linux-kernel - Re: [GIT PULL] CRC updates for 6.14

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Message-ID: <CAHk-=wiVRnaD5zrJHR=022H0g9CXb15OobYSjOwku3m54Vyb4A@mail.gmail.com>
Date: Thu, 23 Jan 2025 12:52:30 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Eric Biggers <ebiggers@...nel.org>
Cc: "Theodore Ts'o" <tytso@....edu>, linux-crypto@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>, Chao Yu <chao@...nel.org>, 
	"Darrick J. Wong" <djwong@...nel.org>, Geert Uytterhoeven <geert@...ux-m68k.org>, 
	Kent Overstreet <kent.overstreet@...ux.dev>, 
	"Martin K. Petersen" <martin.petersen@...cle.com>, Michael Ellerman <mpe@...erman.id.au>, 
	Vinicius Peixoto <vpeixoto@...amp.dev>, 
	WangYuli <wangyuli@...sls0nwwnnilyahiblcmlmlcaoki5s.yundunwaf1.com>
Subject: Re: [GIT PULL] CRC updates for 6.14

On Thu, 23 Jan 2025 at 10:18, Eric Biggers <ebiggers@...nel.org> wrote:
>
> FWIW, benchmarking the CRC library functions is easy now; just enable
> CONFIG_CRC_KUNIT_TEST=y and CONFIG_CRC_BENCHMARK=y.
>
> But, it's just a traditional benchmark that calls the functions in a loop, and
> doesn't account for dcache thrashing.

Yeah. I suspect the x86 vector version in particular is just not even
worth it. If you have the crc instruction, the basic arch-optimized
case is presumably already pretty good (and *that* code is tiny).

Honestly, I took a quick look at the "by-4" and "by-8" cases, and
considering that you still have to do per-byte lookups of the words
_anyway_, I would expect that the regular by-1 is presumably not that
much worse.

IOW, maybe we could try to just do the simple by-1 for the generic
case, and cut the x86 version down to the simple "use crc32b" case.
And see if anybody even notices...

              Linus