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b=bvFCvDvGwjhK1bqLysdZpbWxrPf+zUMo22+IJ0Rn8J1oJQYzEwqbUalud9GdrM14T dTcuOehBuie3jS5zN+ZxCwZ0nJBEyjtys/YlAcjk2FD/xLF2npSenBpjf/DPZVef2H 6HZGStknwpqLzNP+acWCwkoWAzCnYfJAfhYBRGaM= Date: Mon, 17 Feb 2025 19:13:51 -0800 From: Andrew Morton To: "Colin King (gmail)" Cc: Matthew Wilcox , linux-mm@kvack.org, kernel-janitors@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH][next] mm/mincore: improve performance by adding an unlikely hint Message-Id: <20250217191351.0003a6f07d017d80762fae41@linux-foundation.org> In-Reply-To: <08e4a7fc-bd06-4c8c-96e2-84991c4ce891@gmail.com> References: <20250217170934.457266-1-colin.i.king@gmail.com> <08e4a7fc-bd06-4c8c-96e2-84991c4ce891@gmail.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 36981A0008 X-Rspamd-Server: rspam12 X-Stat-Signature: h1mxomzs395pwbzq9uy1z1gdt81kctwe X-HE-Tag: 1739848434-762458 X-HE-Meta: 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 Rsa6W2vh mhwSfMFbjtYFcRme51y3Xelq69j4gy0hd0Zw+oLXkLz2Z9PUUOsTxGC+ESdIgOr0+q7X46UE5ws+M2uQ6vUBGy5Dzlr0vkf3mkDHIxyPmkBPQsbpjx2JMYWYGwOurSJHSYZyR/yt+hlZZu7qwVFfjv4KlGXRe4BepfXOEOoHKaOcklPNhM96K7HCv4Cw0kEiQ6V9entBMR35kn/befv2fiADjXhErW+96tSTMoxG/3eJ4dFW1E3j/F/un0w/0sy6VIlZB8nuzaOv4nN3XInDKgZTPH5goJl8PGoU3jENteVgRnU40jX6uhyFwyb6E3r+mDOGF3iaoeNHR8PhxBaFERSWKyA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, 17 Feb 2025 18:00:22 +0000 "Colin King (gmail)" wrote: > fOn 17/02/2025 17:58, Matthew Wilcox wrote: > > On Mon, Feb 17, 2025 at 05:09:34PM +0000, Colin Ian King wrote: > >> Adding an unlikely() hint on the masked start comparison error > >> return path improves run-time performance of the mincore system call. > >> > >> Benchmarking on an i9-12900 shows an improvement of 7ns on mincore calls > >> on a 256KB mmap'd region where 50% of the pages we resident. > >> > >> Results based on running 20 tests with turbo disabled (to reduce > >> clock freq turbo changes), with 10 second run per test and comparing > >> the number of mincores calls per second. The % standard deviation of > >> the 20 tests was ~0.10%, so results are reliable. > > > > I think you've elided _just_ enough information here that nobody can > > judge whether your stats skills are any good ;-) You've told us 7ns > > (per call, presumably) and you've told us 0.10% standard deviation, > > but you haven't told us how long the syscall takes, so nobody can tell > > whether 7ns is within 0.10% or not ;-) > > Ugh, my bad. > > Improvement was from ~970 down to 963 ns, so small ~0.7% improvement. > It actually doesn't change the generated code: hp2:/usr/src/25> diff -u mm/mincore.lst.old mm/mincore.lst --- mm/mincore.lst.old 2025-02-17 19:11:34.093727411 -0800 +++ mm/mincore.lst 2025-02-17 19:12:59.797009056 -0800 @@ -1563,7 +1563,7 @@ start = untagged_addr(start); /* Check the start address: needs to be page-aligned.. */ - if (start & ~PAGE_MASK) + if (unlikely(start & ~PAGE_MASK)) b27: 31 ff xor %edi,%edi asm (ALTERNATIVE("", b29: 90 nop