linux-kernel - Re: [PATCH] x86_64: inline csum_ipv6

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Message-ID: <CANn89iJzcb_XO9oCApKYfRxsMMmg7BHukRDqWTca3ZLQ8HT0iQ@mail.gmail.com>
Date: Thu, 13 Nov 2025 10:18:08 -0800
From: Eric Dumazet <edumazet@...gle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H . Peter Anvin" <hpa@...or.com>, "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, linux-kernel <linux-kernel@...r.kernel.org>, 
	Simon Horman <horms@...nel.org>, Kuniyuki Iwashima <kuniyu@...gle.com>, netdev@...r.kernel.org, 
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH] x86_64: inline csum_ipv6_magic()

On Thu, Nov 13, 2025 at 8:26 AM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 11/13/25 07:45, Eric Dumazet wrote:
> > Inline this small helper.
> >
> > This reduces register pressure, as saddr and daddr are often
> > back to back in memory.
> >
> > For instance code inlined in tcp6_gro_receive() will look like:
>
> Could you please double check what the code growth is for this across
> the tree? There are 80-ish users of csum_ipv6_magic().

Hi Dave

Sure (allyesconfig build)

Before patch:

size vmlinux
   text    data     bss     dec     hex filename
886947242 245613190 40211540 1172771972 45e71484 vmlinux

After patch:
 size vmlinux
   text    data     bss     dec     hex filename
886947242 245613190 40211540 1172771972 45e71484 vmlinux

I found this a bit surprising, so I did a regular build (our Google
production kernel default config)

Before:

size vmlinux
   text    data     bss     dec     hex filename
34812872 22177397 5685248 62675517 3bc5a3d vmlinux

After:

 size vmlinux
   text    data     bss     dec     hex filename
34812501 22177365 5685248 62675114 3bc58aa vmlinux

So it would seem the patch saves 371 bytes for this config.

>
> Or, is there a discrete, measurable performance gain from doing this?

IPv6 incoming TCP/UDP paths call this function twice per packet, which is sad...
One call per TX packet.

Depending on the cpus I can see csum_ipv6_magic() using up to 0.75 %
of cpu cycles.
Then there is the cost in the callers, harder to measure...

Thank you.