linux-kernel - RE: Optimising csum

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Date:   Tue, 22 Nov 2022 16:55:27 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Willy Tarreau' <w@....eu>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: RE: Optimising csum_fold()

From: Willy Tarreau <w@....eu>
> Sent: 22 November 2022 16:25
> 
> On Tue, Nov 22, 2022 at 01:08:23PM +0000, David Laight wrote:
> > There are currently 20 copies of csum_fold(), some in C some in assembler.
> > The default C version (in asm-generic/checksum.h) is pretty horrid.
> > Some of the asm versions (including x86 and x86-64) aren't much better.
> >
> > There are 3 pretty good C versions:
> >   1:	(~sum - rol32(sum, 16)) >> 16
> >   2:  ~(sum + rol32(sum, 16)) >> 16
> >   3:  (u16)~((sum + rol32(sum, 16)) >> 16)
> > All three are (usually) 4 arithmetic instructions.
> >
> > The first two have the advantage that the high bits are zero.
> > Relevant when the value is being checked rather than set.
> >
> > The first one can generate better instruction scheduling (the rotate
> > and invert can be executed in the same clock).
> >
> > The 3rd one saves an instruction on arm, but may need masking.
> > (I've not compiled an arm kernel to see how often that happens.)
> >
> > The only architectures where (I think) the current asm code is better
> > than the C above are sparc and sparc64.
> > Sparc doesn't have a rotate instruction, but does have a carry flag.
> > This makes the current asm version one instruction shorter.
> >
> > For architectures like mips and risc-v which have neither rotate
> > instructions nor carry flags the C is as good as the current asm.
> > The rotate is 3 instructions - the same as the extra cmp+add.
> >
> > Changing everything to use [1] would improve quite a few architectures
> > while only adding 1 clock to some paths in arm/arm64 and sparc.
> >
> > Unfortunately it is all currently a mess.
> > Most architectures don't include asm-generic/checksum.h at all.
> >
> > Thoughts?
> 
> Then why not just have one version per arch, the most efficient one,
> and use it everywhere ? The simple fact that we're discussing the
> tradeoffs means that if we don't want to compromise performance here
> (which I assume to be the case), then it needs to be per-arch and
> that's all. At least that's the way I understand it.

At the moment there are a lot of arch-specific ones that are
definitely sub-optimal.

I started doing some patches, my x86-64 kernel in about 4k
smaller with [1].
I was going to post the patches to asm-generic an x86.

	David

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