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=-5.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 32325C43460 for ; Tue, 20 Apr 2021 07:59:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id F110D61077 for ; Tue, 20 Apr 2021 07:58:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229718AbhDTH73 (ORCPT ); Tue, 20 Apr 2021 03:59:29 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:35398 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229521AbhDTH73 (ORCPT ); Tue, 20 Apr 2021 03:59:29 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1618905537; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=DAjeY/T/CPScSPWymCzycaeNgOOPXoZc8Xkk3oZ35yo=; b=NGzp5blfOjjj/hO/aTzl8BWEnwUtaCVx7Rf2O3rG2yDQ3KOWdR+L3RadtmaG2Ctp3cGmjr HlGMN0OgLp+EMRF0804sEJF8N8NiGJoDNJyfVHcJ/RmAFJZ4ri9vj6uITDFIrO9bEVohPS 5odycSR9vPNIZWbzuuUudSM+Zp2t3CI= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-267-ZNEhrwp9NXembJ99KWR84g-1; Tue, 20 Apr 2021 03:58:53 -0400 X-MC-Unique: ZNEhrwp9NXembJ99KWR84g-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 2C17B91E767; Tue, 20 Apr 2021 07:58:52 +0000 (UTC) Received: from T590 (ovpn-13-154.pek2.redhat.com [10.72.13.154]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 735E75D9CA; Tue, 20 Apr 2021 07:58:39 +0000 (UTC) Date: Tue, 20 Apr 2021 15:58:37 +0800 From: Ming Lei To: JeffleXu 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 Subject: Re: [dm-devel] [RFC PATCH 2/2] block: support to freeze bio based request queue Message-ID: References: <20210415103310.1513841-1-ming.lei@redhat.com> <20210415103310.1513841-3-ming.lei@redhat.com> <42c79dce-ad99-4e59-6566-727fa08a66bc@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <42c79dce-ad99-4e59-6566-727fa08a66bc@linux.alibaba.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Precedence: bulk List-ID: X-Mailing-List: linux-raid@vger.kernel.org On Tue, Apr 20, 2021 at 03:21:55PM +0800, JeffleXu wrote: > > > 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? Right. > > > > > > 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. OK, we can simply drop it. Thanks, Ming