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Thu, 01 Apr 2021 15:33:51 -0700 (PDT) Subject: Re: [PATCH for-5.13] io_uring: maintain drain requests' logic To: Hao Xu , Jens Axboe Cc: io-uring@vger.kernel.org, Joseph Qi References: <1617181262-66724-1-git-send-email-haoxu@linux.alibaba.com> <55b29f0f-967d-fc91-a959-60e01acc55a3@gmail.com> <652e4b3b-4b98-54db-a86c-31478ca33355@linux.alibaba.com> <49152a6e-6d8a-21f1-fd9c-8b764c21b2d3@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: <59d15355-4317-99af-c7c4-364d0e7c1682@gmail.com> Date: Thu, 1 Apr 2021 23:29:47 +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: <49152a6e-6d8a-21f1-fd9c-8b764c21b2d3@linux.alibaba.com> 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 01/04/2021 15:55, Hao Xu wrote: > 在 2021/4/1 下午6:25, Pavel Begunkov 写道: >> On 01/04/2021 07:53, Hao Xu wrote: >>> 在 2021/4/1 上午6:06, Pavel Begunkov 写道: >>>> >>>> >>>> On 31/03/2021 10:01, Hao Xu wrote: >>>>> Now that we have multishot poll requests, one sqe can emit multiple >>>>> cqes. given below example: >>>>>       sqe0(multishot poll)-->sqe1-->sqe2(drain req) >>>>> sqe2 is designed to issue after sqe0 and sqe1 completed, but since sqe0 >>>>> is a multishot poll request, sqe2 may be issued after sqe0's event >>>>> triggered twice before sqe1 completed. This isn't what users leverage >>>>> drain requests for. >>>>> Here a simple solution is to ignore all multishot poll cqes, which means >>>>> drain requests  won't wait those request to be done. >>>>> >>>>> Signed-off-by: Hao Xu >>>>> --- >>>>>    fs/io_uring.c | 9 +++++++-- >>>>>    1 file changed, 7 insertions(+), 2 deletions(-) >>>>> >>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c >>>>> index 513096759445..cd6d44cf5940 100644 >>>>> --- a/fs/io_uring.c >>>>> +++ b/fs/io_uring.c >>>>> @@ -455,6 +455,7 @@ struct io_ring_ctx { >>>>>        struct callback_head        *exit_task_work; >>>>>          struct wait_queue_head        hash_wait; >>>>> +    unsigned                        multishot_cqes; >>>>>          /* Keep this last, we don't need it for the fast path */ >>>>>        struct work_struct        exit_work; >>>>> @@ -1181,8 +1182,8 @@ static bool req_need_defer(struct io_kiocb *req, u32 seq) >>>>>        if (unlikely(req->flags & REQ_F_IO_DRAIN)) { >>>>>            struct io_ring_ctx *ctx = req->ctx; >>>>>    -        return seq != ctx->cached_cq_tail >>>>> -                + READ_ONCE(ctx->cached_cq_overflow); >>>>> +        return seq + ctx->multishot_cqes != ctx->cached_cq_tail >>>>> +            + READ_ONCE(ctx->cached_cq_overflow); >>>>>        } >>>>>          return false; >>>>> @@ -4897,6 +4898,7 @@ static bool io_poll_complete(struct io_kiocb *req, __poll_t mask, int error) >>>>>    { >>>>>        struct io_ring_ctx *ctx = req->ctx; >>>>>        unsigned flags = IORING_CQE_F_MORE; >>>>> +    bool multishot_poll = !(req->poll.events & EPOLLONESHOT); >>>>>          if (!error && req->poll.canceled) { >>>>>            error = -ECANCELED; >>>>> @@ -4911,6 +4913,9 @@ static bool io_poll_complete(struct io_kiocb *req, __poll_t mask, int error) >>>>>            req->poll.done = true; >>>>>            flags = 0; >>>>>        } >>>>> +    if (multishot_poll) >>>>> +        ctx->multishot_cqes++; >>>>> + >>>> >>>> We need to make sure we do that only for a non-final complete, i.e. >>>> not killing request, otherwise it'll double account the last one. >>> Hi Pavel, I saw a killing request like iopoll_remove or async_cancel call io_cqring_fill_event() to create an ECANCELED cqe for the original poll request. So there could be cases like(even for single poll request): >>>    (1). add poll --> cancel poll, an ECANCELED cqe. >>>                                                    1sqe:1cqe   all good >>>    (2). add poll --> trigger event(queued to task_work) --> cancel poll,            an ECANCELED cqe --> task_work runs, another ECANCELED cqe. >>>                                                    1sqe:2cqes >> >> Those should emit a CQE on behalf of the request they're cancelling >> only when it's definitely cancelled and not going to fill it >> itself. E.g. if io_poll_cancel() found it and removed from >> all the list and core's poll infra. >> >> At least before multi-cqe it should have been working fine. >> > I haven't done a test for this, but from the code logic, there could be > case below: > > io_poll_add()                         | io_poll_remove > (event happen)io_poll_wake()          | io_poll_remove_one >                                       | io_poll_remove_waitqs >                                       | io_cqring_fill_event(-ECANCELED) >                                       | > task_work run(io_poll_task_func)      | > io_poll_complete()                    | > req->poll.canceled is true, \         | > __io_cqring_fill_event(-ECANCELED)    | > > two ECANCELED cqes, is there anything I missed? Definitely may be be, but need to take a closer look >>> I suggest we shall only emit one ECANCELED cqe. >>> Currently I only account cqe through io_poll_complete(), so ECANCELED cqe from io_poll_remove or async_cancel etc are not counted in. >>>> E.g. is failed __io_cqring_fill_event() in io_poll_complete() fine? >>>> Other places? >>> a failed __io_cqring_fill_event() doesn't produce a cqe but increment ctx->cached_cq_overflow, as long as a cqe is produced or cached_cq_overflow is +=1, it is ok. >> >> Not claiming that the case is broken, but cached_cq_overflow is >> considered in req_need_defer() as well, so from its perspective there >> is no much difference between succeed fill_event() or not. >> >>>> >>>> Btw, we can use some tests :) >>> I'll do more tests. >> >> Perfect! >> >>>> >>>> >>>>>        io_commit_cqring(ctx); >>>>>        return !(flags & IORING_CQE_F_MORE); >>>>>    } -- Pavel Begunkov