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Wed, 14 Oct 2026 04:51:27 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ThreadId: AAmuA3xfg9N3 Date: Wed, 14 Oct 2026 10:50:41 +0200 From: "Arnd Bergmann" To: "Andrew Morton" , "Biplab Raut" Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, "Gao Xiang" , "Nick Terrell" , yann.collet.73@gmail.com, guoxuenan@huawei.com, "Nathan Chancellor" , "Nicolas Schier" , "Kees Cook" Message-Id: In-Reply-To: <20261012203257.614d88c12146544aa6487b4d@linux-foundation.org> References: <20261013032010.2453893-1-Biplab.Raut@amd.com> <20261012203257.614d88c12146544aa6487b4d@linux-foundation.org> Subject: Re: [PATCH] lib/lz4: add software-pipelined prefetch to decode fast loop Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On Tue, Oct 13, 2026, at 05:32, Andrew Morton wrote: > On Tue, 13 Oct 2026 03:20:10 +0000 Biplab Raut w= rote: >>=20 >> Kernel LZ4 median scan: 1.0994 s >> Patched median scan: 1.0472 s >> Median paired speedup: 5.09% (range 4.19%=E2=80=936.99%) >> Patched was faster in all 15 rounds. > > Well that's nice. I can't actually recall a case where prefetch made a > measurable difference, although I'm sure there are such cases. > > We used to use prefetches often (10+ years ago) but it seems to fallen > out of favor. This code is a prime candidate. mm/ barely uses > prefetch. Maybe it should, Unfortunately the patch causes a build regression on x86 when using lz4 compressed bzImage files: x86_64-linux-ld: error: unplaced orphan section `.altinstructions' from = `arch/x86/boot/compressed/misc.o' x86_64-linux-ld: error: unplaced orphan section `.altinstr_replacement' = from `arch/x86/boot/compressed/misc.o' I see that the zero-day bot has also reported this[1]. The problem is that the prefetch() helpers use SMP alternatives that are not available in the decompressor. This has been a problem in other patches before [2]. Not sure if there is an easy way to stop the decompressor from using the alternatives here, for 7.4 I assume the only option is to drop the patch and try again once that is resolved. Arnd=20 [1] https://lore.kernel.org/all/202610140857.6jRIqXf1-lkp@intel.com/ [2] https://lore.kernel.org/all/CAHk-=3Dwhrc-ruKs4Kt90EGzKd+pYhZFKs6bgBV= CV=3D55BK+p1nzg@mail.gmail.com/