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Mon, 23 Sep 2019 09:37:53 -0700 (PDT) Received: from [192.168.43.245] ([91.135.68.130]) by smtp.gmail.com with ESMTPSA id s24sm2470091edx.5.2019.09.23.09.37.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Sep 2019 09:37:53 -0700 (PDT) Subject: Re: [PATCH v2 1/2] sched/wait: Add wait_threshold To: Peter Zijlstra Cc: Jens Axboe , Ingo Molnar , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <20190923071932.GD2349@hirez.programming.kicks-ass.net> 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== Message-ID: <3e359937-5b19-8a4c-4243-ba2edff68504@gmail.com> Date: Mon, 23 Sep 2019 19:37:46 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.0 MIME-Version: 1.0 In-Reply-To: <20190923071932.GD2349@hirez.programming.kicks-ass.net> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="afwh8Yn56RhJBUf9WB5Zf5rnPoIZsPbEd" 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) --afwh8Yn56RhJBUf9WB5Zf5rnPoIZsPbEd Content-Type: multipart/mixed; boundary="sioDMMgZJs6jpfgK812OHuEHse6sCnGbc"; protected-headers="v1" From: Pavel Begunkov To: Peter Zijlstra Cc: Jens Axboe , Ingo Molnar , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Message-ID: <3e359937-5b19-8a4c-4243-ba2edff68504@gmail.com> Subject: Re: [PATCH v2 1/2] sched/wait: Add wait_threshold References: <20190923071932.GD2349@hirez.programming.kicks-ass.net> In-Reply-To: <20190923071932.GD2349@hirez.programming.kicks-ass.net> --sioDMMgZJs6jpfgK812OHuEHse6sCnGbc Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable Just in case duplicating a mail from the cover-letter thread: It could be done with @cond indeed, that's how it works for now. However, this addresses performance issues only. The problem with wait_event_*() is that, if we have a counter and are trying to wake up tasks after each increment, it would schedule each waiting task O(threshold) times just for it to spuriously check @cond and go back to sleep. All that overhead (memory barriers, registers save/load, accounting, etc) turned out to be enough for some workloads to slow down the system. With this specialisation it still traverses a wait list and makes indirect calls to the checker callback, but the list supposedly is fairly small, so performance there shouldn't be a problem, at least for now. Regarding semantics; It should wake a task when a value passed to wake_up_threshold() is greater or equal then a task's threshold, that is specified individually for each task in wait_threshold_*(). In pseudo code: ``` def wake_up_threshold(n, wait_queue): for waiter in wait_queue: waiter.wake_up_if(n >=3D waiter.threshold); ``` Any thoughts how to do it better? Ideas are very welcome. BTW, this monster is mostly a copy-paste from wait_event_*(), wait_bit_*(). We could try to extract some common parts from these three, but that's another topic. On 23/09/2019 10:19, Peter Zijlstra wrote: > On Sun, Sep 22, 2019 at 11:08:50AM +0300, Pavel Begunkov (Silence) wrot= e: >> From: Pavel Begunkov >> >> Add wait_threshold -- a custom wait_event derivative, that waits until= >> a value is equal to or greater than the specified threshold. >=20 > This is quite insufficient justification for this monster... what exact= > semantics do you want? >=20 > Why can't you do this exact same with a slightly more complicated @cond= > ? >=20 --=20 Yours sincerely, Pavel Begunkov --sioDMMgZJs6jpfgK812OHuEHse6sCnGbc-- --afwh8Yn56RhJBUf9WB5Zf5rnPoIZsPbEd Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEE+6JuPTjTbx479o3OWt5b1Glr+6UFAl2I9NoACgkQWt5b1Glr +6Vv+BAAn48KtuECZJP43/AXvRPRyf3HrK7yvgmvlYX8AutwGPo4qO+0SGjwo5kM YLdodOixGzYWPUCHPgsdrBTyVhe5LMvUeFrNRGRHxzC+BwAkpAf5vOyiMlN+P+YG i0IvOffLgdHR8z14lQjbwwThxlaK7INW1+H/jUBll7sAF3NceH9zjJbDW5f0l2rK jPRpVEwahcM9nmSGc4s1XLxiSQpJLuGTStwhQfSilzbFod5A6moj1u63zBUuJ+gc oeqN0fZ32uv3DZmegJrQ1yY0QNRN7ck4cAnO0i5aPhOYVwGqrwgUEXNnpxiM6geM jnkAErcD7Y5OPFNngnkCY5Zrqucm9x4zy1+Jg8xPHajtC1z+a771v0XGyi5B3WjW YrnM8bJhOFj+4C3sam6DCXTSbHgSgsPLvIhYjx3d/JgwcSMbRt9jD3l8caqnv7XH Z3/E317auOBfZldTwmAx8WeaBsNw5QcpF+s1HrCnvFBoiso6wXv3baunrK4T8ab0 uQhwLbJwQ4w6bLhz2MpZzqzi47zc7sPDiSMmMBc5lEyA1P2Scyp6GPvShRgI3bVK fYwA+1I+Xdsr2FuUTi87Hi4J2Jif+UTc4gLlE4ib+mRF35mvyj0lt/h5JRb/dsiW yz1lA3bqY5uPJGVvhxAlguNo9bOJsNjcFu3csiwa4yxeckwMEcE= =HQjJ -----END PGP SIGNATURE----- --afwh8Yn56RhJBUf9WB5Zf5rnPoIZsPbEd--