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Mon, 24 Feb 2020 09:38:11 -0800 (PST) Received: from [192.168.43.177] ([109.126.137.65]) by smtp.gmail.com with ESMTPSA id t187sm151681wmt.25.2020.02.24.09.38.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 24 Feb 2020 09:38:10 -0800 (PST) To: Jens Axboe , Andres Freund Cc: io-uring@vger.kernel.org References: <20200224010754.h7sr7xxspcbddcsj@alap3.anarazel.de> <20200224033352.j6bsyrncd7z7eefq@alap3.anarazel.de> <90097a02-ade0-bc9a-bc00-54867f3c24bc@kernel.dk> <20200224071211.bar3aqgo76sznqd5@alap3.anarazel.de> <933f2211-d395-fa84-59ae-0b2e725df613@kernel.dk> <20200224165334.tvz5itodcizpfkmw@alap3.anarazel.de> <14eb8dcb-d5ba-9e0a-697d-e4b8fbad3f08@kernel.dk> 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: Deduplicate io_*_prep calls? Message-ID: <706b2cb9-331f-f2d4-135b-ec9abae7e10e@gmail.com> Date: Mon, 24 Feb 2020 20:37:19 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: <14eb8dcb-d5ba-9e0a-697d-e4b8fbad3f08@kernel.dk> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="vPO5x3teOkzsN7oRK9Cj5sjEGKUrg5aFA" Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --vPO5x3teOkzsN7oRK9Cj5sjEGKUrg5aFA Content-Type: multipart/mixed; boundary="pfuM8ygpRrI2aYO2nFaSfzpIDp6z6kpBl"; protected-headers="v1" From: Pavel Begunkov To: Jens Axboe , Andres Freund Cc: io-uring@vger.kernel.org Message-ID: <706b2cb9-331f-f2d4-135b-ec9abae7e10e@gmail.com> Subject: Re: Deduplicate io_*_prep calls? References: <20200224010754.h7sr7xxspcbddcsj@alap3.anarazel.de> <20200224033352.j6bsyrncd7z7eefq@alap3.anarazel.de> <90097a02-ade0-bc9a-bc00-54867f3c24bc@kernel.dk> <20200224071211.bar3aqgo76sznqd5@alap3.anarazel.de> <933f2211-d395-fa84-59ae-0b2e725df613@kernel.dk> <20200224165334.tvz5itodcizpfkmw@alap3.anarazel.de> <14eb8dcb-d5ba-9e0a-697d-e4b8fbad3f08@kernel.dk> In-Reply-To: <14eb8dcb-d5ba-9e0a-697d-e4b8fbad3f08@kernel.dk> --pfuM8ygpRrI2aYO2nFaSfzpIDp6z6kpBl Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 24/02/2020 20:19, Jens Axboe wrote: > On 2/24/20 9:53 AM, Andres Freund wrote: >> Hi, >> >> On 2020-02-24 08:40:16 -0700, Jens Axboe wrote: >>> Agree that the first patch looks fine, though I don't quite see why >>> you want to pass in opcode as a separate argument as it's always >>> req->opcode. Seeing it separate makes me a bit nervous, thinking that= >>> someone is reading it again from the sqe, or maybe not passing in >>> the right opcode for the given request. So that seems fragile and it >>> should go away. >> >> Without extracting it into an argument the compiler can't know that >> io_kiocb->opcode doesn't change between the two switches - and therefo= re >> is unable to merge the switches. >> >> To my knowledge there's no easy and general way to avoid that in C, >> unfortunately. const pointers etc aren't generally a workaround, even >> they were applicable here - due to the potential for other pointers >> existing, the compiler can't assume values don't change. With >> sufficient annotations of pointers with restrict, pure, etc. one can g= et >> it there sometimes. >> >> Another possibility is having a const copy of the struct on the stack,= >> because then the compiler often is able to deduce that the value >> changing would be undefined behaviour. >> >> >> I'm not sure that means it's worth going for the separate argument - I= >> was doing that mostly to address your concern about the duplicated >> switch cost. >=20 > Yeah I get that, but I don't think that's worth the pain. An alternativ= e > solution might be to make the prep an indirect call, and just pair it > with some variant of INDIRECT_CALL(). This would be trivial, as the > arguments should be the same, and each call site knows exactly what > the function should be. And unfortunately, instead of a nice jump table or offset near-jump, it'l= l generate a mess. Though, the attempt itself may prove useful by clarifyin= g handler/core/etc API. --=20 Pavel Begunkov --pfuM8ygpRrI2aYO2nFaSfzpIDp6z6kpBl-- --vPO5x3teOkzsN7oRK9Cj5sjEGKUrg5aFA Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEE+6JuPTjTbx479o3OWt5b1Glr+6UFAl5UCc8ACgkQWt5b1Glr +6XbEA//Xl09ceHJg32+TBKb+ONBdtYXko/x73PIw1zsR49BOaobUuUINTMYBtj4 n1bYVrkGyOtepJHhO8uhFC9Kj0j5h78INUyeKS9B1qRZ54acubSLO+j8vkltNsjM WgMGI6EnqXHhqOUMo47Wvi4r8g2J9Zes33jhAy90XB34WPqHElDiuVCARCubaB/E 0tncAPJzlA+DGc1uZMhxwu1RvdHktxY7+zta2vCX0XWqtdWO2wpATfVeDPBIE+9A rR0z/0cUxkyxCgu+bp2AQ1pJKj/rCbgaKU6EFmaZs7QyFt02eV3CqeHds1BQAgRt lIOxEuxbYNccT/J7C+scnXmGoJYaAeR15wA8O80P/KvpQwg56tgR8fARAG93E7JK l09LAaOpZUtFYUXt0i0i/wYhvx6/UCjBQFpot3aRASZPLsXvmG/Pbz5pKQjrhwfv WUoqvRQjNlp1OctUSJBqkBwsJV0lE5vM5vCJlg+p2kctjRP8VUFdLGrXs9ZKe1dq cdSEH/EvK7XNMMssx917uQxDh/cV7KlVCw/osVidEcdDn0wqfGF5ytTDAAzCadsC V3BrzzVwdv3aOgcn4lvpupLd69jCqLZ7jF892lxwgFPLuZlntCISL9gxXGf9FepP LbqErqaP7g8obg2nGVvcT36ilpBSEiGtr2ikVQdAtsVmSFSwXCM= =043u -----END PGP SIGNATURE----- --vPO5x3teOkzsN7oRK9Cj5sjEGKUrg5aFA--