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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=ham 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 5092DC433B4 for ; Fri, 9 Apr 2021 06:15:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2081E60FF0 for ; Fri, 9 Apr 2021 06:15:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233352AbhDIGPx (ORCPT ); Fri, 9 Apr 2021 02:15:53 -0400 Received: from out30-133.freemail.mail.aliyun.com ([115.124.30.133]:33397 "EHLO out30-133.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231402AbhDIGPx (ORCPT ); Fri, 9 Apr 2021 02:15:53 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R131e4;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_---0UUyWRps_1617948939; Received: from B-25KNML85-0107.local(mailfrom:haoxu@linux.alibaba.com fp:SMTPD_---0UUyWRps_1617948939) by smtp.aliyun-inc.com(127.0.0.1); Fri, 09 Apr 2021 14:15:39 +0800 Subject: Re: [PATCH 5.13 v2] io_uring: maintain drain requests' logic 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> From: Hao Xu Message-ID: <9fd3a4c0-488f-ca82-083a-78d448a1564e@linux.alibaba.com> Date: Fri, 9 Apr 2021 14:15:38 +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: <19183813-6755-52bb-5391-4809a837ec5f@kernel.dk> 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 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 >