight@...lab.com>
Cc: Noah Goldstein <goldstein.w.n@...il.com>, kernel test robot <lkp@...el.com>, 
	"x86@...nel.org" <x86@...nel.org>, 
	"oe-kbuild-all@...ts.linux.dev" <oe-kbuild-all@...ts.linux.dev>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "edumazet@...gle.com" <edumazet@...gle.com>, 
	"tglx@...utronix.de" <tglx@...utronix.de>, "mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>, 
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "hpa@...or.com" <hpa@...or.com>
Subject: Re: x86/csum: Remove unnecessary odd handling

On Fri, 5 Jan 2024 at 15:53, David Laight <David.Laight@...lab.com> wrote:
>
> I'd have to fix his benchmark code first :-)
> You can't use the TSC unless you lock the cpu frequency.
> The longer the test runs for the faster the cpu will run.

They'll stabilize, it has soem cycle result aging code.

But yes, set the CPU policy to 'performance' or do performance
counters if you care deeply.

> On a related point, do you remember what the 'killer app'
> was for doing the checksum in copy_to/from_user?

No. It's a long time ago, and many things have changed since.

It's possible the copy-and-csum it's not worth it any more, simply
because all modern network cards will do the csum for us, and I think
loopback sets a flag saying "no need to checksum" too.

But I do have a strong memory of it being a big deal back when. A
_loong_ time ago.

          Linus