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Fri, 09 Apr 2021 00:54:27 -0700 (PDT) Subject: Re: [PATCH 5.13 v2] io_uring: maintain drain requests' logic To: Hao Xu , Jens Axboe Cc: io-uring@vger.kernel.org, Joseph Qi References: <1617794605-35748-1-git-send-email-haoxu@linux.alibaba.com> <00898a9b-d2f2-1108-b9d9-2d6acea6e713@kernel.dk> <32f812e1-c044-d4b3-d26f-3721e4611a1d@linux.alibaba.com> <119436dd-5e55-9812-472c-7a257bda12fb@linux.alibaba.com> <826e199f-1cc0-f529-f200-5fa643a62bca@gmail.com> <19183813-6755-52bb-5391-4809a837ec5f@kernel.dk> <9fd3a4c0-488f-ca82-083a-78d448a1564e@linux.alibaba.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== Message-ID: Date: Fri, 9 Apr 2021 08:50:21 +0100 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: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 09/04/2021 08:05, Hao Xu wrote: > 在 2021/4/9 下午2:15, Hao Xu 写道: >> 在 2021/4/9 上午12:18, Jens Axboe 写道: >>> On 4/8/21 6:22 AM, Pavel Begunkov wrote: >>>> On 08/04/2021 12:43, Hao Xu wrote: >>>>> 在 2021/4/8 下午6:16, Hao Xu 写道: >>>>>> 在 2021/4/7 下午11:49, Jens Axboe 写道: >>>>>>> On 4/7/21 5:23 AM, Hao Xu wrote: >>>>>>>> more tests comming, send this out first for comments. >>>>>>>> >>>>>>>> Hao Xu (3): >>>>>>>>     io_uring: add IOSQE_MULTI_CQES/REQ_F_MULTI_CQES for multishot requests >>>>>>>>     io_uring: maintain drain logic for multishot requests >>>>>>>>     io_uring: use REQ_F_MULTI_CQES for multipoll IORING_OP_ADD >>>>>>>> >>>>>>>>    fs/io_uring.c                 | 34 +++++++++++++++++++++++++++++----- >>>>>>>>    include/uapi/linux/io_uring.h |  8 +++----- >>>>>>>>    2 files changed, 32 insertions(+), 10 deletions(-) >>>>>>> >>>>>>> Let's do the simple cq_extra first. I don't see a huge need to add an >>>>>>> IOSQE flag for this, probably best to just keep this on a per opcode >>>>>>> basis for now, which also then limits the code path to just touching >>>>>>> poll for now, as nothing else supports multishot CQEs at this point. >>>>>>> >>>>>> gotcha. >>>>>> a small issue here: >>>>>>    sqe-->sqe(link)-->sqe(link)-->sqe(link, multishot)-->sqe(drain) >>>>>> >>>>>> in the above case, assume the first 3 single-shot reqs have completed. >>>>>> then I think the drian request won't be issued now unless the multishot request in the linkchain has been issued. The trick is: a multishot req >>>>>> in a linkchain consumes cached_sq_head when io_get_sqe(), which means it >>>>>> is counted in seq, but we will deduct the sqe when it is issued if we >>>>>> want to do the job per opcode not in the main code path. >>>>>> before the multishot req issued: >>>>>>        all_sqes(4) - multishot_sqes(0) == all_cqes(3) - multishot_cqes(0) >>>>>> after the multishot req issued: >>>>>>        all_sqes(4) - multishot_sqes(1) == all_cqes(3) - multishot_cqes(0) >>>>> >>>>> Sorry, my statement is wrong. It's not "won't be issued now unless the >>>>> multishot request in the linkchain has been issued". Actually I now >>>>> think the drain req won't be issued unless the multishot request in the >>>>> linkchain has completed. Because we may first check req_need_defer() >>>>> then issue(req->link), so: >>>>>     sqe0-->sqe1(link)-->sqe2(link)-->sqe3(link, multishot)-->sqe4(drain) >>>>> >>>>>    sqe2 is completed: >>>>>      call req_need_defer: >>>>>      all_sqes(4) - multishot_sqes(0) == all_cqes(3) - multishot_cqes(0) >>>>>    sqe3 is issued: >>>>>      all_sqes(4) - multishot_sqes(1) == all_cqes(3) - multishot_cqes(0) >>>>>    sqe3 is completed: >>>>>      call req_need_defer: >>>>>      all_sqes(4) - multishot_sqes(1) == all_cqes(3) - multishot_cqes(0) >>>>> >>>>> sqe4 shouldn't wait sqe3. >>>> >>>> Do you mean it wouldn't if the patch is applied? Because any drain >>>> request must wait for all requests submitted before to complete. And >>>> so before issuing sqe4 it must wait for sqe3 __request__ to die, and >>>> so for all sqe3's CQEs. >>>> >>>> previously >>> >>> I think we need to agree on what multishot means for dependencies. Does >>> it mean it just needs to trigger once? Or does it mean that it needs to >>> be totally finished. The latter may obviously never happen, depending on >>> the use case. Or it may be an expected condition because the caller will >>> cancel it at some point. >>> >>> The most logical view imho is that multishot changes nothing wrt drain. >>> If you ask for drain before something executes and you are using >>> multishot, then you need to understand that the multishot request needs >>> to fully complete before that condition is true and your dependency can >>> execute. >> This makes sense, and the implementation would be quite simpler. but we >> really need to document it somewhere so that users easily get to know >> that they cannot put a drain req after some multishot reqs if they don't >> want it to wait for them. Otherwise I worry about wrong use of it since >> the meaning of 'put a drain req after some multishot reqs' isn't so >> obvious: >>     - does it waits for those multishot reqs to complete once >>     - or does it waits for those ones to fully complete >>     - or does it ignore those ones at all >> > I realised that if a drain req has to wait for multishot reqs' fully >  completion, then users have to explicitly cancel all the previous > multishot reqs, otherwise it won't execute forever: >     sqe0(multishot)-->sqe1(drain)-->sqe2(cancel multishot)    stuck And it's not a new behaviour, e.g. read(pipe); drain(); where nobody writes to the pipe will stuck as well. I like that it currently provides a full barrier between requests, are there other patterns used by someone? -- Pavel Begunkov