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Sat, 30 Nov 2019 16:04:59 -0800 (PST) To: Arvind Sankar Cc: Jens Axboe , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <20191130220458.GA297712@rani.riverdale.lan> From: Pavel Begunkov Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Subject: Re: [PATCH v2] block: optimise bvec_iter_advance() Message-ID: <74c78d40-8b0c-2e87-d882-8396411a7be5@gmail.com> Date: Sun, 1 Dec 2019 03:04:36 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.0 MIME-Version: 1.0 In-Reply-To: <20191130220458.GA297712@rani.riverdale.lan> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="CHTm4B0gVqLHJKOFLU00v8vDHQ3itIuQZ" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --CHTm4B0gVqLHJKOFLU00v8vDHQ3itIuQZ Content-Type: multipart/mixed; boundary="nRL8HNEAHx3wbdctGV9uh9llvEhqWdbVD"; protected-headers="v1" From: Pavel Begunkov To: Arvind Sankar Cc: Jens Axboe , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Message-ID: <74c78d40-8b0c-2e87-d882-8396411a7be5@gmail.com> Subject: Re: [PATCH v2] block: optimise bvec_iter_advance() References: <20191130220458.GA297712@rani.riverdale.lan> In-Reply-To: <20191130220458.GA297712@rani.riverdale.lan> --nRL8HNEAHx3wbdctGV9uh9llvEhqWdbVD Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 01/12/2019 01:05, Arvind Sankar wrote: > On Sat, Nov 30, 2019 at 11:23:52PM +0300, Pavel Begunkov wrote: >> bvec_iter_advance() is quite popular, but compilers fail to do proper >> alias analysis and optimise it good enough. The assembly is checked >> for gcc 9.2, x86-64. >> >> - remove @iter->bi_size from min(...), as it's always less than @bytes= =2E >> Modify at the beginning and forget about it. >> >> - the compiler isn't able to collapse memory dependencies and remove >> writes in the loop. Help it by explicitly using local vars. >> >> Signed-off-by: Pavel Begunkov >> --- >> >> v2: simplify code (Arvind Sankar) >> >=20 > Thanks :) >=20 > Signed-off-by: Arvind Sankar >=20 > Btw, I discovered that gcc 9.2 doesn't optimize away the second > comparison in something like That's unfortunate. IDK, whether it could be easily done in gcc, but maybe relay it to compiler guys? >=20 > m =3D min(a,b); > return m>a; >=20 > So the WARN_ONCE bit doesn't get optimized away even in cases like > bio_for_each_bvec where it's guaranteed at compile-time to not trigger.= >=20 That's more like a starting point. The idea is to revise and tune/rewrite iteration helpers including *for_each_bvec(). I assume, all those are really poorly optimisable (especially with -fno-strict-aliasing). So, noted to consider --=20 Pavel Begunkov --nRL8HNEAHx3wbdctGV9uh9llvEhqWdbVD-- --CHTm4B0gVqLHJKOFLU00v8vDHQ3itIuQZ Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEE+6JuPTjTbx479o3OWt5b1Glr+6UFAl3jA5oACgkQWt5b1Glr +6V2vA//Q9Y17xFB7dLIXrxr8SSpVI/+BIg0h0ySn/HcPiW9T4I8anzmPkXCx0FP 0tt1dl2UhlZN+PwudjQnePNOnO4l29XyfU5UYc78aecm3n68zJJ8xQqPq8n1gZH8 Hx1GoHarsrWIXAR6d1XRM+fesHRdm7SbpIomblooCGfS0dXlx0u7gQLDM5sstQXX VOhTipaDpgX/qFyYkce3+fF46JnVwgJevEoYSjPPpmrXqnek0l8RovObjVu5WZYN uAlbmKnDhfpuBrz95lkz2WwobUPdu8Y2U9WYaXXB4pjvnXSH3+jArcTTBhk6/Fie nbqshXb8R6jkSx6lt7RUe+HHjblzqi+sYGaN7sCEJT4Uawk6V/hQf/sw6yONb9ZD ElxvCtTpUntwK5SI1MPogz052wMF/5jklU++9OkQGbP3HU6BcsoSXo96ezklqwx7 8WqRSr7D9RpBFcLXjFL8UNh0W2ZKh6npyi0Viq5nPVshst3ZkFR7oG+EYpNP3tSi UgZ3NswymZPPM9a8Dk/W4jzLxjP1QZOO76V+0+DoxxRoJoJ+Q1mRsdkFPRF+G+Nj IQyE1fckYgiOppobJ7N29t4veIJEWpGTkx23PQyo1vmCs+wuZCCHFbpjQstuOun0 IuifOqdh+z7AzlgU0gRtjpGee9pnbizDDOkpZdjIYzXtmXsOFFI= =tmSe -----END PGP SIGNATURE----- --CHTm4B0gVqLHJKOFLU00v8vDHQ3itIuQZ--