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Tue, 08 Oct 2019 15:05:54 -0700 (PDT) Received: from [192.168.43.219] ([109.126.138.202]) by smtp.gmail.com with ESMTPSA id y19sm210699wmi.13.2019.10.08.15.05.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 08 Oct 2019 15:05:51 -0700 (PDT) To: Jens Axboe , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <936cd758d6c694fe1b8b9de050e24cfecdc2e60d.1570489620.git.asml.silence@gmail.com> <62a8a6c7-9c5b-c9a4-9c73-c77db87c6637@kernel.dk> <99bfb7aa-6980-fc14-32f7-a479dea63eb4@gmail.com> 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] io_uring: remove wait loop spurious wakeups Message-ID: <4004b5bc-edf0-cc8f-8efc-7f848c95f0ba@gmail.com> Date: Wed, 9 Oct 2019 01:05:45 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="yIlVw3XG5ZYr0uTxHwD7mpXh9cod03GJp" 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) --yIlVw3XG5ZYr0uTxHwD7mpXh9cod03GJp Content-Type: multipart/mixed; boundary="V7yMANjARSlJhZwMxRwczcAhzhHvXv8cl"; protected-headers="v1" From: Pavel Begunkov To: Jens Axboe , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Message-ID: <4004b5bc-edf0-cc8f-8efc-7f848c95f0ba@gmail.com> Subject: Re: [PATCH] io_uring: remove wait loop spurious wakeups References: <936cd758d6c694fe1b8b9de050e24cfecdc2e60d.1570489620.git.asml.silence@gmail.com> <62a8a6c7-9c5b-c9a4-9c73-c77db87c6637@kernel.dk> <99bfb7aa-6980-fc14-32f7-a479dea63eb4@gmail.com> In-Reply-To: --V7yMANjARSlJhZwMxRwczcAhzhHvXv8cl Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 09/10/2019 00:22, Jens Axboe wrote: > On 10/8/19 2:58 PM, Pavel Begunkov wrote: >> On 08/10/2019 20:00, Jens Axboe wrote: >>> On 10/8/19 10:43 AM, Pavel Begunkov wrote: >>>> On 08/10/2019 06:16, Jens Axboe wrote: >>>>> On 10/7/19 5:18 PM, Pavel Begunkov (Silence) wrote: >>>>>> From: Pavel Begunkov >>>>>> >>>>>> Any changes interesting to tasks waiting in io_cqring_wait() are >>>>>> commited with io_cqring_ev_posted(). However, io_ring_drop_ctx_ref= s() >>>>>> also tries to do that but with no reason, that means spurious wake= ups >>>>>> every io_free_req() and io_uring_enter(). >>>>>> >>>>>> Just use percpu_ref_put() instead. >>>>> >>>>> Looks good, this is a leftover from when the ctx teardown used >>>>> the waitqueue as well. >>>>> >>>> BTW, is there a reason for ref-counting in struct io_kiocb? I unders= tand >>>> the idea behind submission reference, but don't see any actual part >>>> needing it. >>> >>> In short, it's to prevent the completion running before we're done wi= th >>> the iocb on the submission side. >> >> Yep, that's what I expected. Perhaps I missed something, but what I've= >> seen following code paths all the way down, it either >> 1. gets error / completes synchronously and then frees req locally >> 2. or passes it further (e.g. async list) and never accesses it after >=20 > As soon as the IO is passed on, it can complete. In fact, it can comple= te > even _before_ that call returns. That's the issue. Obviously this isn't= > true for purely polled IO, but it is true for IRQ based IO. And the idea was to not use io_kiocb after submission. Except when we kno= w, that it won't complete asynchronously (e.g. error), that could be checked= with return code, I guess. Anyway, thanks for the explanation! --=20 Yours sincerely, Pavel Begunkov --V7yMANjARSlJhZwMxRwczcAhzhHvXv8cl-- --yIlVw3XG5ZYr0uTxHwD7mpXh9cod03GJp Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEE+6JuPTjTbx479o3OWt5b1Glr+6UFAl2dCD0ACgkQWt5b1Glr +6Xr3w/9F7F/NNIXCXtDSBKW1q/7dmCkK1QBZnywTaKS4tF8tOgxXsDXB8tNn2Yt XGSCxlyQ2IKUiShMsA2DPKIsdyAlwA03kd2jM5FHRLZ3RNAZLwz2nrg5R41Uu9rL WipfLfi2k0gu0wlOQTgGjAlT8pquO7IuGxwznAGqdcoJplAQO56Dh4+mR/q2wvqi uKCmGuGehyH9yHzt8A3a5on81kttgECqxI3H3NRIlEP1hi2K6lyMfa9UcFYcYKLK cg5QUOlbdmCNzRzOnlvGGlAx77leOTHPWEopbuGoC8lIkXhM2aOl2OKB6FSq9v+4 QW+FbCUr3VCoxe19pIjUt2dSEd5JO9+ou9k1DFhWXuYjvQ1j0zzCYhJ2zoGY9707 H4Q1sPY2m057pd5YJaL9Ojm42MGugGIxQsZ4jncsseKqwkBelRlJsgmroJRlBNTT fRlQgmuF+1o6KRcNHPZoRnwUYG08mKOfidsPI/5BSW20fWQx7FqHT8p6leYkMdr+ tX1A6b5hyoTGrLhn2yJ6vfzkxw7FitKPLsi4VytZ9Ruy8QlhdMjN5mVbP84+wdtU 5GbAMTo0/DImBS+7TTh4PodIF5vIFNpHxKu8r2R8AzjnmlJKe9rpv0IrZotM1YWo 3aMeFR78Fj9rhUQg1HLB0DmkUcvmfKrXMKyS/dwCBtwStE1Zp4o= =I4EE -----END PGP SIGNATURE----- --yIlVw3XG5ZYr0uTxHwD7mpXh9cod03GJp--