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=-5.2 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 9A046C433ED for ; Tue, 20 Apr 2021 07:22:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 59DDB60238 for ; Tue, 20 Apr 2021 07:22:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229957AbhDTHWa (ORCPT ); Tue, 20 Apr 2021 03:22:30 -0400 Received: from out30-130.freemail.mail.aliyun.com ([115.124.30.130]:41315 "EHLO out30-130.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229718AbhDTHWa (ORCPT ); Tue, 20 Apr 2021 03:22:30 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R101e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04400;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=10;SR=0;TI=SMTPD_---0UWBJ92o_1618903315; Received: from admindeMacBook-Pro-2.local(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0UWBJ92o_1618903315) by smtp.aliyun-inc.com(127.0.0.1); Tue, 20 Apr 2021 15:21:57 +0800 Subject: Re: [dm-devel] [RFC PATCH 2/2] block: support to freeze bio based request queue To: Ming Lei Cc: Jens Axboe , linux-raid@vger.kernel.org, Bart Van Assche , Mike Snitzer , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, Song Liu , dm-devel@redhat.com, Christoph Hellwig References: <20210415103310.1513841-1-ming.lei@redhat.com> <20210415103310.1513841-3-ming.lei@redhat.com> From: JeffleXu Message-ID: <42c79dce-ad99-4e59-6566-727fa08a66bc@linux.alibaba.com> Date: Tue, 20 Apr 2021 15:21:55 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-raid@vger.kernel.org On 4/19/21 9:50 PM, Ming Lei wrote: > On Mon, Apr 19, 2021 at 08:05:46PM +0800, JeffleXu wrote: >> >> >> On 4/15/21 6:33 PM, Ming Lei wrote: >>> For bio based request queue, the queue usage refcnt is only grabbed >>> during submission, which isn't consistent with request base queue. >>> >>> Queue freezing has been used widely, and turns out it is very useful >>> to quiesce queue activity. >>> >>> Support to freeze bio based request queue by the following approach: >>> >>> 1) grab two queue usage refcount for blk-mq before submitting blk-mq >>> bio, one is for bio, anther is for request; >> >> >> Hi, I can't understand the sense of grabbing two refcounts on the >> @q_usage_count of the underlying blk-mq device, while >> @q_usage_count of the MD/DM device is kept untouched. > > Follows the point: > > 1) for blk-mq, we hold one refcount for bio and another for request, and > release one after ending bio or completing request. Blk-mq has already implemented queue freezing semantics, even without this 'grabbing two refcount'. So is this just for the code consisdency with the bio-based queue? > > 2) for bio based queue, just holding one refcount for bio, and release it > after the bio is ended. OK. > > As I mentioned to you, the current in-tree code only grabs the refcount > during submitting bio for bio base queue, and the refcount is released > after returning from submission, see __submit_bio(). Yes. I ignored that the refcount grabbed in the entry of bio submission has been returned back when the submission completes for bio-based queue. > >> >> In the following calling stack >> >> ``` >> queue_poll_store >> blk_mq_freeze_queue(q) >> ``` >> >> Is the input @q still the request queue of MD/DM device? > > It can be either one after bio based io polling is supported, > queue/io_poll is exposed for both blk-mq and bio based queue. > > However, I guess bio based polling doesn't need such strict bio queue > freezing, cause QUEUE_FLAG_POLL is only read in submission path, so > looks current freezing just during submission is enough. Not actually. blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin) struct blk_mq_hw_ctx *hctx; long state; - if (!blk_qc_t_valid(cookie) || !blk_queue_poll(q)) + if (!blk_queue_poll(q) || (queue_is_mq(q) && !blk_qc_t_valid(cookie))) Here QUEUE_FLAG_POLL is still checked in blk_poll() for bio-based queue, at least in your latest patch for bio-based polling. -- Thanks, Jeffle