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Tue, 20 Oct 2026 05:06:48 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface X-Archived-By: MIME-Version: 1.0 X-ThreadId: ATLhSvAouVZo Date: Tue, 20 Oct 2026 11:06:06 +0200 From: "Arnd Bergmann" To: "Marc Kleine-Budde" Cc: "Ian Rogers" , "Thomas Gleixner" , "Andy Lutomirski" , "Vincenzo Frascino" , "Stefan Kerkmann" , "Christophe Leroy" , "David Laight" , "Arnaldo Carvalho de Melo" , "Namhyung Kim" , "Eric Biggers" , "Alexander Viro" , "Jason A . Donenfeld" , linux-arm-kernel@lists.infradead.org, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org Message-Id: In-Reply-To: <20261020-zealous-warping-quetzal-84479a-mkl@pengutronix.de> References: <20261019201257.3805356-1-irogers@google.com> <31fe09f6-0a14-40fc-8ad8-c03998fb1f5c@app.fastmail.com> <20261020-succinct-zebu-of-vigor-6cb2cd-mkl@pengutronix.de> <20261020-zealous-warping-quetzal-84479a-mkl@pengutronix.de> Subject: Re: [PATCH v1 1/2] vdso: Add VDSO_UNALIGNED_PACKED config option for ARMv5 Content-Type: text/plain Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20261020_020703_731842_C2224043 X-CRM114-Status: GOOD ( 21.51 ) X-Spam-Score: -2.1 (--) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Tue, Oct 20, 2026, at 10:41, Marc Kleine-Budde wrote: > On 20.10.2026 10:24:51, Arnd Bergmann wrote: >> On Tue, Oct 20, 2026, at 08:44, Marc Kleine-Budde wrote: >> >> That would be better, but I still hope we can just get back to the >> portable code across all architectures for the in-kernel version. >> >> I have one more request for you: can you check /proc/cpu/alignment >> on the system that shows the performance regression with and without >> the patch? > > I asked Stefan Kerkmann (who wrote the original regression report) the > same question, but we haven't had time for it yet. We'll get around to > it. Ok, thanks! >> In your original report, you have shown that Ian's patch >> caused the drop in performance and that it caused the added function >> calls, but I don't see any sufficiently hot code path that would >> actually explain the scale of the regression, > > Don't underestimate the overhead of function calls on those old CPUs > with tiny I-caches. What I meant is that I tried to look for calls to get_unaligned() in the code path you'd hit with UBIFS on nand flash, and I only found a possible call once per page written (ebu_nand_write_page_hwecc), which should have a much lower overhead than what you observe. While ubifs does use crc32, it uses the lib/crc/ version rather than the crypto/crc32c.c that you listed, and there is no get_unaligned() in the lib/ one. If you have NOR flash, the caller is likely do_write_buffer() calling map_word_load(). That one could explain the problem and could also be optimized further if it turns out to be the bottleneck. Arnd