From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B06C1C433ED for ; Fri, 9 Apr 2021 07:06:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6F30261107 for ; Fri, 9 Apr 2021 07:06:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231308AbhDIHGd (ORCPT ); Fri, 9 Apr 2021 03:06:33 -0400 Received: from out4436.biz.mail.alibaba.com ([47.88.44.36]:18368 "EHLO out4436.biz.mail.alibaba.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230412AbhDIHGb (ORCPT ); Fri, 9 Apr 2021 03:06:31 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R141e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04400;MF=haoxu@linux.alibaba.com;NM=1;PH=DS;RN=4;SR=0;TI=SMTPD_---0UUyCxSo_1617951939; Received: from B-25KNML85-0107.local(mailfrom:haoxu@linux.alibaba.com fp:SMTPD_---0UUyCxSo_1617951939) by smtp.aliyun-inc.com(127.0.0.1); Fri, 09 Apr 2021 15:05:39 +0800 Subject: Re: [PATCH 5.13 v2] io_uring: maintain drain requests' logic From: Hao Xu To: Jens Axboe , Pavel Begunkov 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> Message-ID: Date: Fri, 9 Apr 2021 15:05:39 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.9.0 MIME-Version: 1.0 In-Reply-To: <9fd3a4c0-488f-ca82-083a-78d448a1564e@linux.alibaba.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org 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 > > >>