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Tue, 14 Apr 2020 09:32:01 -0700 (PDT) To: Jens Axboe , Hrvoje Zeba Cc: io-uring@vger.kernel.org, "zhangyi (F)" References: <0df2f436-0968-c708-84e2-da0c3daa265c@kernel.dk> <6835cec5-c8a5-dc49-c4e3-0df276c8537a@gmail.com> <05510b01-4d0f-28c4-b987-999e4e91ce66@gmail.com> <2eba8624-b487-cc5a-61d0-9c046ad88eec@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: Odd timeout behavior Message-ID: Date: Tue, 14 Apr 2020 19:31:01 +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: <2eba8624-b487-cc5a-61d0-9c046ad88eec@kernel.dk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 14/04/2020 19:04, Jens Axboe wrote: > On 4/14/20 9:46 AM, Pavel Begunkov wrote: >> On 14/04/2020 03:44, Jens Axboe wrote: >>> On 4/13/20 1:09 PM, Pavel Begunkov wrote: >>>> On 13/04/2020 17:16, Jens Axboe wrote: >>>>> On 4/13/20 2:21 AM, Pavel Begunkov wrote: >>>>>> On 4/12/2020 6:14 PM, Hrvoje Zeba wrote: >>>>>>> On Sun, Apr 12, 2020 at 5:15 AM Pavel Begunkov wrote: >>>>>>>> >>>>>>>> On 4/12/2020 5:07 AM, Jens Axboe wrote: >>>>>>>>> On 4/11/20 5:00 PM, Hrvoje Zeba wrote: >>>>>>>>>> Hi, >>>>>>>>>> >>>>>>>>>> I've been looking at timeouts and found a case I can't wrap my head around. >>>>>>>>>> >>>>>>>>>> Basically, If you submit OPs in a certain order, timeout fires before >>>>>>>>>> time elapses where I wouldn't expect it to. The order is as follows: >>>>>>>>>> >>>>>>>>>> poll(listen_socket, POLLIN) <- this never fires >>>>>>>>>> nop(async) >>>>>>>>>> timeout(1s, count=X) >>>>>>>>>> >>>>>>>>>> If you set X to anything but 0xffffffff/(unsigned)-1, the timeout does >>>>>>>>>> not fire (at least not immediately). This is expected apart from maybe >>>>>>>>>> setting X=1 which would potentially allow the timeout to fire if nop >>>>>>>>>> executes after the timeout is setup. >>>>>>>>>> >>>>>>>>>> If you set it to 0xffffffff, it will always fire (at least on my >>>>>>>>>> machine). Test program I'm using is attached. >>>>>>>>>> >>>>>>>>>> The funny thing is that, if you remove the poll, timeout will not fire. >>>>>>>>>> >>>>>>>>>> I'm using Linus' tree (v5.6-12604-gab6f762f0f53). >>>>>>>>>> >>>>>>>>>> Could anybody shine a bit of light here? >>>>>>>>> >>>>>>>>> Thinking about this, I think the mistake here is using the SQ side for >>>>>>>>> the timeouts. Let's say you queue up N requests that are waiting, like >>>>>>>>> the poll. Then you arm a timeout, it'll now be at N + count before it >>>>>>>>> fires. We really should be using the CQ side for the timeouts. >>>>>>>> >>>>>>>> As I get it, the problem is that timeout(off=0xffffffff, 1s) fires >>>>>>>> __immediately__ (i.e. not waiting 1s). >>>>>>> >>>>>>> Correct. >>>>>>> >>>>>>>> And still, the described behaviour is out of the definition. It's sounds >>>>>>>> like int overflow. Ok, I'll debug it, rest assured. I already see a >>>>>>>> couple of flaws anyway. >>>>>>> >>>>>>> For this particular case, >>>>>>> >>>>>>> req->sequence = ctx->cached_sq_head + count - 1; >>>>>>> >>>>>>> ends up being 1 which triggers in __req_need_defer() for nop sq. >>>>>> >>>>>> Right, that's it. The timeout's seq counter wraps around and triggers on >>>>>> previously submitted but still inflight requests. >>>>>> >>>>>> Jens, could you remind, do we limit number of inflight requests? We >>>>>> discussed it before, but can't find the thread. If we don't, vile stuff >>>>>> can happen with sequences. >>>>> >>>>> We don't. >>>> >>>> I was too quick to judge, there won't be anything too bad, and only if we throw >>>> 2^32 requests (~1TB). >>>> >>>> For the issue at hand, how about limiting timeouts' sqe->off by 2^31? This will >>>> solve the issue for now, and I can't imagine anyone waiting for over one billion >>>> requests to pass. >>> >>> I'm fine with that, but how do we handle someone asking for > INT_MAX? >> >>> INT_MAX is allowed, but I want to return -EINVAL instead. >> If you mean UINT_MAX, then sqe->off is u32, so can't happen. > > No, I mean count > INT_MAX, what you're suggesting we just don't support. Got it. That's what my question was about. > If there are apps right now using that, how do we handle it? 1. if # of inflight requests is limited (empirically/naturally or not), then we can extend req->seq to >= 33 bits (leaving sqe->off u32), and it'll just work. e.g. if req->seq is u64, then we need 2^64 - 2^32 inflight requests to hit the issue. And I don't expect anybody creating requests worth of 1ZB (2^30 TB). 2. or to think about something I'll send 1., and then out of curiosity give a thought to 2. -- Pavel Begunkov