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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 03F34C433EF for ; Mon, 9 May 2022 11:53:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233271AbiEIL5E (ORCPT ); Mon, 9 May 2022 07:57:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55806 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233314AbiEIL5D (ORCPT ); Mon, 9 May 2022 07:57:03 -0400 Received: from out30-57.freemail.mail.aliyun.com (out30-57.freemail.mail.aliyun.com [115.124.30.57]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 60C23248E3 for ; Mon, 9 May 2022 04:53:09 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R241e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=alimailimapcm10staff010182156082;MF=xiaoguang.wang@linux.alibaba.com;NM=1;PH=DS;RN=6;SR=0;TI=SMTPD_---0VCkPC54_1652097185; Received: from 30.82.254.134(mailfrom:xiaoguang.wang@linux.alibaba.com fp:SMTPD_---0VCkPC54_1652097185) by smtp.aliyun-inc.com(127.0.0.1); Mon, 09 May 2022 19:53:06 +0800 Message-ID: <2ed84b17-e9cf-973f-170c-f56eb90517ba@linux.alibaba.com> Date: Mon, 9 May 2022 19:53:05 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Thunderbird/91.8.1 Subject: Re: Follow up on UBD discussion Content-Language: en-US To: Ming Lei Cc: ZiyangZhang , Gabriel Krisman Bertazi , joseph.qi@linux.alibaba.com, linux-block@vger.kernel.org, Jens Axboe References: <874k27rfwm.fsf@collabora.com> <8a52ed85-3ffa-44a4-3e28-e13cdc793732@linux.alibaba.com> <55edea6e-e7dc-054a-b79b-fcfc40c22f2f@linux.alibaba.com> From: Xiaoguang Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org hi, > >>>> Second, I'd like to share some ideas on UBD. I'm not sure if they are >>>> reasonable so please figure out my mistakes. >>>> >>>> 1) UBD issues one sqe to commit last completed request and fetch a new >>>> one. Then, blk-mq's queue_rq() issues a new UBD IO request and completes >>>> one cqe for the fetch command. We have evaluated that io_submit_sqes() >>>> costs some CPU and steps of building a new sqe may lower throughput. >>>> Here I'd like to give a new solution: never submit sqe but trump up a >>>> cqe(with information of new UBD IO request) when calling queue_rq(). I >>>> am inspired by one io_uring flag: IORING_POLL_ADD_MULTI, with which a >>>> user issues only one sqe for polling an fd and repeatedly gets multiple >>>> cqes when new events occur. Dose this solution break the architecture of >>>> UBD? >>> But each cqe has to be associated with one sqe, if I understand >>> correctly. >> Yeah, for current io_uring implementation, it is. But if io_uring offers below >> helper: >> void io_gen_cqe_direct(struct file *file, u64 user_data, s32 res, u32 cflags) >> { >>         struct io_ring_ctx *ctx; >>         ctx = file->private_data; >> >>         spin_lock(&ctx->completion_lock); >>         __io_fill_cqe(ctx, user_data, res, cflags); >>         io_commit_cqring(ctx); >>         spin_unlock(&ctx->completion_lock); >>         io_cqring_ev_posted(ctx); >> } >> >> Then in ubd driver: >> 1) device setup stage >> We attach io_uring files and user_data to every ubd hard queue. >> >> 2) when blk-mq->queue_rq() is called. >> io_gen_cqe_direct() will be called in ubd's queue_rq, and we put ubd io request's >> qid and tag info into cqe's res field, then we don't need to issue sqe to fetch io cmds. > The above way is actually anti io_uring design, and I don't think it may > improve much since submitting UBD_IO_COMMIT_AND_FETCH_REQ is pretty lightweight. Actually I don't come up with this idea mostly for performance reason :) Just try to simplify codes a bit: 1) In current implementation, ubdsrv will need to submit queue depth number of sqes firstly, and ubd_ctrl_start_dev() will also need to wait all sqes to be submitted. 2) Try to make ubd_queue_rq simpler, it maybe just call one io_gen_cqe_direct(). > > Also without re-submitting UBD_IO_COMMIT_AND_FETCH_REQ command, how can you > commit io handling result from ubd server and ask ubd driver to complete > io request? No, I don't mean to remove COMMIT command, we still need io_uring async command feature to support ubd COMMIT or GETDATA command. I have another concern that currently there are may flags in ubd kernel or ubdsrv, such as: #define UBDSRV_NEED_FETCH_RQ (1UL << 0) #define UBDSRV_NEED_COMMIT_RQ_COMP (1UL << 1) #define UBDSRV_IO_FREE (1UL << 2) #define UBDSRV_IO_HANDLING (1UL << 3) #define UBDSRV_NEED_GET_DATA (1UL << 4) Some of their names looks weird, for example UBDSRV_IO_FREE. I think more flags may result in more state machine error. Regards, Xiaoguang Wang > > > Thanks, > Ming