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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 27 Jun 2023 16:02:40 -0700
Message-ID: <CAHk-=wgtrxkRDR95y=U685LMd_ZR5TrLW0_LGoQi3v8Or11Yrw@mail.gmail.com>
Subject: Re: [GIT PULL] x86/misc for 6.5
To:     Arjan van de Ven <arjan@linux.intel.com>
Cc:     Borislav Petkov <bp@alien8.de>,
        Noah Goldstein <goldstein.w.n@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        x86-ml <x86@kernel.org>, lkml <linux-kernel@vger.kernel.org>
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On Tue, 27 Jun 2023 at 15:51, Arjan van de Ven <arjan@linux.intel.com> wrote:
>
> I'm not surprised though; running 2 parallel streams (where one stream has a fixed zero as input,
> so can run OOO any time) .. can really have a performance change like this

How much do people care?

One of the advantages of just having that single "update_csum_40b()"
function is that it's trivial to then manually unroll.

With a 4-way unrolling, I get

  02: 184.0 / 184.5 cycles (8b414316) Upcoming linux kernel version
  04: 184.0 / 184.2 cycles (8b414316) Specialized to size 40
  06: 89.4 / 102.5 cycles (512daed6) New version
  22: 184.6 / 184.4 cycles (8b414316) Odd-alignment handling removed

but doesn't most network hardware do the csum on its own anyway? How
critical is csum_partial(), really?

(The above is obviously your test thing modified for 1500 byte
packets, still. With 40-byte packets, the 4-way unrolling obvious
doesn't help, although it doesn't noticeably hurt either - it's just
one more compare and branch)

                  Linus